# X-Y Plotter Table For The TI 99/4A Computer - Final

Posted by Vorticon, 25 May 2018 · 657 views

So I've been mulling the idea of creating an X-Y table for my TI computer, which could be used to draw a bitmap image or perhaps do some laser engraving. I searched the web for inspiration and found this ingenious video by HomoFaciens where he uses the stepper platform that moves the laser head in optical drives to create a very effective but small X-Y plotter.

So I went ahead an found a couple of used DVD-ROM drives on Ebay and ripped them apart, only to find that only one of them had a stepper motor assembly. According to HomoFaciens, only about 50% of the optical drives actually use stepper motors. Nonetheless, I figured I could start by experimenting in controlling the stepper platform with the TI's parallel (PIO) port. Below is the actual stepper motor assembly from one of the drives:

The stepper motor in this assembly is a 4 wire stepper motor, which means it has 2 coils with each pair of wires going to one coil. It's easy to identify each pair by simply doing a continuity test on the wires using a multimeter. To move the motor in one direction, one of the coil has to cycle it's polarity repeatedly between positive and negative while the other coil is idle. To reverse the motion, simply repeat the process using the second coil.
In order to be able to control the polarity on the coil, one could use relay circuits like the one on my robotic arm controller, or more commonly use an H-bridge circuit as below:

Based on the input to the 4 input points on the circuit marked A, B, C and D, the polarity can be switched at will. The table below shows how to do this.

Given that we have 2 coils in the stepper motor, we obviously will need 2 H-bridges to control it, one for each coil. While you can purchase ready made stepper motor controllers on most hobbyist sites, it's really pretty simple to build for just pennies, so I went ahead and breadboarded a couple of H-bridges for test purposes.

As for the connection to the TI, I decided to allocate one data pin on the PIO port to each input port on the H-bridges, which means that all the data pins will be used (Eight). This is problematic because the X-Y table will need 2 stepper motors at a minimum, one for the X axis and one for the Y axis, and I obviously will not have enough data pins for that. The solution is going to involve some kind of multiplexing, but we'll come back to that. For now let's make sure this thing actually works!

Here's the connection layout:

Coil 1:
A --> D7
B --> D6
C --> D5
D --> D4

Coil 2:
A --> D3
B --> D2
C --> D1
D --> D0

So in order to control the coils, I just need to send the appropriate byte pattern to the PIO port. For example, in order to run the stepper motor forward, coil 1 needs to have a bit pattern of 1100 corresponding to the DCAB H-bridge inputs, and coil 2 needs to be inactive, which translates to a bit pattern of 1010 (refer to the table above). Putting it all together we get:

```   Coil 2           Coil 1
D   C   B   A    D   C   B   A
D0  D1  D2  D3   D4  D5  D6  D7
1   0   1   0    1   1   0   0
```

Converting this 8-bit binary number to decimal we get 172, which we can send to the PIO port via the CALL LOAD command in Rich Extended Basic. Then we reverse the polarity of coil 1 while keeping coil 2 inactive to keep the stepper motor moving forward and so on and so forth.
Here's the control program on the TI. I opted to use Rich Extended Basic for its ease of use and unique ability to allow low-level access to hardware. Below is the test program listing:
```10 CALL CLEAR
20 CRU=2432 ! RS232 CARD CRU ADDRESS IS >1300 (4864). RXB USES CRU/2
30 CALL IO(3,1,CRU,1) ! ACTIVATE THE RS232 CARD
40 CALL IO(3,1,CRU+7,1) ! TURN ON THE LED ON THE CARD
50 CALL IO(3,1,CRU+1,0) ! SET THE PIO PORT TO OUTPUT
60 FOR I=1 TO 30 ! 30 FORWARD STEPS
70 CALL LOAD(20480,172) ! COIL 1 POSITIVE POLARITY - COIL 2 INACTIVE -- PIO ADDRESS IS 20480
80 GOSUB 500 ! DELAY TO SLOW DOWN THE MOVEMENT
90 CALL LOAD(20480,163) ! COIL 1 NEGATIVE POLARITY - COIL 2 INACTIVE
100 GOSUB 500
110 NEXT I
120 FOR I=1 TO 30 ! 30 REVERSE STEPS
130 CALL LOAD(20480,202) ! COIL 1 INACTIVE - COIL 2 POSITIVE POLARITY
140 GOSUB 500
150 CALL LOAD(20480,58) ! COIL 1 INACTIVE - COIL 2 NEGATIVE POLARITY
160 GOSUB 500
170 NEXT I
180 GOTO 60 ! REPEAT THE ENTIRE PROCESS
500 FOR D=1 TO 200::NEXT D::RETURN
```
And here's what it looks like in action:

