Interfacing with a PDP-11/05: Becoming RS232

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


Using which SCL signals?

After sorting out the serial wires mechanically, (see previous page), it is now time to care about voltage levels and polarity.

Electrically, the SCL connector on the PDP-11/05 carries these signals:

  • the 20mA serial signals RCV+,RCV-, XMT+, XMT-.
  • special 20mA signals to control a paper tape reader: READER RUN+, READER RUN-
  • TTL level signals for serial receive and transmit
  • Clock control: you can disable the internal baud rate generator with CLK DISAB and provide an own baudrate clock over the CLKIN TTL signal.
  • GND, -15V and +5V, without any fuses.
  • there are no RS232 signals
  • there are no modem control signals

Warning: Because of the mix of TTL signals and -15V, you can damage a lot if the wrong wires touch.
And remember: The +5V have the full power supply behind it!

A try with the 20mA interface

I first tried to connect a PC with the 11/05 over a industrial RS232-to-20mA converter, but this failed.

A 20mA interface works by one side providing a 20mA current, which drives receiver and transmitter of the closed loops for Transmit and Receive.

But the 20mA interface of the PDP-11/05 is not a proper one: the receiver is more like a low impedance voltage sensor, while the transmitter simply switches voltage at the levels +3.5V to -15 V.  At best you call the PDP-11/05 serial interface a "TTY interface": it is well suited to read data generated by mechanical switches to GND, and driving solenoids for transmit data.

Using TTL signals

Then I decided to use the SERIAL IN/OUT TTL signals over a RS232-to-TTL converter. You can buy these as kit for a few Euro, they are based on the popular MAX232 chip.

pdp1105 sclconnector to rs232 adapter


This worked immediately!

A RS232 adapter with baudrate generator

Later I build my own adapter, because I wanted run the PDP-11/05 with higher baudrates than 2400. Here is the schematic:

rs232 schematic

Level converter

In the upper half you see the MAX232 level converter circuit. The MAX232 isolates PC and PDP-11/05 from each other, and converts the TTL levels (0 to +5V) to RS232 levels (+/- 9V) .

You will notice that the RS232 RxD signal goes through one extra inverter (between R1IN and R1OUT ). A look into "PDP-11/05-S,11/10-S systems engineering drawings", sheet "DPH" revealed, that the signal "Serial IN H" is in fact three times inverted, so it should be labled "Serial IN L".

The adapter has a DSUB 9 male connector ("X1") and acts like a RS232 "DTE" (data terminal equipment), just like a PC. Connect it to a terminal or a PC with a null modem cable.

Baudrate generator

The baudrate generator occupies the bottom half  of the schematic. Different baudrates are generated by an ATmega48 microcontroller, which is totally underemployed by this task.  Input is a base frequency of 4.9152 MHz and a ten-position select switch. Output to the PDP-11/05 is the "CLK IN" frequency (baudrate x 16)  and the "CLK DISAB" signal. Only if CLK DIAB is low the '05 runs with  the external baudrate.

If the selector switch is at "0", the PDP-11/05 runs with its internal baudrate. Other positions select 110, 300, 600, 120, 2400, 4800, 9600, 19200 and 38400 baud. The UART is not specified to run faster than 38400, and and even at 19200 baud the '05 processor is limiting the speed in console mode operation.