Rs-422 Interface; Rs-485 Interface; Bias Resistors; Termination Resistors - ABB RS485 Installation And Startup Manual

Each signal that transmits in an RS-232 data transmission system appears on the
interface connector as a voltage with reference to a signal ground. The RS-232 receiver
typically operates within the voltage range of +3 to +12 and -3 to -12 volts.

3.4 RS-422 interface

An RS-422 interface requires at least four wires. Two wires each are used for Transmit
and Receive. A fifth wire is usually required for Signal Ground, when connecting non-
isolated devices together. Handshaking lines may also be supported by some hardware.
This interface is full duplex, allowing use of the same software drivers as for RS-232. The
differential drivers allow for distances up to 1200 meters (4000 feet). The receivers of an
RS-422 device are always enabled.

3.5 RS-485 interface

For multi-drop operation, drivers must be capable of tri-state operation.
An RS-485 interface requires at least two wires. In a two-wire configuration, the same
pair of wires is used for Transmit and Receive. The two-wire configuration utilizes half-
duplex communications. Transmit driver circuits are always taken off-line or tri-stated,
when not in use. This tri-state feature reduces the load on the network, allowing more
devices, without the need of special hardware. This interface also uses differential
drivers, supporting distances up to 1200 meters (4000 feet).
In a differential system the voltage produced by the driver appears across a pair of signal
lines that transmit only one signal. A differential line driver will produce a voltage from 2 to
6 volts across its A and B output terminals and will have a signal ground (C) connection.
Although proper connection to the signal ground is important, it isn't used by a differential
line receiver in determining the logic state of the data line.
A differential line receiver senses the voltage state of the transmission line across two
signal input lines, A and B. It will also have a signal ground (C) that is necessary in
making the proper interface connection. If the differential input voltage Vab is greater than
+200 mV the receiver will have a specific logic state on its output terminal. If the input
voltage is reversed to less than -200 mV, the receiver will create the opposite logic state
on its output terminal.

3.6 Bias resistors

RS-422 and RS-485 networks often require bias, or pull-up and pull-down resistors.
These resistors are used to stabilize the network. By definition, in a MODBUS RTU
network, it is the responsibility of the Master to provide this function.
Some systems may function without these stabilizing resistors, but may be more
susceptible to communication errors. Though the pull-up and pull-down resistors are the
same, the value of these resistors varies from device to device. The actual recommended
resistance may be calculated, and varies with the number of devices on the bus.

3.7 Termination resistors

Termination resistors are often used to reduce reflections on the network. This problem
occurs most with long wires and high baud rates. Due to variations in wire and
equipment, whether or not to use these terminators is usually determined by system
testing. The general rule is to add them only if needed. The resistors are typically 120
ohms, and installed across the Transmit and Receive wire pairs. Normally, one resistor is
installed at each end of each pair of wires.
RS485  Serial interface considerations - 7