Omega A2400 manual RS-485 Multidrop System, Data+

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 220Ω 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