Positions 7 and 8 of SW1 determine when the RS-422/485 driver and receiver are enabled. Position 7 controls the driver and Position 8 controls the receiver. For RS-422 operation, set both switches to the “Off” position. For multi-drop RS-485 four-wire systems, position 7 should be “On” and position 8 should be “Off.” This allows the receiver to be enabled all of the time and eliminates some possible timing problems. For RS-485 two-wire systems, both switches should be in the “On” position. This disables the RS-422/485 receiver whenever the driver is enabled, preventing data from being echoed back to the fiber side of the FOSTCDR. Table 2 illustrates the switch settings for typical setups.
Table 2: 422/485 Switch Settings
| | Position 7 | Position 8 |
| | TX Enable | RX Enable |
| RS-485 2-Wire Mode | ON | ON |
| (half duplex) |
| | |
| RS-485 4-Wire Mode | ON | OFF |
| (full duplex) |
| | |
| RS-422 Mode | OFF | OFF |
| (full duplex) |
| | |
Multi-Drop Operation
A multi-drop configuration can be created by forming a ring of FOSTCDRs. Each transmitter is tied to the following converter’s receiver, starting at a master node and continuing around to each slave and back to the master. By setting SW1:6 to the “On” position on the slaves, all data sent from the master or preceding slaves is echoed back out the fiber transmitter to the rest of the slaves and eventually back to the master node.
Because all data is echoed back, there are special considerations when constructing a multi-drop system. The master will see its own transmitted data. This means that the master device must be full-duplex (RS-232, RS-422, or four-wire RS-485) and that it must be capable of ignoring or otherwise accepting its own echoed transmission. Slaves must also be able to accept data from previous slaves in the loop.
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