Omega A2400 - Manual (Page 20)

Communication 3-4
RS-485 Multidrop System
Figure 3.1 illustrates the wiring required for multiple-module RS-485
system. Notice that every module has a direct connection to the A2400. Any
number of modules may be unplugged without affecting the remaining
modules. Each module must be setup with a unique address and the
addresses can be in any order. All RS-485 modules must be setup for no
echo to avoid bus conflicts (see Setup). Also note that the connector pins
on each module are labelled with notations (B), (R), (G), and (Y). This
designates the colors used on standard 4-wire telephone cable:
Label Color
(B) GND Black
(R) V+ Red
(G) DATA- Green
(Y) DATA+ Yellow
This color convention is used to simplify installation. If standard 4-wire
telephone cable is used, it is only necessary to match the labeled pins with
the wire color to guarantee correct installation.
To minimize unwanted reflections on the transmission line, the bus should
be arranged as a line going from one module to the next, starting with the
A2400. ‘Tree’ or random structures of the transmission line should be
avoided. For wire runs greater than 500 feet total, the end of the bus should
be terminated with a 220
Ω resistor connected between DATA+ and DATA-
. The A2400 has a resistor built in to terminate the start of the bus.
When using a bi-directional RS-485 system, there are unavoidable periods
of time when all stations on the line are in receive mode. During this time,
the communications lines are left floating and are very susceptible to noise.
To prevent the generation of random characters, the lines are biased in a
MARK condition as shown in Figure 2.1. The 1K resistors are used to keep
the DATA+ line more positive than the DATA- line when none of the RS-485
transmitters are on. When enabled, the low impedance of an RS-485 driver
easily overcomes the load presented by the resistors.
Special care must be taken with very long busses (greater than 1000 feet)
to ensure error-free operation. Long busses must be terminated as de-
scribed above. The use of twisted cable for the DATA+ and DATA- lines will
greatly enhance signal fidelity. Use parity and checksums along with the ‘#’
form of all commands to detect transmission errors. In situations where
many modules are used on a long line, voltage drops in the power leads