14 Apr
2001
14 Apr
'01
10:53 a.m.
From: Tony Duell <ard(a)p850ug1.demon.co.uk>
>
> I've revisited the RS232 signal definitions and can't see one called
"busy". I
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.
There are RS232 lines to arbitrate TX/RX data flow. Though the better
Serial
interfaced printers not only allowed for this they also used Xon/Xoff
serial
protocal for buffer management.
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 :
I have one and it's one very nice printer still. It's used is mostly on
the
DEC and CP/M systems that didn't support parallel IO or as a connector
convenience on my part. All of those systems the driver supports the
Xon/Xoff flow control as it was very easy to do.
'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...
It is but look at the EIA line there and its name and function.
Different
name is the problem. It was used for HALFduplex modems to arbitrate
flow.
The DEC parlance prefered the three wire signaling with signal common
and TXD and RXD using xon/xoff flow control. If the connection was
used with wider signals then DTR/CTS were added to signal mostly
that something was connected.
Allison