Casio PB-100 / Tandy PC-4 versions and documentation

[Jeff_Birt]

I recently picked up a Tandy PC-4 which is a Casio FP-100 in Tandy clothing. It only needed minor repair (I did a video, to be released December 29, 2021.) After fixing it I was looking at what it would take to build a RAM expansion module for it using the Tandy PC-4 service manual as a guide. Then when editing the video, I realized that I had a later revision of the main PCB.

The original PCB for RS Catalog #26-3650 (PB-100) had a small board bodged on to correct a timing issue which could scramble the expansion RAM. They went on to release 26-3650A and 26-3650B which did away with the mod board and moved that functionality into the RAM module. I found this out thanks to someone who used to work for RS still having the Tech Bulletin about it. I'm not sure what the difference between A and B might be. I think the 'B' model is what Casio called the PB-100F.

I found a cheap PB-100 (original I think, and not functional) on eBay this morning and grabbed it. I'm hoping to document what the differences are and would love to find the Tandy or Casio service manual for the later revisions. I have attached the Tandy Tech Bulletin and the link to the Tandy service manual for original version is below. If anyone else happens to have more documentation I would greatly appreciate it. Thanks -

https://archive.org/details/manualzilla-id-6027413

[Dan C]

Hi, sorry i dont have much info on this matter, but i just wonder if you are referring to
Casio PC-100, Casio FP-100 or Casio PB-100 ?

seems to be PB-100? the other 2 is not CASIO's i think?

[Jeff_Birt]

(12-29-2021 06:19 PM)Dan C Wrote:  Hi, sorry i dont have much info on this matter, but i just wonder if you are referring to
Casio PC-100, Casio FP-100 or Casio PB-100 ?

seems to be PB-100? the other 2 is not CASIO's i think?

The PB-100, thanks for pointing that out. A typo on my part. (I managed to spell it correct in the first paragraphs but stuffed it up in the other two.)

I did later find the following webpage which covers the FX-700P but comes with 2K on board. He has reverse engineered the protocol used for the RAM which will make it much simpler to try and duplicate.

http://www.pisi.com.pl/piotr433/hardware.htm

[Dan C]

(12-29-2021 07:12 PM)Jeff_Birt Wrote:  

(12-29-2021 06:19 PM)Dan C Wrote:  Hi, sorry i dont have much info on this matter, but i just wonder if you are referring to
Casio PC-100, Casio FP-100 or Casio PB-100 ?

seems to be PB-100? the other 2 is not CASIO's i think?

The PB-100, thanks for pointing that out. A typo on my part. (I managed to spell it correct in the first paragraphs but stuffed it up in the other two.)

I did later find the following webpage which covers the FX-700P but comes with 2K on board. He has reverse engineered the protocol used for the RAM which will make it much simpler to try and duplicate.

http://www.pisi.com.pl/piotr433/hardware.htm

Nice info! I love all info on old Casio calculators!
But maybe you should change the subject to Casio PB-100 then? if its possible.

[Jeff_Birt]

I was able to change the thread title on all of my postings. The broken PB-100 from eBay arrived and it is broken indeed. The little 2K nibble RAM chip is drawing 5ma and returning garbage to the processor.

The interesting part is that while it did not boot to the 'READY P0' screen the 'DEG' annunciator was on and the cursor was blinking. If I pulled the Phase 2 clock line (output of processor and only goes to RAM) the cursor no longer blinks! It seems that the routine that periodically blinks the cursor is the same one that periodically reads from the RAM. So, if the RAM does not return anything the cursor does not blink.

At least this unit can be used to test a 4-bit SRAM simulator