PDP-11/44 Restoration

This winter RICM has undertaken the restoration of a PDP 11/44.

The PDP 11/44 was the last PDP 11 designed with discrete logic for the processor.


    • Installed a DD11-DK 9-slot Unibus backplane that was removed from a spare BA-11 chassis.

    • Installed a M9300 Unibus terminator in slot 9 of the DD11-DK backplane.

    • Interconnected the two Unibus backplanes with a M9302 Unibus jumper.

    • Installed G727A jumpers in the Unibus slots where the NPR jumper was intact and installed G7273 boards in the Unibus slots where the NPR jumper was cut.

    • Both RA81 disks power up. The lower one has a flakey circuit breaker. They don't go ready so heads are probably locked.

    • The system powers up and the RUN LED and the LED on the M9076 MFM board both go on for a few seconds. Looks promising.


    • Installed two M8722 MS11-MB 256KW memory boards from MT's collection.

    • Installed the two boards for a UDA50 from MT's collection, the red SDI cables, and the SDI bulkhead from RICM spares.

    • The console cable was missing so we made one from a spare DL11 console cable and the connector housing from an Ethernet cable. We lucked out when we found that the plastic Ethernet connector housing and the one for the console cable were the same. We wired the console cable for DCE and used a male DB-25 connector so we don't need to use a null-modem cable to connect it to the terminal.

    • We tried two VT220 terminals and found that both work. We will use the green screen one for the console.

    • The console baud rate switches on the M9076 MFM board looked wrong so we took the easy path and set them all open for 19,200 baud.

    • We checked the settings of the M7090 Console Interface board. The jumpers look OK, but we found that one of the leads on the transistor Q1 was broken. Transistor Q1 is for the 20mA interface so we gave it a try as is.

    • On power up we saw nothing on the console, but the led on the MFM board indicated that data was being sent to the terminal. We installed an M7090 from MT's collection. On power up we saw some garbage on the screen and then ?22 CP HUNG. Major progress! All of the console commands resulted in the ?22 CP HUNG error. The manual said this means that the console told the CPU to do a Unibus transfer, but it didn't happen.

    • The LEDs on both UDA50 boards were cycling so they are alive.

    • We pulled the UDA50 boards and replaced them with G7273 boards. On power up we saw some garbage on the console and then just the >>> prompt. We could write and read from memory and registers, so the 11/44 CPU must be mostly functional.

    • The LED on the M9076 Console Interface board goes on when writing to the terminal. The other M9076 didn't do that.

    • Reinstalling the UDA50 boards resulted in the ?22 CP HUNG error so the M7485 board must have a problem. Time to look for another one.

    • We followed the procedures in the 11/44 User's Guide to see what boot PROMs were installed. The results were strange so we pulled the M7098 Unibus Interface board to see what was physically installed. Nothing! We will install DL (RL02), DU (RA81), and DY (RX02) boot ROMs.


    • Installed a working UDA-50.

    • Installed boot ROMs for RX02, RL02, and MSCP (RA60, RA80, RA81, RA90).

    • We tried to spin up the two RA81 drives. Both start to spin and then stop. Probably needs replacment starting capacitors. There are more RA81 drives in the warehouse that we can try.


    • I tried to spinup the RA80 drive. It smelled like something was cooking and popped the breaker. I tried the last RA81 in the collection that that has the same spinup problem as the other two drives. At least this drive has one of the newer black HDAs. Time to find some startup capacitors.

    • I found three M8256 RX211 floppy disk controllers in the warehouse and tried to boot the 11/44 from the RX02 that came with the 11/83. It made a lot of strange noises and didn't boot. I tried the RX02 from the 11/70. That sounded more promising, but still didn't boot. I connected that RX02 into the 11/34 and the RY0: could read a floppy. RY1: didn't work at all. After a little surgery I found that the drive belt for RY1: had come off. From the dust coating the belt it had been off for a very long time. I cleaned the heads, but RY0: still would not boot. RY1: seems to work OK. I swapped RX211 boards and verified that all three spare controllers are OK. I connected the RX02 to the 11/44 and got it to boot from RY1. The boot procedure seems a little strange; put the switch in HALT, enter "B DY1" and hit enter, move the switch to CONT.


    • Did some more debugging on the RA81 drives. I got to try the Termiflex that I bought years ago.We have 5 RA81 and 1 RA80 drive that all behave the same; they start to spinup and then stop. I fixed an RA81 that had the same symptom a few years ago by replacing the starting capacitor. We will try to find a replacement starting capacitor this week.

    • Found a bad spot in the cable that connects the RX211 controller to the drives. I will cut off about a foot of damaged cable and crimp on a new connector.

    • The UDA50 controller did not have the cycling LEDs display. We found that wiggling the larger ribbom cable that interconnects the two boards fixed it. I will replace the cable next week.

    • We tried to connect an RA70 disk to the UDA50. The drive works fine at home when it is installed in a VAX-3500, but will not spin up without the operator control panel connected. Maybe I need a jumper on the connector for the operator control panel?

    • Booted RT11 again this week. At least that works.

    • Next week we will try installing the RL02 controller and booting from an RL02 disk.

This is the special special Termiflex that can be used to debug an RA80, RA81, or TU81. With a combination of three shift buttons on the side you effectively have a full keyboard. There is 4k of memory and a scroll switch on the left side.

The RA81 drives pass the basic non-spinning tests.

This is the firmware and hardware version and the serial number of the RA81.

The Master and Slave Diagnostic Status.