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What is this custom board in a Tandy 1000HX on ebay.

Should be built in joystick ports on the HX. Also without the RAM expansion module you don't get DMA support on the PLUS BUS.
 
Cute, I made that guy an offer on it because I'm writing a game engine in TP7 that I want to make sure is fast enough for a 4.77mhz 8088, he accepted... I google for more info on the HX, and I find a thread about the unit I won!

That custom card peaked my interest too, so I'll report what it is as soon as I have it in hand. As it looks like a five pin I can think of a few possibilities:

1) Keyboard adapter. Entirely possible since there is no external keyboard jack, maybe the internal keyboard is bamjacked?

2) Cassette port. Some early PC software did come on cassette, or perhaps it's to transfer data off old TRS-80 tapes?

3) Midi. "Dumb UART" midi is just 31250 baud serial that can be implemented with a handful of chips. I think that is the most likely candidate.

I'm going to start digging through my old 80 micro's (I have most of 1982 through 1988) to see if there's anything similar in there for projects. The '87 and '88 issues were mostly about the 1K anyhow. Could even be a "commercial" product as a lot of the stuff advertised in '80 Micro were often slapped together on perf board in someones basement.

IF it works this will make a excellent testbed for the range of capabilities I want my 'retro game' engine to be able to target... I'm aiming for 160x100x16 cga (that also works tandy/ega/vga in that mode, which I figured out how to do by forcing REAL mode 3), 320x200x16 ega/tandy/pcJr and 640x480 VGA all from one game engine (with ease of porting to SDL/Free Pascal) -- much like how the old Sierra games were adaptable on the fly.

Quite a retrocomputing month for me -- I got a stack of Coco's last month, been programming 6809 ASM for coco and 8088 under DosBox, now I've got a HX on the way.

I'll check back in to report on that board tomorrow after I get it.
 
Alright, I've got that board in front of me now.

The chips on it are as follows:

AD7574JN - 8 bit DAC
74LS00N - 2 input NAND gate
SN74LS74AN - Edge triggered flip-flops
SN74LS27N - 3 input positive NOR gate
SN74LS244N - Low power schottky

It's all wire-wrap and bus-wire on the backside in not the most organized manner, but I'm thinking I'll try and put together a circuit diagram on it.

Definately NOT Midi despite the five pin connector. The DAC has me thinking it's an extra analog soundcard or something... though it using all five wires (four running right to the ISA bus) makes me wonder if maybe it is some form of cassette adapter. Normally you'd not need a DAC unless you were reading TRS-80 Color Computer tapes (since it used a 6 bit DAC0.

It is one big question mark. Anybody got a clue?
 
Wow... the powers of the internets!

Good to hear it went to someone who has some idea of how to investigate what it is. I saw the pictures in the listing and wondered if it could have been some sort of casette adapter. The 244 is almost certainly used to interface with the HX's data bus, and the flip flops... who knows... could be used to save state on writes to the IO . Seems like the '27 and '00 might be used for address decoding, but it's hard to say.

Do you know which signals wire straight to the bus from the connector? Maybe it's a piece of test gear.

Strange that it has a DAC, but no amplifier circuitry of any kind.
 
No, the AD7574 is an 8-bit ADC--that is, input is analog and output is an 8-bit quantity.

I suspect that most of the "glue" ICs are used for address decoding and device selection.

At some I/O port or memory address, it will be represented (you can probably determine which with a logic probe). Basically the conversion cycle is 15 uSec. and restarts upon being read. In other words, you just read the ADC every 15 uSec or greater and you get a value.

You can determine the hookup of the DIN connector with continuity checker--the input will be connected to pin 4 (perhaps with a divider or other scaling) on the AD7574.
 
First off, my bad. The AD7574JN is a ADC, not a DAC. (I finally got the right data sheet for it -- I my google-fu failed me)

Going through the wiring on the 5 pin DIN, and ... well, damn. It's mostly all power connects.

Shell (brown wire) - Ground on bus (B1)
Pin 1 (orange wire) - multiple chips and +12v on bus (B9)
Pin 2 (yellow wire) - ties to several chips and -12v on bus (B7)
pin 3 (red wire) - also runs to several chips and +5v on bus (B3)
pin 4 (blue wire) - AD7574JN pin 2 (reference voltage?!?)
pin 5 (green wire) - 74LS74AN pin 5 (Q output)

So we've got ground, +12, -12, +5, status read on one of the flip-flop outputs, and reference voltage into a ADC. Wierd part is Analog in is amongst the wires tied to ground, the resistors, and the clock pin on the same chip.

So this isn't cassette either... Starting to think you might be right on the test equipment notion. It looks like the device changes the reference voltage while the compare input is tied to +12...

The outputs from the ADC run direct to D0...D7, so the analog in is definately being decoded to the bus. I'm going to investigate the address lines to see where it's mapped to...

-- edit -- yeah, chuck(g) beat me to the punch on the correction.
 
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