I recently purchased a 1995 vintage piece of test equipment that
utilized either 1M or 4M dynamic ram modules (70nS) of the old 30pin
SIMM configuration. The embedded processor board that used this ram is
based on a Motorola 68020 controller and designed to read and write data
to the DRAM using a 16bit wide bus. The board is designed to accommodate
two banks of dram. Each bank will consist of two 8 bit simm drams. I’m
sure many of you remember this typical motherboard configuration of this
era.
I am attempting to determine why I cannot get 4M memory to work is this
unit(I’ve tried three types). There is a dip switch which you select
either 1M or 4M memory, but the ram self test fails miserably when using
4M modules. The 1M modules work just fine.
I’ve studied the R/W timing signals (RAS/CAS, etc) as well as refresh
timing and all looks to be OK, with lots of margin. What I am not sure
of is how to determine the signal integrity, such as overshoot, ringing,
etc... One thing that I did was to determine if my power supply was
holding up during the R/W process, understanding that there is
considerable power requirements for dram during R/W. Using a scope and
testing the power supply while triggering off of the R/W lines, I could
see no sagging or spikes occurring. I also thought initially that there
could be some logic issues when the switch being set to the 4M position.
So, what I did was to use some insulating tape to tape up the A10 PCB
contact on the 4M simms and pull up the A10 to +5 on the simm to commit
this address line. Then set the switch to the 1M position and see if
the dram would then work as a 1M part. No luck, still failed the memory
test.
I could use some help understanding the difference(s) in the dram (other
than size). If anyone would like to help out, I would certainly
appreciate it as I am quite lost at the moment.
Thanks
Jim
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