digital equipment corporation · maynard, massachusetts
AX08 Laboratory Peripheral

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.

AX08 Documentation

AX08 Specifications
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


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.

Thumbnail Picture Selector

Current size small. Select picture size for links below   Small  Medium  Large

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)

Boards Used
QuantityBoard TypeDescription
A401 Sample and Hold  
A601 3 Bit DAC  
A704 Reference Supply  
A706 Power Supply  
R002 Diode Network  
10 R107 Inverter  
11 R113 NAND/NOR Gate  
R123 Input Bus Gate  
17 R202 Dual Flip Flop  
R203 Triple Flip Flop  
R205 Dual Flip Flop  
R302 Dual Delay (One Shot)  
R405 Crystal Clock  
R603 Triple Pulse Amplifier  

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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