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IBM PCjr. 128K Memory Attachment

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Great Hierophant

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I just got a PCjr. and everything works except the memory attachment. Its the IBM brand and from a visual inspection everything looks fine. However, when attached to the system, the RAM will count up to 192K and give an ERROR A message and freeze the system. I assume that one of the chips are bad. Is there any way to tell which chip(s) may be bad, or is it time to replace all 16 memory chips, preferably with 256K chips, and turn it into a 512K sidecar.

Can you piggyback a good RAM chip over a bad one?
 
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I don't think the technical reference manual has the wiring diagram for the memory sidecars in it; those came after the technical reference manual was last updated. The maintenance and service manual has the same problem.

If there was just a flaky memory chip you would have bad value being read after it was written. The machine would throw an error but it should continue to boot and let you run diagnostics. Freezing is a very bad sign; I would be looking for something else like bent or broken pins on the sidecar connector, stray pieces of solder causing shorts, etc. And of course, make sure the DIP switches on the back of the memory sidecar are correct.

There are only two banks of memory in the sidecar. Upgrading the sidecar is not rocket science, but it involves desoldering all 16 64Kx1 DRAM chips. And you should install sockets before populating with 256Kx1 DRAM chips. There are detailed write-ups available.

I rarely have memory go bad. But when I do and the chip can be removed easily I have a "SimCheck" device with a single DRAM chip adapter that is really good at spotting bad chips. Memory testers that run in the system can be used but with limitations. Like for example, the memory testing program is living in memory. : - )
 
I took another look at the sidecar today. What I did this time was to attach the sidecar with all the switches in the off position. When the switches are all off, the BIOS will not recognize the sidecar RAM. The machine threw no errors at me. When I got to BASIC, I flipped the appropriate dipswitch on the memory sidecar and did a Ctrl + Alt + Ins to get to the built-in PCjr. diagnostics. The diagnostics routine counted to 256K, which it should. However, I was not able to load a boot disk with JRCONFIG.DSK.
 
Great Hierophant said:
I took another look at the sidecar today. What I did this time was to attach the sidecar with all the switches in the off position. When the switches are all off, the BIOS will not recognize the sidecar RAM. The machine threw no errors at me. When I got to BASIC, I flipped the appropriate dipswitch on the memory sidecar and did a Ctrl + Alt + Ins to get to the built-in PCjr. diagnostics. The diagnostics routine counted to 256K, which it should. However, I was not able to load a boot disk with JRCONFIG.DSK.
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Did it print anything else besides the ERROR A message? Any hex value dumped out too?
 
geoffm3 said:
Did it print anything else besides the ERROR A message? Any hex value dumped out too?
Click to expand...

Nope, just Error A. It counts up to 192KB before giving the error. The fact that the diagnostic program can count the RAM seems to me to indicate that the problem may not be with a faulty memory chip. Maybe the DRAM Controller, the 74S409, is faulty. It handles the memory refresh.
 
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Great Hierophant said:
Nope, just Error A. It counts up to 192KB before giving the error. The fact that the diagnostic program can count the RAM seems to me to indicate that the problem may not be with a faulty memory chip. Maybe the DRAM Controller, the 74S409, is faulty. It handles the memory refresh.
Click to expand...

I think the BIOS only displays the "good" memory, so that sounds like maybe your problem is with the second bank of RAM.

I see in the BIOS code that it should be possible to have it spit out a bank # of the bad bank and bitmap of bad bits within the bank. That may require you to put it in MFG test mode when booting up (this is done using the joystick buttons).

When I was troubleshooting my memory problem, there are some additional manufacturing test routines built into the BIOS that you can access by setting a combination of joystick buttons while powering on. It will spit out some hex codes to the serial port at 9600bps (8O2). I think this should display which bank and bits you're having trouble with if that is of any use to you...

http://www.vintage-computer.com/vcf...-t-boot-beeps-twice-no-display&highlight=pcjr
 
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