Bias Resistors; Termination Resistors; Shielding And Grounding Considerations; Cable Requirements - Hitachi Abb Rvt Installation Und Betriebsanleitung

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 volt-
age 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 stabi-
lize the network. By definition, in a MODBUS RTU network, it is the responsibility of the Master to provide this func-
tion.
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 de-
termined 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.

3.8. Shielding and grounding considerations

The signal ground conductor is often overlooked when ordering cable. An extra twisted pair must be specified to have
enough conductors to run a signal ground. A two-wire system then requires two twisted pairs.
It is often hard to quantify if shielded cable is required in an application or not. Since the added cost of shielded cable
is usually minimal it is worth installing the first time.

3.9. Cable requirements

The type of wire to use will vary with required length. Wire with twisted pairs and an overall shield is used most often.
The shield is tied to earth ground or chassis, and typically at one end only (generally at the Modbus Master side). The
shield is not to be used as a signal common or ground. The table below lists typical cable recommendations.
RS-232
Up to 15m (50ft) virtually any standard shielded twisted pair with drain (Belden 9502 or equivalent).
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