So while it works in principle, as I mentioned in the video there are several issues to contend with if we are to use the DVD-ROM assemblies:

• The quality of the worm gear assembly is highly variable. Mine was pretty bad...
• The weight that could be supported by the carrier is tiny on my drive assembly, so it will not be able to hold a second assembly on top for the Y axis.
• The usable draw area is very small, 40x40 steps on my drive assembly, so not very practical.
I don't know how many optical drives HomoFaciens had to go through before he got the high quality ones he demonstrated, but I'm not terribly inclined to go that route. This of course means that I will have to build my own table.

UPDATE 6/10/18

Pack007 suggested using a 74HC595 8-bit serial shift register chip for multiplexing to PIO data lines, and it worked extremely well. That chip basically takes in an 8 bit number serially one bit at a time starting with the LSB and outputs that number in parallel using 8 output lines. In addition it has an Output Enable pin which places the parallel output pins in a high impedance state when high, effectively inactivating the chip. The control sequence will go like this:
• Make the LATCH pin (12) low to isolate the serial input from the output
• Activate the chip by making the OE pin (13) low
• Present 1 bit to the DATA pin (14) and cycle the CLOCK pin (11)
• Go back to 3 until all 8 bits are in
• Cycle the LATCH pin (12) to present the number to the output pins
• Go back to 1 for the next number
The output 8-bit number will be coded per the coil sequence discussed earlier and connected to the H-bridges accordingly for the stepper motor. Here's the circuit diagram for a single motor:

```PIO Connections:

D7 (2) --> DATA
D6 (3) --> OE for motor 1
D5 (4) --> OE for motor 2
D4 (5) --> OE for motor 3
HSKOUT (1) --> CLOCK
SPROUT (14) --> LATCH

```

Now we will need 3 stepper motors for the plotter: X, Y and Pen. The way this will work is that each motor will have 2 H-bridges connected to a separate 74HC595 chip. These 3 chips will share the LATCH, CLOCK and DATA pins connections but will each have their own OE pin connections. That way, the computer will be able to select the desired motor by simply making the corresponding OE line low and the other 2 OE lines high. All in all, only 6 output lines from the PIO port will be needed instead of the 24 lines required without multiplexing. Problem solved! This leaves me with several lines that could be used to detect axis end of travel through the use of micro-switches.

