Omega manual RS-485 Termination, A2400 Operation, Character Filter

Functional Description 2-4

bus. This bus connects to multidrop RS-485 devices such as D series modules.

8)	(G)DATA- This is the negative polarity of the differential RS-485 bus.
9)	(R)V+	A2400 power connection. The A2400 operates on 10 to 30
volts dc.
10) (B)GND		This is the ground connection common to all circuits. The

A2400 does not have isolation between power and the two communications ports.

Note that pins 7 through 10 are designated Y, G, R, and B respectively. This corresponds to the Yellow, Green, Red, and Black colors normally found in common telephone cable. All D series RS-485 devices carry this nomencla- ture. If all yellow connections are connected together, and green to green, etc., the RS-485 wiring will be correct.

RS-485 Termination

The RS-485 port lines are terminated as shown in Fig. 2.1. The 220 ohm resistor is used as a transmission line terminator to improve signal fidelity. A similar 220 ohm resistor should be installed at the far end of the RS-485 bus. The 1 K ohm resistors are used to bias the signal lines to a Mark condition. This is necessary when all transmitters of the RS-485 bus are off, which is the most prevalent condition. The resistor bias helps to prevent noise pickup.

The RS-485 port is protected from potentially destructive voltages with positive temperature coefficient thermistors and transient suppressors.

A2400 Operation

A typical installation is shown in Figure 3.1(see Chapter 3). The A2400 is used to provide an interface between a modem and a string of standard modules. Each module and the A2400 has a unique address. In a typical communications sequence the host computer and radio (not shown) will transmit a module command over the air. The modem receiver picks up the message data and presents it to the A2400 on the RS-232 port.

Character Filter

Due to the nature of radio data transmission, noisy data is almost always present at the output of the modem. This is due to inadequate squelch or noise generated when radio transmitters are turned on and off. The A2400 uses noise reduction techniques to reduce the possibility of bad characters