Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!mcnc!rti!bcw
From: bcw@rti.UUCP (Bruce Wright)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Looking for a Sync/Async modem for PC/XT
Summary: Hayes modems do not support true sync communications
Keywords: Synchronous modem
Message-ID: <3061@rti.UUCP>
Date: 2 Jul 89 05:46:04 GMT
References: <64114@yale-celray.yale.UUCP> <595@pegasus.ATT.COM> <1688@eric.mpr.ca>
Organization: Research Triangle Institute, RTP, NC
Lines: 32

In article <1688@eric.mpr.ca>, durham@handel.mpr.ca (Paul Durham) writes:
> 
> The Hayes 2400B internal modem handles both sync and async ( as does
> the external version ). It handles this by doing an internal sync to
> async conversion and moving the data through the 8250 UART ( I am 
> not making this up ). You need a special driver called the Hayes
> Synchronous Driver or some such thing.

Unfortunately the Hayes modems do not support standard synchronous
communications protocols - You can't run BISYNC on them, for example,
unless the modem on the other end is also a Hayes modem.  It can't talk
to standard synchronous modems.

If you think about this it should be obvious that in the general case
there is no way to put a simple sync modem on an async line - even if you 
have a UART on the modem to read the line, you don't have any way for the
computer to transmit a frame or to identify an incoming frame;  often
this is important (you don't want to see just streams of bytes in many
synchronous protocols, you want to see complete frames).  The only way
to do it would be define a higher-level (or lower-level, depending on
your point of view) protocol which you could use to talk to the modem
and which would identify the start and end of the frames.  The Hayes
modems have not done this - the data looks more-or-less like a stream of 
bytes with no frame markers;  also they use the wrong modulation for a
standard synchronous modem.

Not to say anything particular against the Hayes modems - they are good
products - but they really are not meant as general-purpose synchronous
modems;  the mode is provided so that communication between identical
Hayes modems can be sped up using the sync mode.

						Bruce C. Wright