Hi
I'm a software engineer wondering about some hardware. We have a circuit board with RS485 circuitry and Motorola 68302 processor using an LTC491 RS485 chip and MOC5007 optocoupler for electrical isolation. The RS485 circuit is arranged to run as either 4 wire multidrop (with contention detection circuitry) or as two by 2 channel half duplex point to point, with each channel 2 wires.
The contention circuitry seems to cause some complications, so assuming that it's not there, what is the fastest speed I could expect to run 2 wire point to point RS485 with these chips over 1000 metres using reasonable cable?
On two boards, I've been able to do 250 Kbps with no problems and 200 Kbps over 1400 metres. A fellow hardware engineer tells me that the RS485 chip and optocoupler may vary significantly from chip to chip and also over time/ temperature so that predicting a reliable baud rate is difficult. Currently we use 57600 bps. I would like to go at twice that.
Does the fact that 250 Kbps worked indicate that 115200 bps should be reliable?
TIA