Piggyback OS ROM

[hueyjones70]

Instructions for homebrew switchable OS

[hueyjones70]

I recently saw instructions for a homebrew method of piggybacking two different operating systems for the XL/XE (not 1200XL). I don't remember if I saw it on one of the forums or elsewhere. I remember that it involved bending up 2 pins, attaching 2 resistors, and using a 3 position switch. Can anyone point me towards those instructions?

[+Nezgar]

Here's one method from my archives:

 

    Subject: UPGRADE: The Dual OS Switch

------------------------------------------------------------------------


      The Atari OS Switch
      (based on an article in ANTIC)

      Remember, do this at your own risk!

      PARTS: DPDT mini switch (1)
             28 pin socket (1)
             Thin wire (about a foot or so)
             Second OS ROM chip.

      Step 1:	Remove the OS ROM from your XL/XE computer

      Step 2: Solder a 28 pin socket on top of the OS ROM, pin to pin.

      Step 3: This is a diagram of the DPDT switch.
              All letter references [A] refer to the positions
              marked below.

      Switch: +---+
              |A B|
              |C D|
              |E F|
              +---+

      Step 4: Connect [A] to [F]

      Step 5: Connect [B] to [E]

      Step 6: Bend up pin 20 on both of the ROMs.

      Step 7: Connect pin 28 on the ROM/socket to [E]

      Step 8: Connect pin 22 on the ROM/socket to [F]

      Step 9: Connect pin 20 of the original ROM to [C]

      Step 10:Connect pin 20 of the new ROM to [D]

      Step 11:Plug the new ROM into the socket.

      Step 12:Plug the package of ROMs into the motherboard.


      You can now switch between OSes.  I used inline pins to go
      to pin 20 on both ROMs; lets me make changes.

      You'll have to reboot if you try switching with power on.

      Hope this helps,

      Dave Paterson

      davepat@total.net

------------------------------------------------------------------------
Craig Lisowski <http://aegis.mcs.kent.edu/~clisowsk/info.html>
mailto:clisowsk@mcs.kent.edu

 

[+Nezgar]

And another graphic from pigwa: http://ftp.pigwa.net/stuff/collections/atari_forever/Extension/US-DOUBLER/two_os.gif

[hueyjones70]

This looks much easier than what I remember. I know the one I saw before and have pictures of had resistors connecting pins 20 and either 15 or 28.

[BillC]

There are easier methods though they do come with a pricetag.

 

An advantage of the following is there is no soldering required when changing ROM/EPROM chips.

 

Dropcheck sells a re-imaged Rambo XL($35) that allows switching between 3 OS ROM/EPROM chips, and also a 4-in1 OS adapter($15) that allows selecting 16K banks on a 27512 with multiple OSs programmed into it.

Edited July 28, 2019 by BillC

[+Nezgar]

If you use 2, 4, or more OS's on a single larger EPROM, that involves resistors between address lines to hold them high or low without any floating (not connected). I understand you're only interested in the pickybacking method, and not condensing everything into 1 chip... (which is easier with adapters, but costs a bit more as per @BillC just now)

 

It would seem to me that maybe the above methods aren't as 'perfect' since some pins would be left floating. Someone with better electronics background may be able to better comment.

 

The above mods also seem to only fiddle with the CE (chip enable) and OE (output enable) signals, rather than cutting off Vcc to the deselected chip completely... (is that better? worse?)

[_The Doctor__]

I never cut the pins in any of the quick and dirty early hacks like was posted, I always curved and bent the leads up... not only did it look nice, kept them from touching,  the attached wire flow was nice as well.

 

Depended on the chips really, but in my experience leaving the chip powered up and using the enables was the correct way to do it.

[ACML]

You could just use a 27256 (32Kx8) EPROM and it all will still fit under the RF shield.

