Max Side Cars

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Max Side Cars

Post by chuckphd53 »

I started a discussion on Mikes brd about the number of addon cars that IBM had seemed was ok...

With the JRIDE rev'B' and a parallel side car attached, I had two separate systems that exhibited the same issue.

That was the system, at boot, would hang just when the JRIDE started to load bios...
I had investigated this and found that the PCJR drive ability for the DATA bus lines was being pushed to the max
with the JRIDE installed as it uses 6 fan outs to its circuits. This is on top of the fan out the the PCJR main board devices

With the JRIDE closest to the JR and the Parallel card on the outside, the system would boot ok and seem fine, but every now and then
the Parallel port ( I had a zip drive attached) would not get Identified.
This indicated that the data bus fanout thru the additional side car connector was causing signal integrity issues.

So using a long known solution, (back to the early S100 bus days, yes I was around then) I decided to add a PULLUP/PULLDOWN
solution to help the signal transition times. Note: This is a solution used in many designs and has its origins back to the early days
and to be assured it still has its place in the realm of design.

I modified two JRIDE boards I tested and both now worked fine in either side car position as well as the LPT port was also no reliable.

Looking at the signals with a scope and capturing the edge transitions, the signal went from a rather strained leading and falling edges
to a much cleaner, transition with very little ringing.

As I mentioned the correct design would be to use proper Databus drivers on the JRIDE, but as many of the folks have these boards
a user solution was needed so I posted my resistor fix... see below

If you have a JRIDE and discover some strange BOOT issues, or other anomolies, this could be your fix.
It is not pretty, but it is functional and sound!
Let me know your issue and results if you perform this addon...

“I never commit to memory anything that can easily be looked up in a book.” (AE)
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