Mos 6502 Question

[SassiestPen]

I need to replace the cpu for an Atari 5200 

Can it be be replace with an MOS 6502B instead of an MOS 6502C?

 

Thanks

Edited August 26, 2019 by SassiestPen

[+DrVenkman]

The "6502C" is the "SALLY" cpu - it's got a pin or two moved around compared to a stock 6502 from MOS or any of the licensed fab houses (Rockwell, Syntertek, AMI, National Semiconductor ...), and also has a dedicated HALT line to stop the CPU and allow the Atari ANTIC chip to use the system bus. Early Atari 400's/800's used discrete logic to implement this HALT signal but Atari redesigned things along the way, added a dedicated HALT line to the 6502, redesigned the 400/800 CPU boards to use it in lieu of the discrete logic, and then used the same 6502C "SALLY" chip for the 5200, all the XL/XE machines and the 7800.

 

So tl;dr - without digging up the schematics in the 5200 Field Service Manual, I think the answer is no. 

[atari-dna]

Correct. The Sally is not directly compatible with a stock 6502 (opcode differences). There is an eBay seller that has these available. There’s also a board you can design to utilize stock chips but it’s fairly advanced tinkering.

https://atariage.com/forums/topic/277464-sally-replacement-by-standard-6502/

[+DrVenkman]

45 minutes ago, atari-dna said:

Correct. The Sally is not directly compatible with a stock 6502 (opcode differences). 

I don't that's correct. To the extent there are any opcode differences, they'd be illegal (undocumented) opcodes that risked causing compatibility problems if and when chips were updated and improved. The main difference is implementing the HALT pin.

 

As I noted above, the 400/800 originally used standard 6502 processors, implementing the necessary HALT state via discrete logic for ANTIC to access the bus during DMA. SALLY merely implements in on-die silicon rather than the discrete logic used on the first version of 400/800 OS boards. Later 400/800's all have SALLY chips themselves, as do the XL, XE, XEGS and 7800. 

 

 

[atari-dna]

So, it’s quasi compatible—depending on the hardware implementation. If you need a halt signal you need to tinker, if not then it’s drop-in?

[+DrVenkman]

2 hours ago, atari-dna said:

So, it’s quasi compatible—depending on the hardware implementation. If you need a halt signal you need to tinker, if not then it’s drop-in?

And 8-bit Atari computer (and fundamentally, that's what the 5200) needs a HALT signal for ANTIC DMA while it's processing the display data for GTIA to implement on the video output. That signal has to come either from the chip itself,  6502C SALLY or implemented in the logic circuitry of the CPU board as in the 400/800 models that originally used the 6502B.

 

On the SALLY, that HALT signal is implemented on pin 35, which is normally not connected on a stock 6502. There is also another change from the stock 6502 pinout - for most 6502 chips, the R/W signal is on pin 34. On SALLY, that signal is moved over to pin 36 (which itself is usually not connected on a stock 6502). Also, I think the arrangement of the Vss and Vcc pins may have been swapped around but I don't recall off-hand. 

 

So they're op-code compatible with one another (software), but they are *not* drop-in replacements for each other as implemented in either the A8 computers, the 5200 or the 7800.  Hope that makes sense. 

[atari-dna]

Yes this makes sense. Thank you for the explanation!