henry@zoo.toronto.edu (Henry Spencer) (08/27/90)
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
george@wombat.UUCP (George Scolaro) (08/29/90)
In article <1990Aug27.165124.17604@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes: >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 ^^^^ But it can be done without violating the real world... >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. Sorry if I implied that the above tests were within spec and that the MAX232 didn't meet specs. But if the 14c88/14c89 pair (on each end) can quadruple my long distance transmission speed then that is what I'll use. This is on a commercial product where the baud rate is variable, the customer backs the baud rate down to an error free (but fastest possible) speed. The point is that if you want to transmit long distance and you want RS232 (cheap) then whatever you can get will have to do! >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 Sure, I'd agree with that. But then if you plan to use RS232, the the implication is that you will be using cheap cable. If I planned to run low capacitance co-ax (expensive) cable etc, then I wouldn't use RS232. best regards, -- George Scolaro george@wombat.bungi.com [37 20 51 N / 122 03 07 W]