This is a DEC AX08 Laboratory Peripheral manufactured on October 28th 1970, serial number 327. It has the following functions 9 bit multiplexed input analog to digital (A/D) converter with up to 24 input channels, crystal and R/C clocks, up to 3 adjustable Schmitt trigger pulse input, up to 8 digital pulse inputs, and up to 4 digital outputs. It also had a point plot display output. Digital first started selling these units in 1967.
The point plot output was used to drive an oscilloscope. Later X/Y deflection monitors like VR14 were used. The display is generated by the processor specifying an 9 bit X and Y location and then requesting that a spot be drawn on the CRT at that location. It then repeats this process to draw the entire display with out of a bunch of dots. On the VR14 link above are pictures and videos of the display from the similar VC8E point plot display for the PDP-8/E. The entire display would need to be redrawn at a 30-60Hz rate to prevent flicker.
The successive approximation (more) type of A/D converter uses a digital to analog (D/A) converter as part of its operation. To save cost the same D/A converter is used for the analog to digital converter and the point plot display. The AX08 gives precedence to the A/D conversion so the point plot display can't be refreshed while an A/D conversion was in process.
The timing of the AX08 is either controlled by a crystal clock with a period of 100 microseconds or a RC clock with an adjustable period of 20 microseconds to 2 Seconds in 11 ranges. These clocks can be used for software timing and be used as the A/D converter sample clock.
The Schmitt trigger digital pulse inputs have an adjustable threshold of -.5 to -2.5 volts and a .7 volt hysteresis. A bit in a register is set when they cross the lower threshold generate an interrupt to the processor if enabled. The 8 "contingency" inputs set bits in the external register when they are at ground.
| General | |
|---|---|
| Size (HeightxWidthxDepth) | 10.5x19x18.75 |
| Weight | TBD |
| Power | Type 728 or 779 Power supply 10V TBD A, -15V TBD A |
| Operating Temperature | 0 to 50C | A/D Converter Specifications |
| Word Length | 9 bits |
| Type | Successive approximation |
| Accuracy | +- 1/2 LSB +- .2% |
| Speed | <= 17 microseconds |
| Sample & Hold Speed | full scale track in 2 microseconds |
| Input | +-1.024V full scale 50K input impedance |
| Speed | <= 17 microseconds |
| Format | 2's complement |
| Input Channels | 4 standard 24 maximum |
Point Plot Specifications |
| Resolution | 9 bits |
| Intensity Control | 1 bit standard 3 bits optional |
| X & Y Output | 0 to -10V |
| Intensity Output | TBD |
Repairs
I did some repairs restoring and early usage but did not record the details.
At VCF east 2007 I replaced a R107 that was intermittent.
On 10/2/2020 while working on my display for VCF east the point plot display shrank and became distorted. The -10V reference was about -2V. I pulled cards using the reference until I found the one overloading it. It was the A604 in B25 Q9 was shorted emitter to collector. Replacing it and it seemed to work but I noticed the display was still distorted. The schematic called out blue 2N1305 but I used generic. I wondered if that was the problem but replacing Q10 fixed it. It must have been damaged when it was carrying too much current.
The following picture links also have descriptions of what is shown in the pictures.
Front view ( 39K)
Rear of front panel ( 59K)
Top view ( 93K)
Left side ( 53K)
Right side ( 57K)
Bottom view (118K)
Backplane closeup (140K)
Back view ( 34K)
Labels ( 21K)
| Quantity | Board Type | Description |
|---|---|---|
| 2 | A130 | |
| 1 | A202 | |
| 1 | A401 Sample and Hold | |
| 1 | A502 | |
| 2 | A601 3 Bit DAC | |
| 6 | A604 | |
| 1 | A704 Reference Supply | |
| 1 | A706 Power Supply | |
| 1 | R002 Diode Network | |
| 10 | R107 Inverter | |
| 11 | R113 NAND/NOR Gate | |
| 1 | R121 | |
| 1 | R122 | |
| 6 | R123 Input Bus Gate | |
| 2 | R151 | |
| 17 | R202 Dual Flip Flop | |
| 5 | R203 Triple Flip Flop | |
| 5 | R205 Dual Flip Flop | |
| 3 | R302 Dual Delay (One Shot) | |
| 2 | R401 | |
| 1 | R405 Crystal Clock | |
| 1 | R581 | |
| 4 | R603 Triple Pulse Amplifier | |
| 1 | W500 | |
| 2 | W501 |
Feel free to contact me, David Gesswein djg@pdp8online.com with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with. I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation.
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