14 Apr
2001
14 Apr
'01
3:15 p.m.
Actually, I don't think the standard prohibits anything in terms of how the
lines are used. If you sneak power from one or another of them, that's fine. I
must have misunderstood what you wrote to say that there was a signal redefined
as BUSY, while there's no reason you can't use a signal on the DB25 for more or
less any purpose. There are some that are defined in a way that might
discourage you from using a signal as handshake or status signal, but there's
really no reason you have to use CTS or DTR in the usual way.
Dick
----- Original Message -----
From: "Tony Duell" <ard(a)p850ug1.demon.co.uk>
To: <classiccmp(a)classiccmp.org>
Sent: Saturday, April 14, 2001 11:44 AM
Subject: Re: 50 pin SCSI to 50 pin centronics
>
> I've revisited the RS232 signal definitions and can't see one called
"busy".
I
No, and you can't find one called 'mouse power' either, for all many
serial mice take their power from the RS232 port....
Seriously, I guess such printers probably shouldn't be called 'RS232'
because they most definitely use some of the hardware handshake lines in
ways that the standard forbids. On the other hand, just about all 'RS232'
devices do that now.
It has become a de-facto standard to use RTS/CTS and/or DSR/DTR as
hardware handshake lines. In the sense that if you are transmitting, you
look at the state of the CTS input, and only transmit when it's asserted.
This _is_ contrary to the official RS232 standard, agreed.
If you do use RTS/CTS like that, it would be reasonable to call CTS a
READY or BUSY input (the only difference between those 2 for a printer is
the polarity).
> know about software handshaking, and even hardware handshaking, but I don't
know
of a
'busy" signal. Which pin is that?
If you want a silly answer : Pin 11. After all, that's where most
printers seem to put it.
I have just picked up the user manual for the DEC Letterprinter 210
(basically an LA100 in a different box!). There is a table (page 25 in my
edition) that is headed :
'The printer supports the folloing RS232C interface signals
Pin Source Name Function
[...]
11 Printer BUSY Restraint
[...] '
Now, I happen to know that is not part of the official RS232 spec, but it
certainly seems to be a de-facto standard...
Although, interestingly, the next section in the manual (setting the DIP
switches) includes the line :
'Set switch A-1 down for transmission restraint using the EIA BUSY
control line.'
So maybe it did later become part of an EIA spec?
-tony