Warren's Flip-Chip Tester

To aid in the repair of the DEC PDP-8/L and PDP-8/I systems Warren Stearns made a device for testing most the FlipChip boards used in these systems.

The core of the tester is the five Microchip MCP23S17 16-bit input/output port expander with interrupt output chips. Each of these chips has a different address on the SPI bus so they are individually addressable.

Standard 0.1" jumpers connect the pins on the FlipChip socket to the SPI chips so that individual signals can be disconnected for debugging.

Additional resistors are located on the board that can be connected to high-current signals to provide an additional load.

Warren wrote a SPI driver that Bit-Bashes the SPI signals on a standard PC parallel port. Warren also wrote and application that reads a line of I/O codes from a test vector file, configures the I/O pins on the SPI chips, writes the output pins, and compares the input signals to the expected states in the line codes from a test vector file. If it compared OK it reads the next test vector and tries it again.

Some of the simple FlipChips have just a few test vectors. Complicated FlipChips, like the M220, have over 2,000 test vectors.

Future development work could include converting the positive logic signals to negative logic so B/R/S versions of FlipChips could be tested. Many of the FlipChips, mostly the A and G versions, have analog circuitry so analog I/O would be useful.


A group of interested PDP-8 enthusiasts are reviving Warren's Flipchip tester design. Vincent Slyngstad made a PCB based on Warren's original design, and kept the original component locations.

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The PCB version of Warren's tester with an M220 from a PDP-8/I installed.

During testing of this new PCB we found some interesting issues with the original design.
  • The tester needs a real PC parallel port for the interface. Modern PCs and laptops don't have this.
  • The tester software needs the Porttalk driver installed so it can interact with the PC printer port's registers. Modern operating systems and processors don't allow this, so the Porttalk driver is loaded, and then the tester software. This works OK Windows 2000 and Windows XP, but we need to try it on Windows 7. It likely will not work on Windows 10.
  • There is a lot of crosstalk between the SPI reset signal and one of the SPI MISO signals in the printer I/O cable. This may be the cause of the SPI GPIO chips periodically resetting. Cutting the ribbon cable and moving the MISO-A signal reduces the crosstalk.
  • The SPI MISO signals between the MCP23S17 chips and the 7406 driver chip are unterminated. This causes 10MHz ringing on the MISO signals on the printer I/O cable. adding a 10k Ohm pull-down resistor to these signals fixes this.
  • The pull-up resistors on the printer port are undefined. With my PC the pull-up resistors are very light, so the rise time of the MISO signals is about 2us. Adding a 1k Ohm pull-up resistor to +5V fixes this.
  • The MISO-A signal was connected to the printer I/O cable pin 3, but needed to be connected to pin 5.
The tester is working reliably now.

To Do:
  • Install Visual C V1.52 and see if we can recompile the original source code.
  • See if the recompiled tester application works.
  • Make some minor bug fixes to the tester application.
Long Term goals:
  • Replace the parallel printer interface on the tester with a USB interface using an FTDI C232HM Serial/I2C/SPI cable.
  • Write a new GUI tester application in C# using Visual Studio, but keeping the original test vector file format.
  • Use the FTDI CDM cable device drivers and the FTDI ftd2xx.dll, FTD2XX_NET.dll, libMPSSE.dll, FTCSPI.dll to allow the C# application to access the SPI port on the FTDI cable.
  • Wire the MISO and the MOSI signal of all 5x of the SPI GPIO chips in a bus connected to the USB cable. Use the 5x SPI CS signals from the USB cable to select an individual SPI chip.

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The top side of Warren's FlipChip tester.

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The bottom side of Warren's FlipChip tester.