On 25/10/2007, Jinx wrote: > > connect each end of the pair to the TX and RX pins on two PICs, > > or would using say a MAX232 as well help ? > > If it was me, I'd want *something* between the PIC and the wire, > even if it was just a couple of transistors, (a) for protection and > (b) for drive. 1/2 mile is quite a length, and will pick up noise like > RF, EMI, nearby lightning etc if it's in the open > > Since this is a one-off, you don't strictly need to follow any "rules", > just make it work reliably, so any way that gets the job done is OK > > http://www.lammertbies.nl/comm/info/RS-232_specs.html > > Cable length is one of the most discussed items in RS232 world. The > standard has a clear answer, the maximum cable length is 50 feet, or > the cable length equal to a capacitance of 2500 pF. The latter rule > is often forgotten. This means that using a cable with low capacitance > allows you to span longer distances without going beyond the limitations > of the standard. If for example UTP CAT-5 cable is used with a > typical capacitance of 17 pF/ft, the maximum allowed cable length is > 147 feet. > > The cable length mentioned in the standard allows maximum > communication speed to occur. If speed is reduced by a factor 2 or 4, > the maximum length increases dramatically. Texas Instruments has > done some practical experiments years ago at different baud rates to > test the maximum allowed cable lengths. Keep in mind, that the > RS232 standard was originally developed for 20 kbps. By halving > the maximum communication speed, the allowed cable length increases > a factor ten! > > RS232 cable length according to Texas Instruments > > Baud rate Maximum cable length (ft) > > 19200 50 > 9600 500 > 4800 1000 > 2400 3000 > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist