Is there a way to perform the Tandy Mod externally

Hardware questions and modifications

Is there a way to perform the Tandy Mod externally

Postby Corey986 » Thu Jun 05, 2014 11:36 am

I don't want to hack up my motherboard, is there something that can plug into the sidecar or the modem slot to perform the remapping? I do have a jrIDE coming, is there any way to enable the "memory" at B800:4000 on the jrIDE, I think that would also allow tandy games to work since it won't remap, but use real memory...

I'm looking for a reversible solution basically.

Thanks,
Corey
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Re: Is there a way to perform the Tandy Mod externally

Postby geoffdaddy » Fri Jun 06, 2014 6:47 am

I have yet to do the Tandy mod myself, but looking at the schematic of the mod it should be easily reversible just adding a toggle SPDT switch into the circuit.

The mod has you cut the trace going to pin 2 of ZM31, and wire the output of an XOR gate in an 74S86 package (pin 3) to pin 2 of ZM31 instead. All you would have to do to reverse the mod is connect the common pin (usually the center) of the toggle switch to pin 2 of ZM31 and wire one side of the switch to pin 3 of the 74S86, and the other side to pin 9 of ZM30. You'd want to turn off the machine when switching positions.
PCjr: 10MB RACore 01 hard drive expansion, Hotshot 640kB RAM expander, optical mouse, Xircom PE3, speech sidecar.
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Re: Is there a way to perform the Tandy Mod externally

Postby Corey986 » Fri Jun 06, 2014 7:20 am

geoffdaddy wrote:I have yet to do the Tandy mod myself, but looking at the schematic of the mod it should be easily reversible just adding a toggle SPDT switch into the circuit.

The mod has you cut the trace going to pin 2 of ZM31, and wire the output of an XOR gate in an 74S86 package (pin 3) to pin 2 of ZM31 instead. All you would have to do to reverse the mod is connect the common pin (usually the center) of the toggle switch to pin 2 of ZM31 and wire one side of the switch to pin 3 of the 74S86, and the other side to pin 9 of ZM30. You'd want to turn off the machine when switching positions.



Well then the big question is about the trace to pin2 of ZM31, does it only go to pin 9 of ZM30? If the leg of the actual chip can be "cut" and bent up I wouldn't have as big of an issue. I just don't want to cut a trace. To reverse, I can simply desolder and chip and replace it. For some older systems, like the Sol-20, the factory cut traces all the time because of PCB mistakes, but that was the PC world of 1976/77, not mass produced IBM of the 1980's.

Cheers,
Corey
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Re: Is there a way to perform the Tandy Mod externally

Postby Eric » Sat Jun 07, 2014 2:15 am

This probably isn't that relevant, but when I did mine, I desoldered the TTLs, lifted the pins, completely removed the trace, then reinstalled the ICs and jumped everything that needed to be jumped with 30AWG kynar insulated wirewrap. It came out looking pretty clean and the solder mask doesn't have any issue with peeling or anything.


Also, the amount of modification is required is more simple than it would seem, though I'm sure all of you already know that anyway.


I never used an spdt switch for it, though. I feel I can always just desolder the wire from the ORgate and just rejump the two pins to replace the trace if I needed to.

Actually, I don't mean to hijack the thread, but is there any negative effects from a tandy mod on any software? I've seen some artifacting on the Exploring The PCjr software when I have it enabled, so I was wondering if something is screwed up with one of my TTLs, but I currently don't have a logic probe or oscilloscope to test them in case I need to replace them or anything.


I mean, there's gotta be some reason people opt for to include a switch, right?
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Re: Is there a way to perform the Tandy Mod externally

Postby Corey986 » Sat Jun 07, 2014 7:09 am

Eric wrote:This probably isn't that relevant, but when I did mine, I desoldered the TTLs, lifted the pins, completely removed the trace, then reinstalled the ICs and jumped everything that needed to be jumped with 30AWG kynar insulated wirewrap. It came out looking pretty clean and the solder mask doesn't have any issue with peeling or anything.


Which chips did you desolder? ZM31 and ZM30? or just one of them. I don't have a PDF of the schematics, but looking at the data sheet for the 74ls157, something else most likely connects to pin 2 of 74ls157 because the chip ignores the state of pin 2 unless the select(pin 1) is low and then it ignores pin 3. When select (pin 1) is high then it ignores pin2 and reports pin 3. So it seems to me there might be an easier even setup using a pull down or pull up resistor, the 74s86 and no PCB cut. This all depends on what pin 2 is connected to. If I can keep it to no cut, then we could even make a chip clip with a clip wire adapter. That would be cool...

You wouldn't happen to have a PDF of the schematics of that area?

Thanks,
Corey
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Re: Is there a way to perform the Tandy Mod externally

Postby Corey986 » Sun Jun 08, 2014 3:25 am

OK, still can't seem to find the schematics on line. I'm going to check the museum I docent at to see if we have a hardcopy of the technical manual.

Meanwhile I noticed the docs for the PCjr combo card. Unfortunately they are missing the section on exactly how to hook up the JR1000 mod, but what I could get out of the document is the following...

there are connections to pin #2 and #3 of the 74LS157, Pin #9 of the 74LS273 and there is a reference to pin #20 of the 8255A-5. There is also a connection to a VIA, which Via is specifically not stated. This sounds promising and confusing at the same time. Promising in that is uses more wires than the solution that has the cut trace, so something is different. Confusing because pin #2 on the 74LS157 and pin #9 on the 74LS273 are connected by a trace that is cut in the normal hack, so why the two wires unless a trace is cut...

Maybe I'm on a quest for the for the Lockness monster, it just doesn't exist... But you can't blame a guy for trying...

Cheers,
Corey
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Re: Is there a way to perform the Tandy Mod externally

Postby GHiero » Sun Jun 08, 2014 9:14 am

Eric wrote:Actually, I don't mean to hijack the thread, but is there any negative effects from a tandy mod on any software? I've seen some artifacting on the Exploring The PCjr software when I have it enabled, so I was wondering if something is screwed up with one of my TTLs, but I currently don't have a logic probe or oscilloscope to test them in case I need to replace them or anything.

I mean, there's gotta be some reason people opt for to include a switch, right?


Scubaventure doesn't like the mod, see here : http://nerdlypleasures.blogspot.com/201 ... rades.html
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