ADDS Viewpoint A2


19 Jan 2021, 06:10

When I saw this keyboard listed, I hesitated.
ADDS-Viewpoint-case.jpg (272.9 KiB) Viewed 505 times
ADDS-Viewpoint-back.jpg (1.31 MiB) Viewed 505 times
First, it's a little beat up. Both the keycaps and the case, which is also plastic, have some gouges. And it looks like someone started to deal with the rather annoyingly high barrier between the typewriter keys and the numpad. There are pictures of another example's insides on Bitsavers. From these, it was evident that it was Keytronic, so that meant replacing the foil as a first step. And that it used a 4P4C modular jack and had a 5V regulator. So, I figured that it would need 12V and ground and then use the other two lines for TTL-level serial ASCII.

But looking closer at the PCB, I can see that one of the signals just goes to a screw, so it's a chassis ground. The keyboard can't be output-only, because it has a buzzer and the DIP switch settings listed there confirm that one could enable key clicks. Furthermore, the buzzer itself is 12V, so the power isn't trivial, either.

So, I got it for reasonably cheap, half shipping. And dealt with the foam and foil.
ADDS-Viewpoint-foil.jpg (472.37 KiB) Viewed 505 times
And confirmed the details of the circuit.
ADDS-Viewpoint-top.jpg (1.74 MiB) Viewed 505 times
ADDS-Viewpoint-bottom.jpg (1.55 MiB) Viewed 505 times
And built an interface for power and the one signal line.
ADDS-Viewpoint-converter.jpg (429.21 KiB) Viewed 505 times
When connected and powered, the signal is pulled high. And typing characters does nothing.

After fooling around with having the host pull it low for a while, I found that a single pulse would cause it to toggle the line a few times on its own. And that multiple pulses would cause it to make noise, which turned out to be a quite loud alarm kind of sound. It also makes this for a few seconds when 12V is cut, as some capacitors discharge.

The most noticeable thing about these pulses was that the low segments were sometimes short (26µs) and sometimes long (126µs) and there seemed to always be eight of them. The ESC key in the upper-left generated long-long-short-long-long-short-short-short. Which is ASCII ESC (1B) in LSB-first order.

And, likewise, the 1 key next to it generated 31. So, it was indeed ASCII, but with a funny bit-serial protocol.

So I whipped up a decoder for Pulseview.
vpt-1-pulseview.png (46.53 KiB) Viewed 505 times
But then I discovered that the 2 key sends 80, not 32.
vpt-1-2-pulseview.png (47.69 KiB) Viewed 505 times
And 3 is 81; 4 is 82; 5 is 35, as expected; and 6 is 83 again. In fact, there are 16 keys that send 80-8F, 90-9F with shift, and A0-AF with ctrl.

2 3 4 6 Q W Y A Z M ` [ ] \ ; '

I don't think those are a single row in the matrix.

It's almost as though the keyboard were in some kind of raw code + shifts mode. But it's completely reliable for just those keys and no others. And there doesn't seem to be enough room to do all the keys that way. Moreover, C4 is used for LOCK and CD-CF for F1-F3.

There is a similar Viewpoint 60 and Bitsavers has the firmware. It seems to be roughly similar. Even though it's an 8035 instead of an 8021, it doesn't use the INT line but does everything with T1. Nor does it enable the timer interrupt. Without working it all through, I don't see anything in there peculiar about the mapping of some keys. All the ASCII characters, unshifted, shifted, and ctrl-shifted, are right there as expected.

This keyboard was clearly really used at some point. And it's a masked ROM not PROM.

It's easy enough to build a sketch to correct for the anomaly and get a consistent virtual serial port.

I feel like I'd really need to compare with another keyboard of this kind to further determine whether this weird behavior is specific to this board.

Also, I don't think this is related to the "ADDS Protocol" that ADDS ASCII terminals speak. I know everyone just sets those to XT-mode and uses Soarer's. But I can't find any information on what the alternative looks like. I had thought to get one from elecplus, but they've had to shut down temporarily due to the pandemic.


21 Jan 2021, 02:50

That's a really interesting board. What kind of terminal system do you think it was designed for?


21 Jan 2021, 03:43

I'm not sure. And the photos in Terminals Wiki weren't captured by the Wayback Machine before they went offline. ... ewpoint_A2

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