Interfacing with a PDP-11/05: the UART

Parent Category: How-to's Category: Interfacing with a PDP-11/05 Written by Administrator


The PDP-11/05

The PDP-11/05 was introduced in 1972.
As the successor of the 11/20, it is the 2nd PDP-11 and the first one with a CPU based on microcode instead of hardwired logic.

And in its slim case version, it is surely one of the sexiest personal computers ever build (don't tell this to Nova fans)!

If you have a PDP-11/05, the first thing is to get her connected to a modern PC over RS232 connection. This is what these pages are about.


The serial interface

The '05 has a serial interface compatible with the KL11/DL11 standard. This interface was implemented by DEC several times and lived as long a there were PDP-11's: from 1970 to the mid 90's.

The primary I/O device on the 11/05 were teletypes, especially the famous ASR33. These serve as printing terminal as well as mass storage device: you could type data in, or feed the PDP-11 with data from a paper tape. Output could be to a printer, or to a paper tape punch to save the output datastream as holes in the paper tape.


(This special ASR33 belongs to my computer club, CCG. Photo by Jens Kirchhoff)

The ASR was very slow, standard baudrates is 110 baud. But for early terminals like the VT05 (called "glass teletypes"), baudrate could be set as high as 2400 baud. Surely there were concerns wether people could stand this immense flow of information!

For early teletypes the serial protocol was not RS232, they used a 20mA current loop interface instead. So there's no RS232 on the PDP-11/05, just a 20mA interface. Bottom line: You can not just plug it to a modern PC over your 9pin DSUB plug.



The 11/05 CPU is split onto two hex-width flip chip boards: The "control" board (module M7261) and the "datapath" board (module M7260).

As usually, "datapath" implements registers, ALU and bus interface, while "control" is the micro machine, which decodes instructions and holds the processor state.

The serial interface of the '05 is not a normal UNIBUS peripheral card, it is deeply constructed into the CPU, and logically a part of datapath (it sits directly before the ALU ALEG input).

Two of the nine datapath schematic sheets in document "PDP-11/05-S,11/10-S systems engineering drawings" (Oct 1974) are dedicated to the serial interface.


The UART is the big white 40pin ceramic chip with the gold cap, nice to look at:

In the early '70, when development of integrated circuits began, there was not much market demand for integrated circuits. The only thing everbody could use was an integrated UART and MOS RAM. It should last for 10 year until DEC used chips with more pins (yes, I'm thinking of J11).

On the picture above, the UART is a TR1402 (with Fairchild logo?). But there are lot of compatible chips. I found a datasheet for the AY-5-1013 from General Instruments, see attachement.

You can still buy this chip. A little g**gle research found several 10000 items "in stock" at various electronic distributors, and some $2.95 offers at e*y.

Download this file (UART AY-5-1013.pdf)UART AY-5-1013.pdf[Datasheet for UART AY-5-1013 and compatibles]404 kB