486 machine crapped out

[Mike_Z]

Today my Windows 3.11 486 machine crapped out on me. It attempts to boot and then will not recognize the keyboard. I tested the keyboard on another machine and it works fine. I removed the motherboard and inspected it. The keyboard plug looks good, it's the large IBM XT type, the large five pin semicircle one. I thought that maybe the socket may have broken a connection to the mother board, but thats not it. I also removed all the extra ISA boards to make a basic machine, drives and video, but that didn't help either. Any suggestions before I look for another motherboard? Thanks Mike

 

[Stone]

Try another keyboard.

Try wiggling the keyboard plug in the socket.

Did the battery leak and corrode the board near the keyboard?

 

[sergey]

I'd double check the connector (I know you said that you've checked it already). I've seen enough cold joints... you might just want to resolder it .
Check that keyboard gets +5V (pin 4 is ground, and pin 5 is +5V). Some computers have polyfuse on keyboard power supply net, some might have a regular fuse... If you're not getting +5V try tracing that signal on the PCB, you might find a burnt PCB trace or a fuse.
Other than that you might try replacing the keyboard interface ICs. Usually there is a 7407 open collector buffer and a keyboard controller - look for an IC in 40-pin DIP package. Finding a replacement keyboard controller might be a bit tricky. In early AT days Intel UPI-42 (8042/8242/8742) or compatible micro-controllers were used, and they were normally programmed with a firmware developed by the BIOS vendor - AMI, Phoenix. Later on, in 90's, specialized keyboard controller ICs were used, e.g. VIA VT82C42N, Holtek HT6542B, Winbond W83C42, etc.

 

[Mike_Z]

The battery is OK and the plug and socket seem OK. I'll try re-soldering a little later. But I did find the 5 volts between pin 4 and 5. Right next to the socket is something labeled F1, probably the fuse? It's definitely not a regular fuse, it must be something in the board itself. But since I have the +5 on pin 5 it must be OK. There appears to be four traces, that I can see, (failing eye sight is a problem for me), Looks like one goes to the power pins and I think it is the ground. The three others run to a 40 pin IC in a socket. It's labeled Regional HT6542. What are the other pins of the keyboard socket? It appears that pin 1 goes to pin 39 of the IC and pin 3 goes to Pin1 of the IC. I can not determine where pin 3 of the plug goes. Thanks Mike

 

[Mike_Z]

I tried re seating the HT6542 chip but it didn't help. I found this pinout for the chip.

Looks line pin 1 is KBCI, could that be Keyboard common input?
Pin 39 is KBDI/MSCI, might that be Keyboard data input?
Maybe I try to find a HT6542 replacement before I try a new motherboard. Thanks Mike

 

[sergey]

I tried re seating the HT6542 chip but it didn't help. I found this pinout for the chip.
View attachment 23848
Looks line pin 1 is KBCI, could that be Keyboard common input?
Pin 39 is KBDI/MSCI, might that be Keyboard data input?
Maybe I try to find a HT6542 replacement before I try a new motherboard. Thanks Mike

The datasheet for this controller is here
This chip has two modes of operation: AT and PS/2. The main difference is that PS/2 mode supports mouse. The pin assignment is a bit different between these two modes, and the first part of the pin name (before slash) is the signal name for AT mode, while the second part is for PS/2 mode. The mode is auto-detected by the controller - depending on what connected to KBDO output. AT mode uses a non-inverting OC buffer (e.g. 7407) with its output connected to KBDI/MSCI input. PS/2 mode uses OC inverter (e.g. 7406) with its output connected to P10/KBDI input.
Now I am not sure which mode your motherboard uses. Some of these 486 motherboards have keyboard controller configured in PS/2 mode (even though the keyboard connector is an AT-style 5-pin DIN). Such motherboard usually have 5 or 6 pin header for connecting a PS/2 mouse connector.

Anyway if you have a logic probe or oscilloscope you can try debugging the keyboard interface: check if there is clock and data output to the keyboard, or input from the keyboard.

 

[Mike_Z]

Thanks for the datasheet and explanation. I generally use an IBM Model M keyboard with this machine. This Model M keyboard will work with an XT or an AT machine. The computer can also use a much later keyboard like my COMPAQ Presario keyboard. I have a good scope and can look for data. I also found a replacement HT6542 chip on ebay for $12. Let you know what becomes of it. Thanks again, Mike

 

[Mike_Z]

Today, my HT6542 chip showed up in the mail. Before I replaced the chip I checked the power supply voltage on the old chip and it was +5 volts. Then after I replaced the chip and loaded enough cards to see if it would work, it did! So I put all the cards back and everything is fine, except my CMOS setting were screwed up. Fortunately, I had all the setting recorded. Anyway the 486 is working again. Thanks to everyone that helped. Mike

 

[andromeda92]

If i disconnect keyboard i have message:
Keyboard error or no keyboard present

if i connect keyboard, i don't have this error.
so the keyboard is detected byt VT82C42N but it not work, i can't enter to bios for example.

EDIT:
sorry, I got the wrong post