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PDP-8/S #1

Digital Equipment PDP-8/S, S/N 517

 
The PDP-8/S was introduced in 1966 as a low cost version of the PDP-8. The "S" is said to stand for Small, but most say it stands for Slow. It was about 10% of the performance of the PDP-8 and about 20% of the cost. The $10,000 price included 4K of core memory that was expandable to 32K. The successor PDP-8 models that were introduced in the early 70s were much faster than the PDP-8/S and also supported DECtape magnetic tape drives. Most of the (1000 or 1500 depending on the source) PDP-8/S systems were scrapped. There is one at the Computer History Museum, one at the Living Computer Museum, one at the Goodwill Computer Museum, and just a few in private collector's hands.
 
This advertisement is from June, 1966. (Source: http://www.dvq.com)
 



This DEC PDP-8/S was donated to the RICM by a Robin Katz Banks. It was originally used to analyze heart regional and global wall motion data from a very high speed X-ray system at Temple University in Philadelphia, PA.
 
This system has 4K of core memory. There were no disks or tapes on this system, all information was read and saved on paper tape.
 
The PDP-8/S was introduced on 23-AUG-1966. It was designed by Saul P. Dinman and was the first CPU to cost less than $10,000. It is constructed entirely from standard DEC flip-chips and does not contain any integrated circuits. Just the processor contains 1001 transistors. The internal architecture of the CPU is serial making it much slower than other PDP-8 systems. The CPU instruction set is also a little different, so it will not run the same software as other PDP-8 systems.

Click on the image for a larger view.


The assembly at the top left is a PT08B teletype interface that has been modified for variable baud rate.
The Wire-Wrap assemble with the yellow wire is the PC0 controller that holds the Type 750/PC01 paper tape reader controller and the Type 75A paper tape punch controller.
The PC01 below the CPU cabinet is a combination of a PC02 paper tape reader and a PC03 paper tape punch. This reader/punch was also sold as a PC09 for the PDP-9.
 
 

YouTube Video

Warren Stearns demonstrates how to read a test paper tape on a 45 year old Digital Equipment PDP-8/S computer. This is the first time that this has been attempted on this computer in possibly 30 years.
 
A top view of the inside of the CPU. The assembly near the bottom left of this image is the 4K words of core memory. Just to confuse people this core implementation is referred to as the 8K version even though it stores 4K.
 
The bottom of the CPU showing the Wire-Wrap connections between the Flip-Chips. In DEC tradition the blue wires are modifications or repairs.
 
The right side of the CPU, clearly showing the descrete components on the Flip-Chips.
 
A close up showing some the discrete components that make up the CPU.

The front of the CPU chassis.
This is what the processor looks like when it is running. The lights at the right show what type of instuction is being executed.
 
The rear of the paper tape PC0 reader/punch controller.
 
Thanks to Al Kossow from bitsavers.com we now know that the Type 350B incremental plotter controller, Type 34D oscilloscope display, Type 750C paper tape reader, Type 75E paper tape punch, Type CR01C card reader, and the PC01 paper tape reader/punch all use this assembly. All of the modules for the PC02 reader and for the PC03 punch interface are in place. The empty locations are for the other controllers.
 
Some time in the future we may be able to add the modules for the Type 34D oscilloscope display and add an X-Y oscilloscope display. It even supports a light pen!
 
The rear of the PT08 Teletype interface.
 
The inside of the PC02 Paper Tape Reader and PC03 Paper Tape Punch. The 30 CPS punch was made by Royal McBee.
 
The special circuit in the PT08 Teletype interface for variable baud rate. Normally the PT08 has a fixed 110 baud setting. 
 
One of the kits of spare Flip-Chips that came with the system.
 
Inventory tag on the cabinet from Temple University.
 
The label on the PDP-8/S CPU chassis.
 
The label on the PT08-B Rack-Mount Teletype interface.
 
The label on the PC01 paper tape reader/punch assembly that contains the PC02 paper tape reader and the PC03 paper tape punch.
 
The label on the Paper Tape Reader Punch Controller.
 
 
The interface W070 Flip-Chip card for the Teletype cable.
 
 
The 779 power supply in the front of the cabinet. This provides +10VDC and -30VDC to the paper tape reader.
 
The 783A power supply in the rear of the cabinet. This provides +10VDC and -15VDC to the CPU, paper tape reader/punch controller, and the Teletype interface.
The capacitor that is connected to the transformer and is part of the resonance circuit failed after about an hour of operation. The capacitor is a common part so we should have no trouble finding an replacement to repair this power supply.
 
This is a brand new, possibly 40 year old, 783A power supply that replaced the one in the cabinet that has a failed capacitor. Since it probably had not been powered on in 40 years we performed a very slow power-up by reducing the AC input voltage with a Variac. It seems to be working OK in the system now.
 
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The new 738A power supply installed and working.
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