 

1)  Burn the original OS on the 27256 EPROM using address locations 0000 to 3FFF

 

2)  Burn the new OS on the same 27256 using adress locations 4000 to 7FFF

 

3)  Bend up pin 27 of the 27256 so it will not fit in the socket when inserted

 

4)  Insert 27256 EPROM in 28 pin socket makeng sure that pin 27 is bent up and out 

 

5)  Attach a wire from pin 27 of the EPROM and solder it to the center post of the channel 2-3 selector switch

 

DONE !!

 

     This looks more stock (fits under RF shield, no drilling new holes and adding new switches.  The channel 2-3 RF selector switch has been re-purposed to be the OS selector switch.  The Channel 2-3 switch is already conveniently +5VDC ---- Center post ---- GND.  This is nice and neat.  Remember not to flip the OS switch while the power is on.

 

 

 

[ricortes]

12 hours ago, hueyjones70 said:

This looks much easier than what I remember. I know the one I saw before and have pictures of had resistors connecting pins 20 and either 15 or 28.

That could be my fault or one of many others of course. One of the hacks I did and put out in the wild was just a switched a pull up resistor on address lines. One of those, "I only have a single pole single throw switch in my parts bin" kind of things. I probably had 8 or so systems with multiple OS, never the same thing twice. There's really no magic to it, maybe a half dozen ways you can do it.

Edited July 28, 2019 by ricortes

[hueyjones70]

I like to know more than one way to reach a desired result and sometimes enjoy doing things that may seem to be the "hard way". 

[TheMontezuma]

On 7/28/2019 at 3:09 AM, hueyjones70 said:

I recently saw instructions for a homebrew method of piggybacking two different operating systems for the XL/XE (not 1200XL). I don't remember if I saw it on one of the forums or elsewhere. I remember that it involved bending up 2 pins, attaching 2 resistors, and using a 3 position switch. Can anyone point me towards those instructions?

Perhaps it helps:

 

https://atari8bit.net/4xos/

 

 

[Mclaneinc]

Just a little mini add on to this from experience, make sure the toggle switch has a plastic cap on it.....May seem silly but I purchased my OmnimonXL back in the day, fitted it, all good and then one day I was static charged (unknown to me), touched the switch and popped a chip on the PCB...Thankfully it was a dead cheap 74 series chip and I replaced it but it could have been worse..Hence I warn re any switches mounted around the machine just in case there's no protection.

 

Probably does not apply to OS rom chips but better safe than sorry.

[toddtmw]

syscheck 2.2

[ricortes]

One way, no resistors is to just use a 256k EPROM, still 28 pins, and wire the A14 line to the center pole of a 2 position switch with the outside poles going to either 5V or ground. You burn the 16k OS you want to use in either the lower or upper half of the chip. 

 

There was Bob's 1200XL mod of course where he used a solder sucker to remove the 2-24 pin OS ROMs and replace them with 28 pin EPROMs. I think he had a number of iterations that included piggybacked BASIC <EP?>ROM

 

One of my hacks was I used the switch to convert between channel 3 & 4. I have trouble remembering what I did, but I think I just used an inline resistor going to the !CS pin with switching 5V that would put the line in a high state. I'll have to look for that one but I may have used the !PGM or !CS pulled high to stop the chip from going on the buss even when they were both otherwise selected. Really drawing a blank here. Pretty sure that was a piggy back chip hack.

 

Another easy way to do piggy back chips is to just put the center of a switch to the memory decoded output of the memory management chip. I'll upload a schematic, U5 is the OS, U3 is the memory mapper.

 

Also notice both !CS and !OE pins are wired together on the OS chip! All you have to do is bend out both pins of the of EPROMs you want to use, pin 20 for example, and use those pins with a double pole/double throw switch. These are essentially two switches in one body so you can't screw up switching them together. You wire it outside diagonal pins to either 5V or ground with the centers going to U5 pin 20s. 

 

Makes me stupid thinking of all the ways it can be done, but then most people will tell you I have a head start!

