17
Fastcom
®
: G422/X-PCIe
RS-485 Mode
RS-485 is often referred to as a multi-drop or two-wire, half duplex network because the drivers (transmitters) and
receivers share the same two lines. In fact, up to 32 stations can share the same twisted pair. In order for an
RS-485 system to work, only one driver (transmitter) can occupy the network at a time. This means that each
station on the network must control the enabling/disabling of its drivers in order to avoid network conflicts. If two
drivers engage the network at the same time, data from both will be corrupted. In RS-485 mode, the receivers
are always enabled.
The following cable illustration shows four RS-485 Devices sharing the same twisted pair:
TX+
TX-
RX+
RX -
TX+
TX+
TX+
TX-
TX-
TX-
RX+
RX +
RX +
RX -
RX -
RX -
1
2
3
4
Note: The termination resistors from Station #2 and Station #3 have been removed.
Not all RS-422 devices feature RS-485 compatibility; only RS-485 devices can be connected to the RS-485
network.
Note that when in the RS-485 mode, you will need to externally connect TX+ to RX+ and TX- to RX-.
