Title: Using The Xtreme/104 Opto Rev. B In RS-485 Half Duplex Mode
Keywords: Xtreme/104 Opto, Rev B, RS-485, Half Duplex
Date: November 6, 2002
This KDB applies specifically to the Xtreme/104 Opto Rev. B. If you are using and Xtreme/104 Opto Rev. C or an Xtreme Opto Rev. D, these instructions do not apply. If you are using a Rev. C or Rev. D board, please refer to KDB-286, Using the Xtreme/104 Opto Rev. C or Rev. D In RS-485 Half Duplex Mode.
The Xtreme/104 Opto Rev. B requires you need to take extra care when operating in RS-485 half duplex mode. To operate in RS-485 half duplex mode you will need to add BIAS resistors (of about 1.8K in value) to the + and – lines. They should be connected as follows:
+ connect the 1.8K resistor to +5V
– connect the 1.8K resistor to GND.
Note that on Xtreme/104 Opto boards, the customer will have to ensure that the power supplies used to provide the +5V and GND are connected to the SR (GND) of the port under use, since GND is isolated. Some customers have used the +5V and SR (GND) available on the 10 pin headers. +5V is available on pin 10, the customer can use this +5V for the purpose of biasing, they should not use the +5V for anything else.
In the Xtreme/104 Opto manual, documentation specific to the Rev B. board indicates that the customer should connect the TX terminator in a half duplex network. This not necessarily true, unless the customer has a BIAS network either at the “device end” or the “Xtreme end”. The presence of the terminator and lack of BIAS will cause spurious data to be received on the ports. This data is present because the line is tri-stated when not transmitting. This error in the manual will be corrected.
End of KDB-284