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_at_p850ug1.demon.co.uk>
To: <classiccmp_at_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
>
>
Received on Sat Apr 14 2001 - 14:15:56 BST