View Full Version : Compupro Ram 17 in IMSAI problem

May 27th, 2014, 08:04 AM
I have been trying to incorporate a Ram17 into my IMSAI and have had a problem. I was not able to use the front panel to read or write any memory on any board in my system as soon as I plugged this board in.

I verified the switch settings and set it up as a simple 32k board but it would not function by itself or with the other 16 ram board.

So, I started removing chips to see if I could isolate the problem, and here is what I have:

If I remove every chip from the Ram17, leaving only the 5volt regulator, i am still not able to read and write to the other board as long as this 'bare' board is present in the system. As soon as I remove it, all is normal.

Can anyone think of a buss condition that could be altered by simply plugging in this (IEEE696 compliant) board into my IMSAI?
I am convinced there is a signal being held high or low by even the bare board...


May 27th, 2014, 01:26 PM
Hi All;
Thomas, a couple of things..
First, remember that Imsai is NOT IEE696 compliant.. Imsai and Altair were built before IEE696 came out, So You need to look at Imsai and Altair S-100 signals and compare them to the Board that you have Plugged into the Buss..
I have had an old Compupro Ram 'something' that will not work in my Altair either, can't remember which one it is..
Also, Remember that there are a number of signals that are Tied to Ground, in the IEE696 that are Not tied to ground in the Altair / Imsai.. Pins 20 and 70 come to mind.. Also, 20 and 70 Cannot be tied together.. Most likely there are others.. Also, If there are switches on the Board, You neeed to Disable any address higher than 15..


May 28th, 2014, 09:33 AM
Hi Thomas,
As Marty mentioned, the RAM17 is IEEE-696 compliant and puts ground on pin 20. Putting ground on pin 20 will disable the IMSAI front panel through an unimplemented & undocumented feature concerning memory protect. Please see http://www.retrotechnology.com/herbs_stuff/s100_pin20.html for the full story. What I wound up doing was just putting a small piece of scotch tape over pin 20 & pin 70 to isolate them from the bus. I also labeled the card to remind me I put the tape there as it is so hard to see. Also I realized you mention setting the switches properly but also make sure you have switch S1 Paddle 1 "ON" and S1 Paddle 10 "OFF" to enable front panel usage. While my IMSAI is not 100 running yet due to work yet to be completed, I can read & write the entire 64K using the front panel. I hope this helps.

June 6th, 2014, 07:26 AM
Thanks to all that posted - the problem was indeed pin 20 grounded and after isolating it the system works just fine. I had a note from another that recommended just cutting the signal at pin 20 at the front panel so that any board plugged into the buss in the future would not have this same problem.