PDP-12 Restoration at the RICM

tony duell ard at p850ug1.demon.co.uk
Tue Jun 16 23:35:42 CDT 2015

> The PDP-12 came to us with an M452 for the 110 baud TTY console and a
> home-made adjustable baud rate module for the second serial port. The
> adjustable baud rate generator was made by the original user of the system
> in the early 80s, and is just a crystal, rotary switch and a Fairchild
> 4702. I ordered a replacement Intersil IM4702 so we can repair the module.
> The original owner doesn't remember ever having an M405 for the second
> serial port.

Right... (and please stop sending me on wild goose chases)

The 4702 is a well-known baud rate generator IC, basically a crystal oscillator
and divider chain in one package. It generates 16" baud rate outputs as needed
by most common UART chips. As you imply it wasn't really contemporary with
the PDP12, but in this case there is 'provenance'.

I do not believe it is a replacement for just an M405 though. The M405 is a simple
crystal oscillator, the output is at the crystal frequency. Now supposing you wanted
1200 baud. You need 19200Hz at the (16*) input to the serial circuitry. I've never seen
a crystal that slow. There are 32768Hz crystals, of course, and then we jump to 100kHz
and up. So I woulg guess the original design was to use an M405 and another module
(perhaps another M216) as a divider. 


Michael Thompson

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