And here's the RXB test code. It's definitely a slower process than the previous direct parallel method, but it's the price to pay for multiplexing. This is fine for testing, but I might have to switch to assembly for the final control program in order to speed things up. We'll see...
```10 CALL CLEAR
11 OE1=12 ! BIT PATTERN TO ISOLATE MOTOR 1
14 DIM FWD(16),REV(16),OFF(8)
15 CRU=2432 ! CRU BASE OF RS232 CARD DIVIDED BY 2
20 CALL IO(3,1,CRU,1) ! ACTIVATE RS232 CARD
30 CALL IO(3,1 CRU+7,1) ! TURN LED ON
40 CALL IO(3,1,CRU+1,0) ! SET PIO TO OUTPUT
50 FOR I=0 TO 15::READ FWD(I)::NEXT I ! READ FORWARD CONTROL SEQUENCE
60 FOR I=0 TO 15::READ REV(I)::NEXT I ! READ REVERSE CONTROL SEQUENCE
70 FOR I=0 TO 8::READ OFF(I)::NEXT I ! READ IDLE CONTROL SEQUENCE
80 REM MOTOR 1 MOVE FORWARD
90 CALL IO(3,1,CRU+3,0) ! MAKE LATCH PIN LOW
100 CALL LOAD(20480,OE1) ! ISOLATE MOTOR 1
110 FOR I=1 TO 30 ! 30 FORWARD STEPS
120 FOR N=0 TO 15 ! FORWARD BIT SEQUENCE IS 2 8-BIT NUMBERS LONG (16 BITS)
121 REM THE LINE BELOW SENDS A SINGLE BIT OUT THE PIO LSB (D7) PIN
122 REM IF THE BIT IS 1, IT IS ADDED TO THE OE1 BIT PATTERN (D6-D4)
123 REM OTHERWISE ONLY OE1 IS SENT AND D7 REMAINS 0
130 IF FWD(N)=1 THEN CALL LOAD(20480,OE1+1) ELSE CALL LOAD(20480,OE1)
140 CALL IO(3,1,CRU+2,1)::CALL IO(3,1,CRU+2,0) ! CYCLE THE CLOCK PIN
141 REM THE LINE BELOW PRESENTS THE 8-BIT NUMBER TO OUTPUT
150 IF N=7 OR N=15 THEN CALL IO(3,1,CRU+3,1)::CALL IO(3,1,CRU+3,0) ! CYCLE THE LATCH PIN
160 NEXT N
170 NEXT I
180 REM MOTOR 1 REVERSE MOVE
190 FOR I=1 TO 30 ! 30 REVERSE STEPS
200 FOR N=0 TO 15 ! REVERSE BIT SEQUENCE IS 2 8-BIT NUMBERS LONG (16 BITS)
201 REM THE LINE BELOW SENDS A SINGLE BIT OUT THE PIO LSB (D7) PIN
202 REM IF THE BIT IS 1, IT IS ADDED TO THE OE1 BIT PATTERN (D6-D4)
203 REM OTHERWISE ONLY OE1 IS SENT AND D7 REMAINS 0
210 IF REV(N)=1 THEN CALL LOAD(20480,OE1+1) ELSE CALL LOAD(20480,OE1)
220 CALL IO(3,1,CRU+2,1)::CALL IO(3,1,CRU+2,0) ! CYCLE THE CLOCK PIN
221 REM THE LINE BELOW PRESENTS THE 8-BIT NUMBER TO OUTPUT
230 IF N=7 OR N=15 THEN CALL IO(3,1,CRU+3,1)::CALL IO(3,1,CRU+3,0) ! CYCLE THE LATCH PIN
240 NEXT N
250 NEXT I
260 GOTO 110 ! REPEAT THE PROCESS
500 REM MOTOR ACTIVATION SEQUENCES
510 REM FORWARD
520 DATA 0,0,1,1,0,1,0,1,1,1,0,0,0,1,0,1
530 REM REVERSE
540 DATA 0,1,0,1,0,0,1,1,0,1,0,1,1,1,0,0
550 REM IDLE
560 DATA 0,1,0,1,0,1,0,1
```
It is likely I will need to use more beefy transistors for the H-bridges because the current needed by the stepper motors in the final design will be hefty. However, the basic circuit design will remain the same.

Next I'm going to focus on the mechanical assembly.

Update 6/14/18

Soooo, to be perfectly honest I have not been too happy with the performance of the stepper motor extracted from the optical drive. The torque was way too low and the steps were far too large and I had the nagging suspicion I was doing something wrong, and indeed I was!
After further research, it turned out that my step control sequence was incomplete, and I was essentially skipping every other step. The correct sequence for a bipolar 4-wire stepper motor like the one I have turned to be as below:

A and B represent the 2 coils in the motor. To reverse the rotation, simply swap the sequences of A and B. And sure enough when I applied that sequence the torque increased dramatically because both coils were always energized at any one time and the steps were much smaller and smoother. Live and learn

Furthermore, after taking a closer look at the H-bridge circuit I had used and comparing it to other circuits out there, I realized that there was no need to use a mix of NPN and PNP transistors, and that just NPN ones will do the trick, as well as only require 2 input pins for control instead of 4 per H-bridge! And that meant that now I could control 2 stepper motors using only one 74HC595 chip instead of 2 chips! Here's the updated circuit for a single motor:

The new PIO connections become:
```PIO Connections:

D7 (2) --> DATA
D6 (3) --> OE for motor 1 & 2
D5 (4) --> OE for motor 3
HSKOUT (1) --> CLOCK
SPROUT (14) --> LATCH ```
And the control sequence for the steps will be as below, with A and B this time representing each H-bridge input:
```   COIL 2        COIL 1
A      B      A      B
----------------------

