Help identify microcontroller in an XT keyboard

[soon668]

Thanks modem7 for the diagram. I was also trying to do something similar, but got confused

Using your logic probe, when you press a key on the keyboard, do you see activity on the clock and data lines ?

No, the Data and Clock lines remain HIGH when keys are pressed. I have updated your diagram with the result.

[modem7]

Thanks modem7 for the diagram. I was also trying to do something similar, but got confused

I am very experienced in 'recreating' circuit diagrams. But it is still easy to get confused. For example, this morning, I realised that I had resistors R4 and R6 swapped. I updated the diagram.

No, the Data and Clock lines remain HIGH when keys are pressed.

The signals on the clock and data lines are of very short duration. For example, at [here], is an oscilloscope capture of an AA byte being sent to the motherboard by an IBM 83-key keyboard (a keyboard that normally holds the data line low). Top waveform is the clock, and bottom waveform is the data.

Perhaps you should use your logic probe to monitor the clock and data wires on a good keyboard, in order for you to be confident of what your logic probe is expected to show on the faulty keyboard (when it sends a byte).

We know that the microcontroller is at least partially working, because it is what is flashing the LED's when it receives +5V. For your faulty keyboard, when the microcontroller generates a byte and corresponding clock, the clock signal is leaving pin 38 of the microcontroller and the data signal is leaving pin 37 of the microcontroller. If you do not see any signals on either of those pins at power-on and later, when you press a key, then it suggests that the microcontroller is at fault.

[soon668]

Hi modem7, thanks for the help and patience in explaining to me.

Perhaps you should use your logic probe to monitor the clock and data wires on a good keyboard, in order for you to be confident of what your logic probe is expected to show on the faulty keyboard (when it sends a byte).fault.

I will try to source for another keyboard locally or at least from China, it is not easy to find an XT/AT keyboard anymore.

If you do not see any signals on either of those pins at power-on and later, when you press a key, then it suggests that the microcontroller is at fault.

On another thread that I posted about problem with my XT motherboard, I purchased an exact same motherboard from eBay and have waited 2 months to get it, only to realised the keyboard is also not working I was thinking to further troubleshoot the faulty motherboard after I am able to at least try out the new motherboard. But seem luck is not with me...

I will most likely report back 2 months later after I get another keyboard from ebay

[modem7]

I will try to source for another keyboard locally or at least from China, it is not easy to find an XT/AT keyboard anymore.

Consider too the option of an AT class keyboard together with an adapter like the one at [here].

BTW. I updated the circuit diagram even more.

[Chuck(G)]

Based on behavior, I suspect that the keyboard is operational, but for some minor issues. But it would take a 'scope to verify.

[soon668]

Consider too the option of an AT class keyboard together with an adapter like the one at [here].

BTW. I updated the circuit diagram even more.

Thanks for the update modem7. One thing I found which I am not sure it raise any concern to you is, when I plug in the DIN connector to the motherboard (power off), and check continuity using multimeter on C5 or R9, both ends of this components actually reports continuity (beep on multimeter) to the ground. From your diagram, I can understand if one of the end is connected to ground. However, the other end although report continuity, the meter show a small resistance of about 57ohm. If I unplug the connector, it is not behaving this way.

[modem7]

Thanks for the update modem7. One thing I found which I am not sure it raise any concern to you is, when I plug in the DIN connector to the motherboard (power off), and check continuity using multimeter on C5 or R9, both ends of this components actually reports continuity (beep on multimeter) to the ground. From your diagram, I can understand if one of the end is connected to ground. However, the other end although report continuity, the meter show a small resistance of about 57ohm. If I unplug the connector, it is not behaving this way.

C5 and R9 run from ground to +5V. When you have the keyboard plugged into the (unpowered) motherboard, measuring the resistance across C5 (or R9) is also measuring the resistance of the +5V on the motherboard, a measurement that is expected to be quite low.

[soon668]

C5 and R9 run from ground to +5V. When you have the keyboard plugged into the (unpowered) motherboard, measuring the resistance across C5 (or R9) is also measuring the resistance of the +5V on the motherboard, a measurement that is expected to be quite low.

Thanks modem7 for explaining, not something to be concern of then

Based on behavior, I suspect that the keyboard is operational, but for some minor issues. But it would take a 'scope to verify.

Thanks Chuck for your suggestion, will be difficult to justify a scope. Will a "Virtual Oscilloscope Logic Analyzer" like LHT00SU1 do the job?

[Chuck(G)]

How handy are you? There are basic oscilloscope/logic analyzers based on the "Blue Pill" microcontroller boards.
But yes, a USB "virtual scope" would do for this purpose.

[soon668]

Consider too the option of an AT class keyboard together with an adapter like the one at [here].

Hi modem7, I should be able to purchase a Fujitsu FKB8720 keyboard locally. How do I go about building the AT2XT convertor?