DL11-W SN#1014374 Repair

This board was accessible from the front panel when installed in the PDP11/04 but the values read from the receive and tramsit data registers were not as I expected.

On initial power up the receive data register would report 377 octal and the status register 200 octal.

Receiving a character from the attached terminal would change the data register.

Sending a * from the terminal should load 052 octal in the data register.  This board would display 352.

The board was also inconsistent with some characters not being recognized.

Unlike DL11-W SN#1727869 the UART on this board is a genuine M5303 and is socketed.  There is no service tag on the board so I don't know if this came from the factory socketed or has been replaced in service.

Investigating the board with a logic probe shows that the values displayed on the front panel did match the data out pins on the M5303 UART suggesting a fault with the UART.

The M5303 UART is still available from some eBay vendors (2014) but the prices are quite high.  

An alternative part is the General Instruments AY5-1013A.  There were several vendors for this part (2014) with a wide range of prices (USD3 to USD50).

I ordered two from Bulgaria for USD3.  They both worked well when installed.

The board is now operational and will send and receive characters.


DL11-W SN#1727869 Repair

When installed in the PDP11/04 this board caused bits 4, 5 and 8 on the data bus to be set for any Unibus requests. 

The schematic shows that these bits are part of the interrupt vector address.

The "stuck bits" matched the settings for S2-3, S2-6 and S2-8.

Tracking back through the schematic the fault was traced to E63 (7474 flip-flop) that was not clearing correctly leaving /Q low.  E63 was itself not the fault.

The pin 1 gate of E62 (7402 NOR gate) which clears E63 was reading 1.78 volts with no activity when it should be 5v.

Replacing E62 with a new 7402 fixed the problem.

The board is now operational and will send and receive characters.


Making ROMs with SRECORD

The Kaypro / FreHD adaptor uses a 28C256 EEPROM chip that contains multiple ROM images.  The active image is controlled by jumpers on the high address lines.
The challenge I had while doing the project was how to program the EEPROM with multiple ROM images.
The solution was the SRECORD set of utilities and specifically SREC_CAT.
The following features of SREC_CAT were used:
  • Generate a HEX file from a binary ROM file
  • Exclude the footer from the HEX file
  • Offset the load addresses in the HEX file
The following Windows command file shows srec_cat generating HEX files from three Kaypro binary ROM file and combining them together in a single HEX file that was programmed into the 28C256.
@echo off
echo Make Combined Kaypro ROM
del 81-232.HEX
del TROM34.HEX
srec_cat roms\81-232.ROM -binary -o 81-232.HEX -intel -disable=footer
srec_cat roms\KPLUS83.ROM -binary -offset 0x2000 -o KPLUS83.HEX -intel -disable=footer
srec_cat roms\TROM34.ROM -binary -offset 0x4000 -o TROM34.HEX -intel

Kaypro / FreHD Adaptor - Part 2

Following on from the post about the development of the Kaypro / FreHD adaptors this post covers the SHIM board that I designed for the Kaypro 4/83 (81-240-n).

Design objectives for the board were as follows:

  • For use in the Kaypro 4/83 (81-240-n) and possibly the Kaypro II - still to be confirmed.
  • Single board solution that provides a TRS-80 Model III/4 style 50 pin bus for connection to the stock TRS-80 FreHD.
  • Selectable between the standard Kaypro ROM (no hard disk support), KayPlus and hopefully Advent TurboROM if we find it for the /83 machines.

The design was assembled on prototyping board for initial testing.  It was a challenge to find pin headers.  The ones I used for the prototype were expensive but also too tall to allow the Kaypro case to be closed with the shim installed.  That had to be worked out for the final version.

The production board schematic and PCB layout were done using Eagle Light. The layout was arranged to fit within the board size limits of this version.

Production of the boards was done by OSHPARK.

I had heard about OSHPARK in episode 149 of The Amp Hour Podcast but until I used the service didn't appreciate how well it worked.

For Eagle users the board files can be directly uploaded.  Just add a credit card and 21 days later three PCB's arrived in New Zealand.

The pin header problem was solved using headers from eBay and offsetting the header that connects to the Kaypro PCB socket from the sockets on the shim where the Z80 and 28C256 EEPROM are installed.

While most signals come from the Z80 and ROM sockets there are two that need to be patched from the Kaypro motherboard.

The standard Kaypro 4/83 (81-240-n) has a 2732 ROM but this is too small for the KayPlus ROM.  Just adding a larger ROM is not enough because the /CS signal to the ROM will be deasserted after address 0x1000 which is the last address in the 2732.  The solution is to connect U60 pins 14 and 15 to the header on the adaptor.  These are combined by the 22V10 GAL to generate the /CS for the 28C256 EEPROM.


If you have a Kaypro 4/83 (81-240-n) and want an adaptor you will need to make it yourself.

The Kaypro is a really nice machine but doesn't have enough community support to make producing these boards for resale viable.

Here are the files you need:

Kaypro 4/83 Adaptor Schematic and PCB (Eagle)

Kaypro 4/83 Adaptor 22V10 GAL

Kaypro 4/83 Adaptor 28C256 EEPROM File

Kaypro II Adaptor 28C256 EEPROM File

FreHD Version 2.14 Firmware


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Contact Andrew Quinn

jaquinn@ihug.co.nz http://twitter.com/jaquinn