PIC RS 485 See also:
ON 20010226@8:00:57 PM http://204.210.50.240/techref/io/serial/rs485s.htm Modified by:JMN-EFP-786 jamesnewton@efplus.com to says Simon Nield at QUANTEL.COM says:
{the above} assumes you are using 2 wire rs485, which gives one datapath & is therefor half-duplex. 4 wire rs485 gives 2 datapaths and can be full duplex.{...} if you are designing a new system then you may well want to think about 4 wire 485, it'll can be a lot more like 232 from a software point of view, which can make things a lot easier - also means your slaves don't have to spend time listening to each others packets, which can give you a nice code boost. (you use one pair from master to slaves and one pair from slaves to master - so each link is unidirectional). cat5 may be a good choice for cabling; you have plenty of twisted pairs, it's cheap, and its often in the walls already. if you do use cat5 then do check the pairing of the wires as they are a little odd. if you can limit the baud rate then using slew rate limited drivers will make emissions testing a lot easier.
One final point in favour of 4 wire 485 - trying to change direction with the serial ports on a pc with windows is slow as slow can be (assuming you are using an rts controlled converter) as in 20ms or so dead time. this makes polling multiple units a good way to waste loads of bandwidth. one solution if you really do want 2 wire is to use a decent rs485 PCI card. the ones I have tried (from oxford semi) were loads faster at turn around in half duplex (practically instantaneous turnaround at 115200)..... could be that this is due to better optimised drivers for the oxsemi board, or it could be hardware.