ASR 33 Emulation - help!

From: Bob Shannon <>
Date: Sat Oct 20 09:21:53 2001

Derek Peschel wrote:

> On Wed, Oct 17, 2001 at 11:30:33PM -0400, wrote:
> > Our sawmill uses an HP-1000 for process control; it has an infrared paper
> > tape reader and an ASR 33 Teletype. And they all work. But I'd like to
> > replace the teletype with a PC. Does anyone know how to make a PC emulate an
> > ASR 33??
> > Thanks

Ok, practical, how-to info here...

Assuming your using a 12531-series interface in your HP...

No current loop to RS-232 adapter is needed, the 12531 is able to generate and
accept RS-232 signals with minor cable modifications as follows:

Jumper pins 16 and T to pin V. This brings the current loop transmit data into the
RS-232 converter. RS-232 transmit data is not available at pin W.

Jumper pin Y to pins 4 and D. This brings receive data from the RS-232 converter
into the current loop data input. RS-232 data is now applied to pin X.

(These modifications are made inside the hood that attaches to the TTY board)

Now then, data format issues...this gets just a little screwy here....

Older HP software will want to get seven data bits with 'space' parity when a
program is running, but when reading a paper tape the data format switches over to
eight data bits and no parity bit. This is due to HP software, and not the
interface hardware.

Contrary to some advice on the list, Hyperterminal will work just fine, and it does
support ASCII file transfers. The problem is that HP software treats reading a
paper tape differently than when reading keyboard input.

When reading a paper tape, the 12531 interface sends a READ signal by grounding
pins 13 and P through a NPN transistor (Q5). There is no 'easy' way to turn this
signal into an RS-232 compatible signal for automatic format switching without
adding extra hardware.

Lastly, the 12531 A/B interfaces can run up to 2400 baud if given an external clock
signal as a TTL signal on pin L on the interface connector. Jumper W1A must be
moved to the W1B position for this to work. The 12531 C interface can run up to
9600 baud using this trick.

I'm glad this question came up, I need to ship a tested RS-232 cable and 12531
board to a friend on the list...
Received on Sat Oct 20 2001 - 09:21:53 BST

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