1      0      1      0  FORWARD
0      1      1      0
0      1      0      1
1      0      0      1

1      0      1      0  REVERSE
1      0      0      1
0      1      0      1
0      1      1      0
```
Amended RXB control test program. Notice that the trailing 4 zero bits of each byte in the control sequences are reserved for the future control of the seconds stepper motor.
```10 CALL CLEAR
20 OE1=4 ! ISOLATE MOTORS 1 & 2
30 OE2=2 ! ISOLATE MOTOR 3
40 DIM FWD(32),REV(32)
50 CRU=2432 ! BASE RS232 / 2
60 CALL IO(3,1,CRU,1) ! ACTIVATE RS232 CARD
70 CALL IO(3,1,CRU+7,1) ! TURN LED ON
80 CALL IO(3,1,CRU+1,0) ! SET PIO TO OUTPUT
90 FOR I=1 TO 31::READ FWD(I)::NEXT I
100 FOR I=1 TO 31::READ REV(I)::NEXT I
110 REM FORWARD STEPS
120 CALL IO(3,1,CRU+3,1) ! MAKE LATCH LOW
130 CALL LOAD(20480,OE1) ! ISOLATE MOTORS 1 & 2
140 FOR I=1 TO 5
150 FOR N=0 TO 31
155 REM LINE BELOW SENDS DATA BIT BY BIT VIA D7 PIN ON PIO PORT
160 IF FWD(I)=1 THEN CALL LOAD(20480,OE1+1) ELSE CALL LOAD(20480,OE1)
170 CALL IO(3,1,CRU+2,1)::CALL IO(3,1,CRU+2,0) ! PULSE CLOCK
180 IF N=7 OR N=15 OR N=23 OR N=31 THEN CALL IO(3,1,CRU+3,1)::CALL IO(3,1,CRU+3,0) ! CYCLE LATCH
190 NEXT N
200 NEXT I
210 REM REVERSE STEPS
220 FOR I=1 TO 5
230 FOR N=0 TO 31
240 IF REV(N)=1 THEN CALL LOAD(20480,OE1+1) ELSE CALL LOAD(20480,OE1)
250 CALL IO(3,1,CRU+2,1)::CALL IO(3,1,CRU+2,0) ! PULSE CLOCK
260 IF N=7 OR N=15 OR N=23 OR N=31 THEN CALL IO(3,1,CRU+3,1)::CALL IO(3,1,CRU+3,0) ! CYCLE LATCH
270 NEXT N
280 NEXT I
290 GOTO 140
500 REM MOTOR ACTIVATION SEQUENCES
510 REM FORWARD
520 DATA 1,0,1,0,0,0,0,0,0,1,1,0,0,0,0,0,0,1,0,1,0,0,0,0,1,0,0,1,0,0,0,0
530 REM BACKWARD
540 DATA 1,0,1,0,0,0,0,0,1,0,0,1,0,0,0,0,0,1,0,1,0,0,0,0,0,1,1,0,0,0,0,0
```
And here's the whole thing in action:

OK now I can focus on the mechanical assembly

Update 6/17/18

Quick update here: I re-wrote the stepper motor control program in assembly (quick and dirty draft) and the speed difference was remarkable as seen in the video below. I actually could drive the motor much faster than this, but it would likely damage it... And here's where the usefulness of RXB really comes through: it's super efficient to use for rapid prototyping and validation of design ideas because of the ease of editing and running, and once everything is working properly then porting to assembly, if needed for speed, can be done in a fairly straightforward manner.

Update 6/30/18

The mechanical assembly of the X-Y Plotter is now completed. I eventually settled down on an overlapping table design because I felt it would be a bit more compact.
I started by getting my hands on a couple of cheap rod sets on Amazon along with 2 of the ubiquitous Nema 17 4-wire bipolar stepper motors.

Unfortunately, I suppose you get what you pay for, and the rod sets were not well matched at all, with the threaded rod supports being shorter than the sliding rod ones, and thus required a bunch of risers in order to even things out. Furthermore, the threaded rods came with an odd circular nut rather than a flat sided one, making the attachment of a platform to the nut much more of a pain than it had to be. Essentially I had to carve out slots under each platform into which the nut was inserted and glued. The platforms were cut from 5mm thick birch plywood which is pretty stiff.

One issue I noted was that the second level platform tended to sag a little bit when the pen holder platform was all the way to the end, and the whole assembly tended to tip as well. The solution was a simple free coasting wheel on the edge of the second platform for support. For the pen up/down function, I chose to use the stepper motor assembly I had pulled out from the old DVD-ROM drive which I have shown earlier since little torque was going to be needed for that function. I created a simple pen bracket with a set screw which I glued to the lens assembly along with a couple of support brackets.
Time to put the 3D printer to good use!

And here's the final product.

Finally I added limit switches to the X and Y axes as well as the pen control assembly so that I could initialize the plotter to the origin before each print.

Overall I'm pretty happy with the end result. Next step is finalizing the control circuitry for the Nema 17 motors. These draw a lot of current and cannot be driven by the test circuit I demonstrated earlier.

Update 8/1/18
I did some testing to figure out how to properly power and control the Nema 17 stepper motors as seen below. Several components' leads did not fit in the breadboard holes, so I ended up with a huge wire mess... The good news is that is miraculously worked! I opted to reduce the component count by using the L298 H-bridge controller, one for each of the stepper motors.

