Micromod MOD: MODBUS RTU Communications Guide - Manual (Page 7)

MODBUS RTU
COMUNICATIONS GUIDE
3
RS-232 INTERFACE
An RS-232 interface is rated for distances up to 15 meters (50 feet). At least three wires are required for an
RS-232 interface. Wires are required for Transmit, Receive and Signal Ground. Some devices support
additional wires for communication handshaking. RS-232 hardware is a full-duplex configuration, having
separate Transmit and Receive lines.
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. For multi-drop operation, drivers must capable of tri-state
operation.
RS-485 INTERFACE
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. A four-wire
configuration functions much like an RS-422 system, except the 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. A fifth wire is usually required for Signal Ground, when
connecting non-isolated devices together. Additional wires for handshaking may also be supported by some
hardware. This interface also uses differential drivers, supporting distances up to 1200 meters (4000 feet).
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. Functionally, any device on the network may provide the bias stabilization. Biasing may
also be installed external to the devices, with the appropriate resistors and power supply. If the Master is not
the device providing this function, careful consideration must be given to the consequences of that device
failing. If the Master is providing this function, and it fails, there would be no communications anyway. In
some systems, bias resistors may be installed on two slaves, offering redundant biasing. This feature is
available with MOD30ML and Modcell MLP modules (TERM switch). 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. Commander
series instruments use 1.8K
Ω
, while MOD30ML and Modcell use 560
Ω
. The pull-up resistor is connected
from the positive communication line to +5Vdc. The pull-down resistor is connected from the negative
communication line to the power supply common.
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
Ω
, and installed across the Transmit and Receive wire pairs. Normally, one resistor is installed at
each end of each pair of wires. For two-wire installations, one resistor would be installed at each end. If bias
stabilization resistors are not installed, use of these terminations will probably drown the signal, preventing
communications.