Specifications

Communications 3-8

RS-485 Multidrop System

Figure 3.2 illustrates the wiring required for multiple-module RS-485 sys-

tem. Notice that every module has a direct connection to the host system.

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.

DATA* on the label is the complement of DATA (negative true).

To minimize unwanted reflections on the transmission line, the bus should

be arranged as a line going from one module to the next. ‘Tree’ or random

structures of the transmission line should be avoided. When using long

transmission lines and/or high baud rates, the data lines should be termi-

nated at each end with 200 ohm resistors. Standard values of 180 ohms or

220 ohms are acceptable.

During normal operation, there are periods of time where all RS-485 drivers

are off and the communications lines are in an 'idle' high impedance

condition. During this condition, the lines are susceptible to noise pickup

which may be interpreted as random characters on the communications

line. To prevent noise pickup, all RS-485 systems should incorporate 1K

ohm bias resistors as shown in Figure 3.2. The resistors will maintain the

data lines in a 'mark' condition when all drivers are off.

A1000 series converter boxes have the 1KΩ resistors built-in. The resistors

are user-selectable via dip switch located on the rear panel of the A1000.

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