From there, I designed a complete control circuit schematic and a double-sided PCB layout. It's not a professional design, but it's the best I could do with my limited skill set:

The 4 pads at the corners are needed for later alignment.

Next is the actual PCB build...

Update 8/2/18

Printed the layout on transparency sheets using my laser printer. Two copies per PCB side in order to maximize the tracings opacity.

Exposure of a double-sided pre-sensitized PCB for 8 minutes under a fluorescent light. I cannot stress enough the need to use a high quality PCB here. I have had very good luck with MG Chemicals. The cheap Chinese stuff is terrible as I painfully discovered from past experience. Aligning the top and bottom layers was fussy work. I marked and drilled into the PCB the 4 corner pads and used pins to go through the top transparency, the PCB itself, and the bottom transparency and I got near-perfect alignment that way. Only 2 pins were really needed.

And here's the exposed and developed PCB

Next I etched the board, and I have to say it turned out overall pretty nice. It is super important at this point to test each trace for continuity and bridges as well for the possibility of a short. I found 3 trace breaks and couple of bridges that way. It is so much easier to fix the issues at this stage rather than try to debug the board once it is fully populated.

Finally I drilled the pads. i start with a very small drill bit and test fit all the components, drilling progressively larger holes as needed. Again this needs to be done before you do any soldering for component!

This was probably the largest double-sided PCB I have ever built! In case you are wondering whether it was worth the effort, it's really more of a philosophy than an economic question. I personally enjoy the challenge of designing and building my projects from scratch as much as possible, although I would definitely get far more professional results had I farmed them out to a PCB fabrication house and it would likely have been cheaper too...

I'm going to defer the actual soldering and final testing until I get back from vacation at the end of the month. I need a break anyway

Update 10/6/18
I have been quite busy behind the scenes over the past month getting this contraption to work, and I think I finally got it.
The board assembly went without too much difficulty and actually looks overall pretty good for a homemade amateur job although bridging the top and bottom traces was fastidious since obviously the holes are not plated through and through.

After completing the trace bridging

Assembled board. The large heat sinks are for the L298 H-bridges which can run very hot.

Now no prototype design ever survives it's first iteration, and this was no exception. Of course when first powered up nothing worked (surprise!   ). It took me an entire week of extensive testing to figure out all the issues. First, I had forgotten to bridge one of the vias on the board, and there were a few tiny solder bridges as well. Then I found out that I had made a mistake by assigning the PIO pin 11 (ground) as Vss instead of pin 12, so ended up frying both of the 74HC595 chips... And lastly I had neglected to tie the PIO handshake in line to Vss via a pull-up resistor, and thus the end of travel switches did not work. Overall not too bad actually, as the general design was relatively sound...

Debugging in process...

Below are the updated schematics and PCB layouts:

Bottom layer and top component outline

Top layer

But even when I got the controller board finally working, I was not happy with the motions of the stepper motors and they were running very rough. After some more research it turned out that my step sequence for the stepper motors was wrong when running in reverse. You see, there is lots of documentation online on the forward step sequence of bipolar stepper motors, but not a single mention of how to run them in reverse. I finally reverted to looking at the source of the Arduino stepper library and it turned out that all one needed to do was simply run the sequence in the opposite order starting with the last position in the sequence. Doh... Once I fixed that, the motors ran great.

But then another issue surfaced where the stepper motor I was using for the pen up/down function would not run well when supplied with more than 5V, and this voltage was barely enough to run the Nema 17 stepper motors for the X and Y axes. It took me a while to realize that the problem was with the voltage, and I was very close to trashing the pen assembly thinking that the pen stepper motor was damaged. Note to self: always try to match the stepper motors in a project to minimize headaches and preserve hair... In any case, I am now at a point where everything is working as it should and the next step is going to be actually getting the device to draw something. That's coming next.

