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5160 256-640KB Motherboard Showing 1/4 Installed RAM

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KLund1

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Livermore, CA
I have a IBM 5160 256-640 KB motherboard. (the brown one)
It is showing 1/4 the installed ram.
I have just the board with 1 row in bank 0 of tested good 256KB 150ns ram and just a video card. nothing else
The ram counts up to only 64Kb !!
The MB SW1 3, 4 are set to ON so only bank 0 is enabled. The other banks are empty.
I pulled the chips, retested them in a ram tester, cleaned the sockets with a anti-static brush, and ISO. The some canned air to dry the sockets out.
Reinstalled ram, powered it up and get the same results !
I deOxoit'ed the SW1, No change.
I tested SW 3 & 4 with procedure here: all good

But If I put 64Kb 200ns chips in bank 0, The ram count shows 64Kb. ( these chips also tested in separate ram tester, all good)
Results of more testing: (all chips for all banks tested good in separate ram tester)
Bank 0 - 256KB chips=64
Bank 0 - 64KB chips =64

Bank0 -256kb chips +
Bank1 -256kb chips = 128

Bank0 -256kb chips +
Bank1 -64kb chips = 128

Bank 0 - 64KB chips +
Bank1 -64kb chips = 128

This pattern continues. The most I can get with all banks populated properly is 256KB
as shown here near the top:
I have added a little pressure to the switches when powering up to see if that changes anything. but it does not.
I have reseated the other 4 socketed chips about the board.
Not sure if this maters, but all 256 chips are 150ns, the 64's are mixed 150ns & 200ns

I am thinking that a support chip has half failed. But I have no idea which one that might be.

Some help or suggestions please.
Thanks
 
Thanks for the very quick reply.
No I did not do that mod.
It should not be necessary for this board.
That mod is for the 64-256 XT MB's
This is a 256-640kb XT MB and is able to handle 640kb un-mod'ed
according to this page:
I am not using any ram expansion cards.
Just want to max this board full of ram
again thanks
 
Good thought !
Checking ,,,,
They are the 05/09/86 ROM's. That is last version from IBM.
They look original to the board.
These should see 640Kb.

I looked over the board with 10x glass. Other then a little dirt here and there that wont seep away, everything looks good. No scratches over the traces. All the chip legs do not look corroded. Just the usual age tarnish.
I booted dos 6.22, formatted a couple disks, copied some files. no problems. Basic works.
Just 25% ram
 
KLund1 said:
I have a IBM 5160 256-640 KB motherboard. (the brown one)
Click to expand...
Q1. Is this a newly acquired motherboard, or one that was working for you and recently failed ?

Just so that we know what you have, in case needed later:
Q2. Do you have the ability to program a diagnostic ROM (for the U18 socket) ?
Q3. Do you have a logic probe ?
 
modem7
Q1 This a just acquired board.
Q2 Yes, and already have supersoft for XT, one made by landmark, other burned by me a few years ago (supersoft v1.2). and mono video card & screen for test output.
Q3 Yes

Running SS v1.2.....
 
KLund1 said:
Running SS v1.2.....
Click to expand...
KLund1 said:
I think the video card switch set wrong but still works.
Click to expand...
The Supersoft/Landmark Diagnostic ROM (SLDR) does not use the video related switch settings.
Neither does Ruud's Diagnostic ROM (RDR).
It is the IBM BIOS ROM that uses those switch settings.

KLund1 said:
Seems to pass all ram tests.
Click to expand...
We always need to consider that it is not possible for diagnostic like this to fully test hardware.

Note the "TO 40000". So, SLDR thinks that only 256K of RAM is present.

KLund1 said:
Q1 This a just acquired board.
Click to expand...
On this 256-640KB motherboard (not an upgraded 64-256KB one), if (repeat: if) there are jumper pins for contacts 1 and 2 of E3 (as pictured at [here]), confirm for us that the jumper hasn't fallen off.

( Joining contacts 1 and 2 get the U44 ROM to use 256-640KB address-to-bank mapping. Diagram at [here]. )

Other things are required such as a chip at position U84, but that chip may be soldered in .
 
I worked out a 22V10 GAL for U44 to allow for use of the D0000 bank also. Was a couple of years ago, but details should be wandering around here in the archives.
 
E2 !!
Jumper was missing!
All Good Now. 640Kb !!!

I looked over minuszerodegrees pages about jumper settings for 5160 MB.
The info about this E2 jumper is complex.
Perhaps additional simpler note about if this is missing you only get 25% of you ram showing. ( but it is quite possible I missed it)
It was only the last line at this picture page that made me check these pins. That is pretty deep to find that info.

But Still !! all this help is great !!
Many thanks once again to both of you!!!
Another XT motherboard saved !!

(now that I have all this stuff on the bench, I have a dead 5150 64-256 to test....)
 
KLund1 said:
Not sure if this maters, but all 256 chips are 150ns, the 64's are mixed 150ns & 200ns
Click to expand...
For the IBM PC family, it doesn't matter. The situation is somewhat akin to mixing and matching 90 MPH and 100 MPH tyres on your car, but your car is driven at 80 MPH.
 
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