TMS44C256DJ Dram memory chip from early 1990s reliability and testing?

[eduardogomez]

Working on early 1990s 286 PC, and my question is how reliable are these memory chips TMS44C256DJ and is there any way of testing them, I suspect they are the problem



or

https://forum.winworldpc.com/uploads/editor/ag/yowjqo7yuyrl.jpg

[amyk]

Is that corrosion on the PCB? Clean that off first.

[Bud]

This is precious dust, it is forbidden to clean it, the item will lose its antique authenticity and resell value.

[bobxyz]

That looks like a 30-pin 256K by 9bit PC/AT memory SIMM - https://en.wikipedia.org/wiki/SIMM

The chips themselves are pretty reliable, but the sockets may have been damaged over time.  You should be able to find cheap used 8-bit or 9-bit (with parity) SIMMs on ebay.  The BIOS menu on the PC may include a memory test option.  Otherwise you can test by swapping to a different SIMM.

[Stray Electron]

   If that is from a 286 or 386 computer then it (the computer) will almost certainly issue beep codes it it detects a memory problem during boot up.  Get a copy of the instructions for your MoBo and it should explain them.  Or if you have one of common BIOS brands such as AMI, you can (could?) buy books that went into detail about exactly how the BIOS operated.  In addition, if your video sub-system is working it should show the progress of the memory test as the BIOS executes it in preparation to booting.  Once you get the computer running there are plenty of disk based memory tests that will do extensive tests of the memory and they will report the exact memory address and bit where any failure occurs.  Some BIOS will also allow you to interrupt the boot process by pressing certain keys and then put you into a ROM based BIOS setup program and those will frequently have a ROM based memory test program that you can run manually. Get the manual for your MoBo and read it!  It should tell you everything that you need to know.

  That memory is typically very reliable but after 35 years MOL, who knows?  But you need to clean the edge connector on those simms and the sockets!  Those are tin plated connectors and they will develop connectivity issues if they're stored in humid or wet conditions, especially if installed in a socket that uses a dissimilar meter such as a gold plated socket. Because of dissimilar metal corrosion, the rule was the tin plated SIMMS were supposed to only be used in tin plated sockets and gold plated SIMMs in gold plated sockets.  But that was widely ignored. However I found that after a year or so that computers with mis matched materials would often develop memory issues. 

   They did make stand alone memory testers for those but almost no one bought them. It was cheaper, faster and easier to simply swap the SIMMs around or to swap in a known-good SIMM.  We had a tester at the company that I worked for and I got a call about a 386 computer that was giving a memory error. I took the tester out and tested all four SIMMs and they are reported as good but we swapped the SIMMs around and with one particular one we could ALWAYS generate a memory error when we ran a particular Lotus spreadsheet.  That was one of the few times that I saw one of that type SIMM fail but it did, and the tester always failed to detect it as bad! 

[eduardogomez]

They did make stand alone memory testers for those but almost no one bought them. It was cheaper, faster and easier to simply swap the SIMMs around or to swap in a known-good SIMM.  We had a tester at the company that I worked for and I got a call about a 386 computer that was giving a memory error. I took the tester out and tested all four SIMMs and they are reported as good but we swapped the SIMMs around and with one particular one we could ALWAYS generate a memory error when we ran a particular Lotus spreadsheet.  That was one of the few times that I saw one of that type SIMM fail but it did, and the tester always failed to detect it as bad!

Bios checks the memory and displays as good but I think that only checks the capacity of the memory and not the actual function then boot never happens, this means when processor starts moving data to memory it doesn't work. I know the best way of diagnosing the electronics is parts cannon, keep replacing the parts until the unit starts working again. And everyone is giving advice on cleaning corrosion and dust like I don't know any better.

[Stray Electron]

And everyone is giving advice on cleaning corrosion and dust like I don't know any better.

  No one on this forum knows what your skill, experience or knowledge level is, so we didn't assume that you know what you're doing.

[HwAoRrDk]

Bios checks the memory and displays as good but I think that only checks the capacity of the memory and not the actual function then boot never happens, this means when processor starts moving data to memory it doesn't work.

The fact it starts and runs the memory check indicates the CPU is running code from the BIOS ROM, which is good. You're right, the memory check in most BIOSes is typically rudimentary and essentially only checks the amount of memory. Basically, all it does is write a pattern to each word address in memory and reads it back; if it gets back exactly what it reads, it knows there is functioning RAM available at that address; if not, it assumes that's where RAM ends. Bad memory can often result in a BIOS memory check showing a smaller amount of RAM than is fitted.

It might be worthing buying a POST diagnostic ISA card. They're fairly cheap on Amazon/eBay/AliExpress. That will be able to show a two-digit diagnostic code that indicates what stage the BIOS has got to in the boot process. If you don't have a speaker on your system, they also typically feature a speaker on the card and come with a cable to connect it to the motherboard, so you'll be able to hear any beep error codes too.

If it hangs after the memory check, there may be the possibility it is trying to seek the floppy drive but failing because the floppy controller is faulty. Have you tried removing all ISA cards except the display card? Also, is the CMOS battery present? If missing or dead, some BIOSes won't boot. But then again, I've not heard of a 286 system doing that, only later systems.

If you don't have the ability to boot from a disk, and still really want to do a RAM test, there is the Landmark Diagnostic ROM which can be installed in place of the BIOS ROM, and performs a more thorough RAM test (in addition to many other tests). IIRC, it doesn't need fully-functional RAM to run. But you would need the appropriate equipment to burn a new EEPROM chip to use that.

[eduardogomez]

Bios checks the memory and displays as good but I think that only checks the capacity of the memory and not the actual function then boot never happens, this means when processor starts moving data to memory it doesn't work.

The fact it starts and runs the memory check indicates the CPU is running code from the BIOS ROM, which is good. You're right, the memory check in most BIOSes is typically rudimentary and essentially only checks the amount of memory. Basically, all it does is write a pattern to each word address in memory and reads it back; if it gets back exactly what it reads, it knows there is functioning RAM available at that address; if not, it assumes that's where RAM ends. Bad memory can often result in a BIOS memory check showing a smaller amount of RAM than is fitted.

It might be worthing buying a POST diagnostic ISA card. They're fairly cheap on Amazon/eBay/AliExpress. That will be able to show a two-digit diagnostic code that indicates what stage the BIOS has got to in the boot process. If you don't have a speaker on your system, they also typically feature a speaker on the card and come with a cable to connect it to the motherboard, so you'll be able to hear any beep error codes too.

If it hangs after the memory check, there may be the possibility it is trying to seek the floppy drive but failing because the floppy controller is faulty. Have you tried removing all ISA cards except the display card? Also, is the CMOS battery present? If missing or dead, some BIOSes won't boot. But then again, I've not heard of a 286 system doing that, only later systems.

If you don't have the ability to boot from a disk, and still really want to do a RAM test, there is the Landmark Diagnostic ROM which can be installed in place of the BIOS ROM, and performs a more thorough RAM test (in addition to many other tests). IIRC, it doesn't need fully-functional RAM to run. But you would need the appropriate equipment to burn a new EEPROM chip to use that.

it is all soldered directly to the board, that is why I have to diagnose each component. But you do give good advice. It is a machine that was working, then it sat for 20 years, now it won't boot, this happens to all of them. So some component fails just by sitting.