User manual

CMU 100 - RS485 to Multimode Fiber Optic Converter
3.4.2 RS485 INTERFACE BOARD
Description
Single, galvanically isolated, half duplex, RS485 port with Phoenix 3 pin screw connector, with
automatic switch to receive after end of transmission and with additional terminating and
stabilizating resistors.
Hardware settings
For proper functioning of that board some settings must be done:

RS485 bus termination and stabilisation

baud rate
RS485 bus termination and stabilisation
TERMINATION
At high transmission rates or long distance, RS485 bus termination is necessary.
The termination on RS485 bus must be set on both ends of the RS485 bus.
STABILISATION
Some device, to work properly, demands that RS485 must always be in known and valid state.
That is, when + (positive) pin is more than 200 mV positive than – (negative) pin. Pins + and –
are sometimes marked as A and B. In case that no device on RS485 bus is transmitting or in
case of short circuit, there is no voltage difference between those pins and some device do not
work correctly.
On this port board so called „true fail-safe“ RS485 chip is used so board works correctly without
stabilisation at invalid RS485 bus state. But still switches for stabilisation are provided on port
board.
The stabilization on RS485 bus may be set on one device only!
Switches at the lower side of connector:
Switch 4 5 6
Description
RS485 stabilisation RS485 termination RS485 stabilisation
Page: Company: Device: Document: Code: Version: Date:
10 Ediseja 21 CMU 100 / 1.5.6 - 0 User manual CMUMU156 V4 05.11.2014
Picture 4: RS485 BUS schematic