Assembly language plotter control program:
Spoiler

Update 10/19/18
So I ran into some issues during the testing process. First it turned out that the long end of the X platform was sagging quite a bit because the wheel assembly I had 3D-printed turned out to be a little shorter than in should be. I thought I'd take it off and add a little wooden insert under the base, but ended up breaking one of the legs...

I thought I'd replace the wheel with a caster ball assembly I got from the Robotshop, but it ended up causing too much wobbling of the X axis which is has a lot of play laterally. You see, a standard wheel will tend to travel in a straight line and resist rotation, but not so with a free caster ball.

So in the end I re-designed the wheel assembly in OpenScad, lengthening it and beefing it up in the process then 3D printing it. It worked quite well and eliminated most of the sagging.

Now for the actual drawing test, I opted to use the XB environment because my main aim was to be able to draw mathematical graphics using the plotter, and using XB for floating point calculations and trigonometry is light years easier than using straight assembly. So I converted my assembly plotter driver to XB use and was also a source of problems because the stepper motors did not work nearly as smoothly from XB as they did from assembly, and I had to tweak a lot of settings to get to work properly.

Plotter driver:

Spoiler

The best way to make sure a plotter is working is to have it draw a circle. So I created an XB test program to do just that using the standard sine/cosine equations for the circle.
The driver includes the following subprograms callable from XB:
• HOMEXY - home the X and Y axes
• HOMEP - home the pen assembly and position the pen holder ready to receive the pen
• XRIGHT and XLEFT - move the X axis to the right or left one unit
• YUP and YDOWN - move the Y axis up or down one unit
• PUP and PDOWN - raise or lower the pen
```10 CALL CLEAR
20 CALL INIT
60 PRINT "SECURE PEN"
70 CALL KEY(0,K,S) :: IF S=0 THEN 70
80 FOR D=1 TO 5
100 NEXT D
110 CURX=256 :: CURY=100 :: R=50
120 FOR DA=0 TO 360
130 RA=DA*PI/180
140 TX=INT(R*COS(RA))+100 :: TY=INT(R*SIN(RA))+25
150 GOSUB 1000
160 NEXT DA
180 STOP
1000 REM  DRAW POINT
1010 DX=CURX-TX :: IF DX<0 THEN DIR=-1 ELSE DIR=1
1015 IF DX=0 THEN 1050
1020 FOR D=1 TO ABS(DX)
1030 IF DIR=1 THEN CALL LINK("XRIGHT")ELSE CALL LINK("XLEFT")
1040 NEXT D
1050 DY=CURY-TY :: IF DY<0 THEN DIR=-1 ELSE DIR=1
1055 IF DY=0 THEN 1090
1060 FOR D=1 TO ABS(DY)
1070 IF DIR=1 THEN CALL LINK("YUP")ELSE CALL LINK("YDOWN")
1080 NEXT D
1090 CURX=TX :: CURY=TY
1100 RETURN
```
I kept tweaking the driver settings until I got a reasonable circle drawn. Success! From there it should be fairly straightforward to use the callable subprograms from XB to create some interesting mathematical graphics.

So essentially this concludes this project. As far as I am concerned, this was one of the most challenging projects I have tackled to date, particularly the mechanical design side of things. I've learned tremendously though, and that new knowledge will definitely come in handy in the future... There is an added bonus in that the controller board can be very easily hooked up to an Arduino or Raspberry Pi and then be driven by sophisticated drawing software available for these platforms to create smooth artistic patterns like the ones you see online. As it stands now though, while it may seem rather primitive, we have to keep in mind that it is being run by an early home computer over 3 decades old!

Wish list: I still want to have the ability to take a TI Artist image in pattern format and print the bitmap on the plotter. Unfortunately it is way too slow to do in the XB environment, so I am looking at creating a pure assembly program for it although trying to draw every pixel might still be too slow. We'll see how that goes, and if I get decent results I'll post another update

Summary video of the entire project

Update 10/24/18
I managed to use The Missing Link (extension program for TI Extended Basic with bitmap graphics -  The_Missing_Link_2_0.zip (705.81KB)
downloads: 6) to load a TI Artist pattern _P picture file and send it to the plotter. It does require embedding the TMLEXTRASO (provides additional TML commands - included on the TML disk) and PDRIVER (the plotter assembly driver shown above) into the program using the HMLOADER utility provided with TML, a pretty straighforward process well detailed in the manual.
```3 CALL LOAD(8192,250,198)
