Vorticon's Blog - X-Y Plotter Table For The TI 99/4A Computer

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So I've been mulling the idea of creating an X-Y table for my TI computer, which could be use 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 ( . 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

As for 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:

 

 
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:
 

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http://atariage.com/forums/blog/659/entry-14733-x-y-plotter-table-for-the-ti-994a-computer/