User`s guide

Card Setup

OMG-ISO-COMM Page 6

Headers E8 and E9

RS-485 is backwardly compatible with RS-422, however it is optimized for

partyline or multi-drop applications. The output of the RS-422/485 driver is

capable of being active (enabled) or tri-state (disabled). This capability allows

multiple PCs to be connected in a multi-drop bus and selectively polled. Half-

duplex two-wire operation is also possible by connecting TX+ to RX+ and TX- to

RX- in your cable hood. The enable to the driver is connected to the UART

Request To Send (RTS) line for RS-485 communications. This allows the RS-485

driver to be tri-stated when inactive on a multi-drop polled network. The software

that you are using must "know how" to enable the driver when it is answering a

poll. Failure to correctly set this jumper can cause transmitter contention

problems preventing operation by any nodes on the network. The

communication software is compatible with this mode of operation. To enable the

driver with RTS install jumpers at E9-A for port 1, E9-C for port 2, E8-A for port 3

and E8-C for port 4.

The RS-485 ‘Echo’ is the result of connecting the receiver inputs to the

transmitter outputs. Every time a character is transmitted; it is also received. This

can be beneficial if the software can handle the ‘echoing’ (i.e. using received

characters to throttle the transmitter) or it can confuse the system if the software

does not. The RS-485 echo can also be disabled at E8 and E9. To disable the

echo install a jumper at E9-B for port 1, E9-D for port 2, E8-B for port 3 and E8-D

for port 4.

The following example shows the driver in the RS-485 mode and the echo

enabled.

E8 & E9

Figure 8 - Headers E8 and E9 RS-485 Mode Enable