Unpopulated sockets on a keyboard PCB

Newbie here, trying to resuscitate an ADDS terminal keyboard.
Opened it up, and there were two unpopulated sockets. Is this a normal thing for vintage keyboards?

I was able to power the board on and probe around and it looked very quiet. The 8048 microcontroller was present, but there wasn't any activity on any of the IO lines, nor on the two 74LS138 multiplexers.

Checked the obvious culprits like missing clock or a stuck reset, but everything looked okay.

The unpopulated sockets are a 24-pin and a 14-pin.

Any help would be greatly appreciated
mprosk

Unread post27 Nov 2018, 03:59

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mprosk
 
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If you post a picture of the keyboard or PCB, maybe someone here will recognise that model.

It could otherwise be hard to tell. I think it is a common thing on mass-produced electronics in general to find unpopulated circuits on a PCB. It is so that manufacturers would be able to use the same design for several slightly different products. That would be more likely if it the keyboard had been manufactured by another party for ADDS.
Findecanor

Unread post27 Nov 2018, 04:34

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Keyboard looks like it was made by Keytronic (A65-02705)

The empty 24-pin has a lot of connections between the uC and that Exar chip.
Found this post about the 22-908 and its use. Looks to be part of the capacitive sensing system.
http://www.vcfed.org/forum/archive/index.php/t-37296.html

Top View:
Image

Detail View:
Image

Reverse View:
Image

Keytronic:
Image

Assembled:
Image
mprosk

Unread post27 Nov 2018, 05:35

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mprosk
 
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The empty socket was most likely for a ROM chip. Currently the keyboard has an Intel 8048 microcontroller which has on-board ROM for program code, but PCB offers the flexibility of using an 8039 or 8040 microcontroller which does not have ROM and store the program in the separate ROM chip. In addition the ROM can be useful during development to hold diagnostic code. In the time frame of this keyboard, it was quite expensive to make PCB changes, so they probably built in the flexibility to use either microcontroller option.
Engicoder

Unread post27 Nov 2018, 15:20

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Yeah, the data pinout on the large part matches the 27xx EPROMs.

The other part is also on the data bus and only has 4 unaccounted pins. I don't recognize it as a common shift register.
kps
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Unread post27 Nov 2018, 23:28

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