40 CALL LINK("PRINT",176,1,"SECURE PEN THEN <ENTER>")
70 CALL LINK("PRINT",176,1,"PICTURE PATH.NAME?")
100 CURX=240 :: CURY=1
110 FOR Y=1 TO 192
120 FOR X=1 TO 240
140 IF PIXEL=1 THEN GOSUB 1000
150 NEXT X
160 NEXT Y
180 STOP
1000 REM  PIXEL PLOT ROUTINE
1010 DX=CURX-X :: IF DX<0 THEN DIR=-1 ELSE DIR=1
1020 IF DX=0 THEN 1060
1030 FOR D=1 TO ABS(DX)
1040 IF DIR=1 THEN CALL LINK("XRIGHT")ELSE CALL LINK("XLEFT")
1050 NEXT D
1060 DY=CURY-Y :: IF DY<0 THEN DIR=-1 ELSE DIR=1
1070 IF DY=0 THEN 1110
1080 FOR D=1 TO ABS(DY)
1090 IF DIR=1 THEN CALL LINK("YDOWN")ELSE CALL LINK("YUP")
1100 NEXT D
1110 CURX=X :: CURY=Y
1120 IF DX=0 AND DY=0 THEN 1150
1150 RETURN
```
Line 3 is added by the loader utility, and the embedded assembly code is not visible when the program is listed.
Here's the result printing this iconic image:   AEN_P.zip (1.57KB)

I'm having too much fun!

Update 11/25/18
Anders Persson pointed out to me that the stepper motor sequence I was using was driving the motors in full steps only, which limits the resolution of the plotter. He provided me with an updated sequence to allow for half-steps, and indeed this worked great with the motors running more smoothly and quieter as well as essentially doubling the plotting area. Below is the complex sine plot in half step, and you will notice how much smaller the drawing is now:

Here's the updated source file:

Spoiler

Update disk with all the files:   PLOTTER.dsk (180KB)

You may want to look at 3-D printer designs, you may be able to adapt them.  Check out RepRap HELIOS and some other of Nicholas Seward's designs for something more than XYZ boxes.

Plotters were a must have for doing color charts until inkjet printers.  I remember using both a flatbed plotter and a giant roller plotter.  Nowdays I have a color laser printer.

• Report

You may want to look at 3-D printer designs, you may be able to adapt them.  Check out RepRap HELIOS and some other of Nicholas Seward's designs for something more than XYZ boxes.

Plotters were a must have for doing color charts until inkjet printers.  I remember using both a flatbed plotter and a giant roller plotter.  Nowdays I have a color laser printer.

Very cool design, but likely a bit more involved than what I had in mind for this project. I was able to secure a couple of inexpensive sets of rods and sliders which I should be able to adapt fairly easily for my purposes. Details to come.

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I played around with a couple of 28BYJ - 5V Stepper Motors on an Arduino and was going to try and hook 2 up to the Atari joystick ports. I found that I ran out of data pins rather quickly. I was able to use a 74HC595 - 8-bit shift register with output latches(A.K.A serial to parallel converter). I could control the coil sequence for two stepper motors with 3 bits; Data, Clock, and Latch. You might be able to use one of the extra bits can be used for pen up and pen down. Just a thought.
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I played around with a couple of 28BYJ - 5V Stepper Motors on an Arduino and was going to try and hook 2 up to the Atari joystick ports. I found that I ran out of data pins rather quickly. I was able to use a 74HC595 - 8-bit shift register with output latches(A.K.A serial to parallel converter). I could control the coil sequence for two stepper motors with 3 bits; Data, Clock, and Latch. You might be able to use one of the extra bits can be used for pen up and pen down. Just a thought.

Yes running out of data pins is one of the issues I ran into. I'm definitely going to look into the specs of that chip and see what I can do with it. Thanks for the tip!
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Walid,

The next step it to use knives to cut vinyl.  Using the TI to make TI bumper stickers!  If anyone can figure it out, it's you.  Very impressed.

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Now I didn't read everything you've written carefully, but based on years of professional experience with stepper motors, I think you still have the stepper motor drive sequence wrong.

For a 2-phase motor, running at full steps, the sequence is like this:

A+

B+

A-

B-

The notation A+ here means that you run current through the A coil in positive direction, with nothing in the B coil. B- implies you've reversed the B coil with no current in the A coil. If the motor just rocks back and forth, you have one coil connected backwards.

For many stepper motors, this will give you 200 pulses per revolution.

To run the motor in half steps, the sequence is like this

A+

A+ B+

B+

B+ A-

A-

A- B-

B-

A+ B-

