Publicité
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.
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
RS485 ç Serial interface considerations 7
Publicité
