PDP-11 - Tephra Blog

Tephra Blog

PDP-11

A collection of 17 posts

Booting Unixv4

Overview of the timeline of booting UnixV4 on the PDP-11/40 for the first time in 40 years. Most of this work takes place on the ClassicCMP Discord. 20/12/2025 - Unix 4 discussion Around 9pm (NZD), news about the new Unixv4 image started to make its way onto

RK05 Restoration

RK05 Restoration

Here is a very quick list of things I have done and parts I have used for service my RK05 drives. Replacement Foam The first step is to thoroughly clean everything and remove and replace all of the old foam. For replacement foam you can use weather stripping material commonly

PDP-11 Floating Point Instruction Repair - Part 4

PDP-11 Floating Point Instruction Repair - Part 4

Back at it again — working on another issue with the FIS. The twist this time? It passes all the tests…

PDP-11 Extended Instruction Set Repair (Done Live)

PDP-11 Extended Instruction Set Repair (Done Live)

In this page I have recorded my precess of when I fixed the EIS board in realtime while on Discord. The fix was simple and took approximately 2 1/2 hours. 7:28PM - Running Tests So, for some context, I have just run BQEAC1 (Basic PDP 11/40 diagnostic

PDP-11 Timing Repair - SACK Time Out

PDP-11 Timing Repair - SACK Time Out

This time I have the Timing card (M7234) on the workbench. The Timing card has the system clock and bus interface logic. It is comprised of a collection of one shot time is flip-flops and delay lines. This makes diagnosing problems difficult. Symptoms * When pressing "START" on occasion

PDP-11 Floating Point Instruction Repair - Part 3

PDP-11 Floating Point Instruction Repair - Part 3

With basic functionality restored, we need to re-run the floating point diagnostics test (BKEAB0) and address any remaining problems. Running The Diagnostics When running the diagnostics, we unsurprisingly receive an error printed to the console. ADR PS SP ANS1 ANS2 ----------------------------------- 004226 000050 000634 177520 017752 004226 000050 000634 177520

PDP-11 Floating Point Instruction Repair - Part 2

PDP-11 Floating Point Instruction Repair - Part 2

Last time we found and fixed a fault where the KE11-F was reading arguments from the wrong area of memory. That restored some basic functionality and allowed us to do some basic floating-point addition (1.0 + 1.0). Unfortunately, where we left off, the PDP-11 was still not passing the

PDP-11 Floating Point Instruction Repair - Part 1

PDP-11 Floating Point Instruction Repair - Part 1

Next up for repair is the PDP-11/40's Floating Instruction Set (FIS) board, also known as the KE11-F. The PDP 11/40 was unique in that the FIS and EIS (Extended Instruction Set) were optional. The FIS builds upon the functionality of the EIS, adding floating-point addition, subtraction,

PDP-11 IR Decode Repair

PDP-11 IR Decode Repair

One of my two IR Decode boards would not boot the diagnostic software or pass basic tests. Symptoms When trying to run the bootstrap program to load the diagnostic from the emulated disk, the system would malfunction and would not boot. When loading and running the diagnostic software from memory

PDP-11/40 Wait Instruction Repair

PDP-11/40 Wait Instruction Repair

While running the diagnostic software for the KW11-L I found that one of my two M7235 Status cards would cause the tests to fail. Looking into the problem, I identified that the cause of the issues was that the wait instruction was not working. Symptoms When manually trying to run

PDP-11 Line Time Clock Repair

PDP-11 Line Time Clock Repair

The KW11-L is an option card for the pdp-11/40 that adds a 50/60 Hz clock. It can be configured in a polling mode or interrupt mode. Symptoms When running the KW11-L diagnostic software (ZKWAG0), one of my two KW11-L cards would fail very early on in the tests.

PDP-11/40 Running Diagnostics

PDP-11/40 Running Diagnostics

So you have a PDP-11/40 and are now at the stage where you need to start running diagnostics? Well, I have the guide for you! Required Equipment * PDP-11/40 * Memory * DL11 (Can be emulated) * Unibone * Terminal or Terminal Emulator Background Information The primary diagnostics tool for PDP-11s is XXDP.

PDP-11 DL-11 Repair (UART)

PDP-11 DL-11 Repair (UART)

I quickly ran into a separate problem: the UART wasn’t transmitting data reliably. That pointed me toward a likely issue with the UART card itself.

PDP-11 Timing Repair

PDP-11 Timing Repair

The next part of my PDP-11 to fix is the M7234 - Timing card. It's responsible for the system clock and unibus bus operations. In this situation, the issue presented itself as a fault related bus operations. Symptoms Console Any operations involving the unibus did not work. Examine,

PDP-11 uWord Repair

PDP-11 uWord Repair

In December of 2024, my PDP-11 developed a new fault in the uWord board. The system had gone from mostly functional to almost completely dead!

PDP-11/40 Servicing

PDP-11/40 Servicing

On this page, I have documented all the things I have repaired on my PDP-11 and things to watch out for with your PDP-11 Power Supply Modules The first time I powered up the PDP-11 a full full bridge rectifier failed in one of the power supply modules. I have

PDP-11/40 Test Programmes

PDP-11/40 Test Programmes

Here is a list of small test programs to run on the PDP-11/40 to verify its basic functions.