Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: RS232 and imitations thereof
Message-ID: <1990Aug27.165124.17604@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <7574@scolex.sco.COM> <1990Aug25.232127.4269@hayes.fai.alaska.edu> <28835@netnews.upenn.edu> <1164@wombat.UUCP>
Date: Mon, 27 Aug 90 16:51:24 GMT

In article <1164@wombat.UUCP> george@wombat.UUCP (George Scolaro) writes:
>... The main point though is that the MAX232 does not have
>enough drive (+12/-12) to transmit RS232 on long cables at high baud rate.
>On a test using 1000ft of 6 wire flat telephone cable we were <9600 baud to
>get reliable data transmission with the max232, but with the 14c88/14c89
>combo we were up at 38400 baud...

There is no way to send 38400 baud RS232.  Can't be done without violating
the RS232 specs.  Check out the slew-rate limits.  The MAX232 datasheet (at
least, the relatively old one I have) goes into some detail about this.
RS232D -- the latest revision of the spec -- explicitly tells you what
used to be implicit, that 20kbaud is the speed limit.

Did you check the capacitance of that 1000ft of cable?  Betcha it's beyond
the RS232 maximum limit too.  With vanilla kinds of cable, you hit the
RS232 spec limit at circa 50 feet.  No RS232 driver is required to meet
specs when driving more than 2500pF.

The MAX232 is possibly the first RS232 driver that really *complies* with
RS232 without external components.  This means you can't cheat with it
the same way you can cheat with the older drivers, which (for example) did
not do slew-rate limiting internally.  It also doesn't exceed the specs
to quite the same extent that older drivers did.  So a lot of people who
got used to playing fast and loose with the rules got tripped up by it.
-- 
Committees do harm merely by existing. | Henry Spencer at U of Toronto Zoology
                       -Freeman Dyson  |  henry@zoo.toronto.edu   utzoo!henry
