Help identify microcontroller in an XT keyboard

[soon668]

I have an XT keyboard (DataComp DFK-700FI) which the LED blink during boot, but after that does not response with any key press, not even cap lock. I have also ensured the keyboard lock jumper on the XT motherboard is set to unlock. I have tested the keyboard on 2 motherboards which was known to have worked before with this keyboard.

I suspect it could be the keyboard's micro controller that is bad. I would like to replace the IC but could not figure out what is the correct one to get. The IC is marked C35054E DATACOMP 8616. I have attached a photo for reference. The marking is a bit faded, but I am able to read under bright light.

ic.jpg

keyboard-pcb.jpg


Would appreciate any help to identify the IC and recommend a replacement if possible.

Thanks!

[modem7]

At [here], 'Datacomp' appears on a keyboard's controller chip that was made by Signetics. It is as though Datacomp paid Signetics to supply a Signetics chip preprogrammed with Datacomp supplied code.

As for your chip, presumably, the logo will reveal the chip maker and C35054E is the part number. If so, C35054E may be an in-house number, and may contain Datacomp supplied code.

[soon668]

At [here], 'Datacomp' appears on a keyboard's controller chip that was made by Signetics. It is as though Datacomp paid Signetics to supply a Signetics chip preprogrammed with Datacomp supplied code.

Hi modem7, thanks for finding the website. I found the Tatung_TCS-5000 resemble the keyboard that I have, except mine does not have the AT/XT switch. It is also have a DFK-700F2 model name, in which mine is DFK-700FI.

Another similar one is DFK-777F which have a INTEL P8049AH - 8-Bit Microcontroller.

If it is not possible to replace the microcontroller, do you have any suggestion to test out the keyboard? I don't have a logic analyzer that I can probe. Also, do you think XT bios make a different? as I recently replaced the Bios.

Thanks again!

[soon668]

Hi modem7,
I have just acquired a logic probe EM4610 , will you be able to guide me on what to check?

Thanks!

[modem7]

I have an XT keyboard (DataComp DFK-700FI) which the LED blink during boot, ...

Some keyboards (perhaps most) do some sort of self test as soon as they receive power, with that self test momentarily flashing on the LED's.

With the keyboard disconnected from the computer, supply +5V to the keyboard's DIN connector (ground to ground pin, +5V to +5V pin).
If the LED's momentarily flash on, then you can be confident that the controller in the keyboard is doing a self test on receipt of +5V.

... but after that does not response with any key press, not even cap lock.

Is the computer's power-on self test (POST) displaying a 301 error (keyboard error) or displaying an error that points to the keyboard ?

Also, do you think XT bios make a different? as I recently replaced the Bios.

No, because earlier your wrote, "I have tested the keyboard on 2 motherboards which was known to have worked before with this keyboard."
Although I am assuming that you only changed the BIOS on one of the motherboards.

But, if you changed the BIOS on both motherboards, and the keyboard problem started after that, then it is time to put back in the original BIOS', just in case.

I have just acquired a logic probe EM4610 ...

The first thing to do is to understand what your probe shows you in varying conditions:
* Measuring a LOW. Can be done by probing a ground pin.
* Measuring a HIGH. Can be done by probing a +5V pin.
* Measuring an alternating signal ('activity'). There will be plenty of those on a functional motherboard.

... , will you be able to guide me on what to check?

First, I am assuming that the motherboard's POST is displaying a 301 error, or displaying an error that points to the keyboard.
Refer to the diagram at [here], a diagram that shows what happens during the motherboard's POST.
That diagram is for the IBM PC, but it is sure to apply to other PC and XT class motherboards as well.

The LED's on your keyboard are momentarily flashing, so we know that the keyboard (particularly the keyboard's controller chip) is getting +5V.

Before jumping in with a logic probe, use a multimeter to verify that:
1. In the keyboard, there is continuity between the controller's clock pin/s and the clock pin of the keyboard's DIN connector.
2. In the keyboard, there is continuity between the controller's data pin/s and the data pin of the keyboard's DIN connector.

(Because the controller chip in your keyboard is unknown, you may need to examine the circuitry to work out the pins.)

If continuity, then use the logic probe to see if there is activity on the controller's clock pin/s and data pin/s during the POST.

And after the POST, I would expect that whenever you pressed a key, that you would see activity on the controller's clock pin/s and data pin/s (all the way out to the clock and data pins on the keyboard's DIN connector). Try many keys in case some are working and some are not.

[soon668]

Hi modem7,

First, I am assuming that the motherboard's POST is displaying a 301 error, or displaying an error that points to the keyboard.

No, the POST does not display an error. When boot, the LED blink for 3 times, then stop, POST continue as normal, and I am able to get to DOS prompt. Keyboard does not response to any key pressed.

1. In the keyboard, there is continuity between the controller's clock pin/s and the clock pin of the keyboard's DIN connector.
2. In the keyboard, there is continuity between the controller's data pin/s and the data pin of the keyboard's DIN connector.

My best guess is the controller chip used should be similar to the one used in DFK-777F, which is P8049AH, datasheet here. On the keyboard circuitry, I traced the DATA pin from the DIN connector to PIN 39 of the controller, and I got continuity. However, I can't get continuity of the CLK from DIN to any of the PIN of the controller. According to the datasheet, PIN 1 is suppose to be the CLK output. When I trace the CLK connection from DIN, I see there is a 4.7 K Ohm resistor on the way to PIN 1 of the controller.

[modem7]

No, the POST does not display an error. When boot, the LED blink for 3 times, then stop, POST continue as normal, and I am able to get to DOS prompt.

That is odd. I would have thought that at least one the two motherboard you have used would have displayed a keyboard related error.

If you remove the keyboard, does the POST display a keyboard related error ?

According to the datasheet, PIN 1 is suppose to be the CLK output. When I trace the CLK connection from DIN, I see there is a 4.7 K Ohm resistor on the way to PIN 1 of the controller.

A P8049AH is a generic device. If I was to look, there may even be one in my old microwave oven. That even applies if the microcontroller is an 8048, or many other microcontrollers produced for different applications. The engineer decides which pins on the microcontroller to use, and what other circuitry is required.

But if you are sure about the circuit you have determined, then maybe the engineer put the 4.7 K Ohm resistor into the clock line for a purpose.

[soon668]

If you remove the keyboard, does the POST display a keyboard related error ?

Yes, it display "Keyboard bad", then proceed with memory count, and booted successfully to DOS again.

[soon668]

But if you are sure about the circuit you have determined, then maybe the engineer put the 4.7 K Ohm resistor into the clock line for a purpose.

Here is the snapshot of the circuit.



The DATA line goes to Pin 39 of the microcontroller, passing by HD7407P.
The CLK line goes to a 4.7kohm resistor, and further go through a jumper before ended in Pin 40 of the microcontroller.

Here is the snapshot of the front of the board, flipped so that it is easier to match with the circuit

[modem7]

If you remove the keyboard, does the POST display a keyboard related error ?

Yes, it display "Keyboard bad", ...

Suggesting that when the keyboard is attached, the motherboard is seeing an AA byte from the keyboard.

Here is the snapshot of the circuit.

First, look at [here], the circuit diagram for the keyboard that IBM supplied with the IBM PC and IBM XT. Note that between the microcontroller and keyboard is U2, an open-collector driver.

With the photos that you provided, I started to trace out some of the circuit, which I show at [here]. R5 is a pull-up resistor.

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