Title: Using The Xtreme/104 Opto In RS485 Half Duplex Mode
Keywords: Xtreme/104 Opto, Rev B, RS485, Half Duplex
Date: July 30, 2002
On the Xtreme/104 and the Xtreme/104 Opto Rev B you need to take extra care when operating in half duplex mode. To Operate in half duplex mode you need to add bias resistors of about 1.8K in value attached 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 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 Xtreme Opto port under use. This is because the 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.
A misprint in the Xtreme Opto manual for the Rev B indicates that the customer should connect the TX terminator in a half duplex network. This is wrong, 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.
The Xtreme Opto RevC has a complete jumper selectable termination and bias network available. It is recommended that the customer USE THIS NETWORK in just about all cases. The only exeption would be where the Xtreme is in multi drop mode, but is not a master. The manual for Rev C and greater has an error in this regard. It says to jumper pin JB4-7 D&E, for half duplex you should jumper F&G!!! This uses the formentioned BIAS and Terminction network.
End of KDB-284a