16 Jun
2015
16 Jun
'15
4:29 p.m.
Date: Mon, 15 Jun 2015 19:42:42 -0400
From: Michael Thompson <michael.99.thompson at gmail.com>
To: cctech <cctech at classiccmp.org>
The M452 creates a 220 Hz clock for the TTY transmitter and a 880 Hz clock
for the TTY receiver.
The M405 for the DP12-B serial port generated a clock that is 16x the baud
rate which is then divided by an M216 module.
Michael Thompson
Just to prolong this discussion...
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.
--
Michael Thompson
16 Jun
16 Jun
9:35 p.m.
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.
-tony
--
Michael Thompson
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