Line Drivers: RS-232/422/485

The RMC SERIAL module supports three different line drivers: RS-232, RS-422, and RS-485. The line driver is selected through RMCWin. The following chart compares these three line drivers:

	RS-232	RS-422	RS-485 (2-wire)	RS-485 (4-wire)
Duplex	Full	Full	Half	Half/Full*
Differential?	No	Yes	Yes	Yes
Topology	Point-to-point	Point-to-point	Point-to-point, Multi-drop	Point-to-point, Multi-drop
Wires	3 or 5**	4 + CMN	2 + CMN	4 + CMN
Max Length***	50-100 ft	4000 ft	4000 ft	4000 ft
Flow Control	None, or Hardware	None	None	None

* Four-wire RS-485 can be full-duplex if the topology is point-to-point. It can also be somewhat full-duplex in a multi-drop scenario in that the master can transmit while a slave is transmitting. However, most multi-drop protocols do not take advantage of this capability.

** RS-232, as supported by the RMC SERIAL, uses only three wires (RxD, TxD, and GND) if flow control is disabled, but five wires (RTS, CTS) if flow control is enabled.

*** The maximum cable lengths vary depending on the baud rate, termination (for RS-422/485), and capacitance of the cable. See RS-232 Wiring for the RMC SERIAL and RS-422/485 Wiring for the RMC SERIAL for details.

Note: From the RMC SERIAL perspective, RS-422 and 4-wire RS-485 are identical. Therefore, the RMC SERIAL offers three drivers: RS-232, RS-422/RS-485 (4-wire), and RS-485 (2-wire).

Each of the above features is described below:

Duplex (Full or Half): Full-duplex means that each device on a serial network can send and receive at the same time, effectively doubling the bandwidth of the network. Half-duplex means that only one device on the network can send data at one time. For the above drivers, full-duplex requires separate send and receive wires.
Differential: Differential wiring uses two wires per signal which allows common mode noise rejection. RS-232 does not use differential wiring, but instead has one wire per signal plus a ground. Differential wiring allows for longer cable distances and greater noise immunity.
Topology: Topology describes the layout of the network. Point-to-point means that exactly two devices are wired together. Multi-drop means that two or more devices are chained together. Notice that "multi-drop" with only two devices becomes point-to-point. See Serial Network Topologies for details.
Wires: This item refers to how many wires need to be connected between nodes. Notice that 2-wire and 4-wire RS-485 actually requires three and five wires respectively because of the Common in addition to the differential signal wires. See RS-232 Wiring for the RMC SERIAL and RS-422/485 Wiring for the RMC SERIAL for details.
Max Length: The maximum cable lengths vary depending on the baud rate, termination (for RS-422/485), and capacitance of the cable. See RS-232 Wiring for the RMC SERIAL and RS-422/485 Wiring for the RMC SERIAL for details.
Flow Control: Flow control can be used with RS-232 to ensure that one device does not overrun the other device. That is, if one device is sending data and the receiving device's buffers get full, then it can use flow control to pause the first device's sending until it has room in its buffers.