From: tnixon@hayes.com
Newsgroups: comp.dcom.modems
Subject: Re: Hardware vs xon/xoff flow control.
Date: 13 Aug 92 10:54:37 EDT
Distribution: usa
Organization: Hayes Microcomputer Products, Norcross, GA

In article <1992Aug11.185428.13958@tvnews.tv.tek.com>,
dougs@tvnews.tv.tek.com (Doug Stevens) writes: 

> We're having a disagreement about how 'standard' use of hardware flow control
> (that is, RTS-CTS signalling) is for serial communications. I know that the
> the RS232 'standard' is not really a standard (in the sense of having all 
> details fully specified), but how off-the-wall is use of hardware
> flow control? 

Hmmm.  As a member of the TR-30.2 standards committee on Data 
Transmission Interfaces, which is the formulating committee for 
EIA-232, I must first object (in a good-natured way, of course) to 
the characterization of EIA-232 as being "not really a standard" or 
"[not] having all details fully specified".  I can only imagine that 
comment coming from someone who has not read the document. Certainly 
there are a lot of devices in the field which use the term "RS-232" 
very loosely, but that does NOT imply any defect in the standard.

EIA-232-E most definitely includes specific provisions for hardware 
flow control, as does CCITT Recommendation V.24 and ISO 2110 (the 
"international" EIA-232).  Flow control of transmitted data is done 
by the Clear to Send circuit (V.24 circuit 106), and flow control of 
received data is done by the Ready for Receiving circuit (V.24 
circuit 133).  In a typical implementation, circuit 133 uses the 
same physical wire as circuit 105 (Request to Send) when such flow 
control is being used, and circuit 105 is assumed to be in the ON 
condition.

> We need to be able to operate to 115 Kbaud and transfer binary data. Does
> anyone know whether packages that operate at this speed and do binary transfer
> (LapLink and Commute come to mind as examples) use hardware flow control?
> Do any packages use xon/xoff to do this?

I doubt that any of these packages use XON/XOFF.  They most likely 
use flow control at a block level (since they're all 
protocol-based) or hardware flow control.  XON/XOFF would not be 
acceptable because it is not transparent to all data patterns unless 
you apply some type of transparentization scheme, but that would 
reduce throughput.

> The statement has been made that many communication packages commonly used on 
> the PC have no provision for dealing with hardware flow control. How true is 
> this? 

Perhaps it was true at one time, but these days virtually all comm 
programs support RFR/CTS ("RTS/CTS") hardware flow control.

> Another statement was made that QuickBASIC has no provision for using
> hardware flow control. I don't use BASIC; is this true?

That wouldn't surprise me, but I can't say for sure.

-- 
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