Newsgroups: comp.dcom.modems
Path: utzoo!utgpu!cunews!hobbit.gandalf.ca!dcarr
From: dcarr@hobbit.gandalf.ca (Dave Carr)
Subject: Re: USR Courier V.32bis questions etc....
Message-ID: <1991Apr21.185752.2200@hobbit.gandalf.ca>
Organization: Gandalf Data Ltd.
References: <6465@husc6.harvard.edu> <1991Apr21.032236.21724@netcom.COM> <cs352a41.672247174@zaphod>
Distribution: na
Date: Sun, 21 Apr 1991 18:57:52 GMT
Lines: 36

In <cs352a41.672247174@zaphod> cs352a41@cs.iastate.edu (Adam Goldberg) writes:

>>I think the reason most modem manufacturers are creating modems with a max
>>RS232 speed of 38400 are doing it for three reasons:  (a) It doesn't require
>>exotic, costly processors,  (b) Real life data transfers won't be more than
>>2.7:1 compressible 98% of the time,  and (c) Few computers can handle data
>>rates above 38400 bps.  In short, they're designing a modem that will meet
>>your data transfer needs without exceeding your budget needs.  

(D) RS-232 has its limits.  You wouldn't want to advertise you can do 56
    or 64K over RS-232, otherwise the end-user starts screaming at your
    technical support people that he is getting errors on his "short" 10
    foot cable.

>Since it is (theoretically) possible for an effective data transfer rate of
>57,600, but it would take 'exotic, costly processors', it struck me:  Hmmm,
>is it possible (or has anyone attempted) to:

Exotic, no.  Current, Yes.  One dedicated 80960CA ($75US), can COMPRESS
a 256K bps link using V.42 bis compression.  Compressing a 14.4 K bps
link is almost archaic.

>  o Construct such a very, very fast modem.
>  o Design it as an internal card with DMA--ie, it would take a very strange
>    RS232 port to support such high speeds, and the processor would be 
>    completely taken over by it, so wouldn't it make sense to have the modem
>    work like a disk drive and deposit data directly to memory?  Clearly if
>    a HST modem has a 68k inside of it...

No need to get this fancy at this speed.  Just keep the modem cable short.
RS-232 should :-) work at 56K for 2 feet or so.

No need for DMA.  Just deeper FIFO's on the USART.  Put a Z16C30 USC with
32 byte FIFO's, and the service routine would only need to run every 20
milliseconds.  CPU overhead lower than most serial ports use at 9600.