 

Edited August 2, 2019 by ricortes

[BillC]

4 hours ago, ricortes said:

One way, no resistors is to just use a 256k EPROM, still 28 pins, and wire the A14 line to the center pole of a 2 position switch with the outside poles going to either 5V or ground.

There is a reason that it's recommended to use pull-up or pull-down resistors. The inputs used for switching should not be left floating since a device may become unstable if inputs(and sometimes outputs) are left floating.

 

If a switch contact is faulty, or the wire to the switch breaks, it would leave that input floating if wired the way you recommend. Using a pull-up resistor and a switch to ground means that it won't switch to the desired function if the switch/connection fails, but it will be stable since that input simply remains high.

[ricortes]

9 hours ago, BillC said:

There is a reason that it's recommended to use pull-up or pull-down resistors. The inputs used for switching should not be left floating since a device may become unstable if inputs(and sometimes outputs) are left floating.

 

If a switch contact is faulty, or the wire to the switch breaks, it would leave that input floating if wired the way you recommend. Using a pull-up resistor and a switch to ground means that it won't switch to the desired function if the switch/connection fails, but it will be stable since that input simply remains high.

Still trying to remember some of the stuff I did back in the day. Also a good idea to use a break before make switch just in case someone hot switches but that would still cause the floating condition<vs short out the entire power supply!> and ~same result you mention with bus contention if you hot switched and reset vs power down.

 

Thinking not remembering. If I used a pull down<or up> on bent out pin A14 of a 256k EPROM with a single toggle to 5V using the channel switch to 5V<or ground> it would make sense. I may have Dremeled the traces on the switch since I pretty much never used anything but a monitor. I think there may have been one other problem like another pin that had to be bent out and tied to a different voltage level but it was a pretty simple hack, only required a resistor and new EPROM since switch was re-purposed. In my dreams it was just a simple on off two pole switch on-open with the A14 line tied through a ~1k resistor to ground. I'm sure I have that system around here somewhere but don't want to take anything apart: Worrying about getting it back together. I was really into reduced part count back then after working in price sensitive products in my day job. I also grew up a farming until I was about 22 years old so repairs with what was on hand and immediate was more important then letting your crops rot. A resistor + EPROM from parts drawer sounds about my MO.

[hueyjones70]

On 7/27/2019 at 10:38 PM, ACML said:

You could just use a 27256 (32Kx8) EPROM and it all will still fit under the RF shield.

 

1)  Burn the original OS on the 27256 EPROM using address locations 0000 to 3FFF

 

2)  Burn the new OS on the same 27256 using adress locations 4000 to 7FFF

 

3)  Bend up pin 27 of the 27256 so it will not fit in the socket when inserted

 

4)  Insert 27256 EPROM in 28 pin socket makeng sure that pin 27 is bent up and out 

 

5)  Attach a wire from pin 27 of the EPROM and solder it to the center post of the channel 2-3 selector switch

 

DONE !!

 

     This looks more stock (fits under RF shield, no drilling new holes and adding new switches.  The channel 2-3 RF selector switch has been re-purposed to be the OS selector switch.  The Channel 2-3 switch is already conveniently +5VDC ---- Center post ---- GND.  This is nice and neat.  Remember not to flip the OS switch while the power is on.

 

 

 

I am going to try this method because it seems really simple and clean. I wish I understood the circuitry involved. It seems that the video circuits would interfere with the signal. 

[ACML]

On 8/15/2019 at 9:54 PM, hueyjones70 said:

I am going to try this method because it seems really simple and clean. I wish I understood the circuitry involved. It seems that the video circuits would interfere with the signal. 

Actually, there are video signal involved with the switch.  It's a discrete signal (+5VDC or GND) used to select what channel.  The video signal is elsewhere.

[Kyle22]

On 8/25/2019 at 1:41 AM, ACML said:

Actually, there are video signal involved with the switch.  It's a discrete signal (+5VDC or GND) used to select what channel.  The video signal is elsewhere.

He MEANT to say "There are NO video signals involved with the switch"...

 

:)