Now you get 400 steps per revolution instead (or at least twice as many, if you have a different resolution than 1.8° per step).

In the second case, when you have power in both coils at the same time, current should be reduced to 0.7 times the normal current.

To get the best torque at higher speeds, you need a chopping driver with current control. You feed a 5 V motor with a higher voltage, but control the current and chop the voltage so you regulate the current that way. The higher drive voltage will help reversing the current flow faster, which is essential for high torque at higher speeds.

With a good chopping driver and sufficiently high voltage, you can count on running the motor up to 500 r/min without much torque derating. Going above 1000 r/min is usually difficult.

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Everything I've seen online, including the Arduino stepper library, use the sequence below, which is what I'm using:

A+ B+ /
A- B+ /
A- B- /
A+ B- /

That seems to work...
I'm curious to see how the motors would run using your half-step sequence. Easy enough to change, so I'll test it out in the next few days and get back to you.
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Now I didn't read everything you've written carefully, but based on years of professional experience with stepper motors, I think you still have the stepper motor drive sequence wrong.

For a 2-phase motor, running at full steps, the sequence is like this:

A+

B+

A-

B-

The notation A+ here means that you run current through the A coil in positive direction, with nothing in the B coil. B- implies you've reversed the B coil with no current in the A coil. If the motor just rocks back and forth, you have one coil connected backwards.

For many stepper motors, this will give you 200 pulses per revolution.

To run the motor in half steps, the sequence is like this

A+

A+ B+

B+

B+ A-

A-

A- B-

B-

A+ B-

Now you get 400 steps per revolution instead (or at least twice as many, if you have a different resolution than 1.8° per step).

In the second case, when you have power in both coils at the same time, current should be reduced to 0.7 times the normal current.

To get the best torque at higher speeds, you need a chopping driver with current control. You feed a 5 V motor with a higher voltage, but control the current and chop the voltage so you regulate the current that way. The higher drive voltage will help reversing the current flow faster, which is essential for high torque at higher speeds.

With a good chopping driver and sufficiently high voltage, you can count on running the motor up to 500 r/min without much torque derating. Going above 1000 r/min is usually difficult.

You were right! Adding the A+, B+, A- and B- in between the other steps in the sequence you mentioned indeed doubled my resolution on the plotter by moving the motors in half-steps, and they run quieter as well. It's interesting that this specific sequence is not commonly mentioned on-line... Thank you for pointing this out to me. This will allow me to create even more elaborate graphics.

I'm going to update the post accordingly.

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I've not looked at what they recommend in conjunction with Aurdino projects, or other similar hobby sites. What I wrote is based on my professional experience, where we used some 1500-2000 stepper motors every year, mostly of frame 34 size (3.4" square flange). We were typically driving them with voltages from 42 up to 60 V. You need a current controlling chopping driver to do that, or they burn immediately.

Nowadays, speed demand for our machines has increased to such values that we hardly use steppers any longer, but full servo motors instead.

It's acutally the "current in one coil at a time" that's the simplest full step sequence. Sending current through two coils at a time is the half steps in between. But if you use only them, as you started with, you get a full step sequence with more torque, but also requiring more power. There are also microstepping drives, which can vary the current in the coils in very small step. They typically create 10000 steps per revolution, or even more. This microstepping makes steppers run more smoothly.

A current regulating chopping drive will normally reduce the current in the windings at half step positions to 0.7 times the current at full steps. Since two coils are excited at the same time, the power dissipation then becomes the same for all steps.

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Here's a link to a description of the two full step sequences, and them combined making a half step sequence. Microstepping is also explained.

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