Multi-Tech Systems 7500 - Manual (Page 128)

RS-485 Connections ION 7500 / ION 7600 User’s Guide
Page 128 Chapter 5 - Features and Applications
RS-485 Connections
Refer to the “Hardware Reference” chapter for the ION 7500 and ION 7600 meters’
RS-485 specifications.
RS-485 connections are made via the captured-wire connectors on the rear of the
meter. Up to 32 devices can be connected on a single RS-485 bus. Use a good
quality shielded twisted pair cable for each RS-485 bus, AWG 22 (0.5 mm
2
) or
larger. The overall length of the RS-485 cable connecting all devices cannot exceed
4000 ft. (1219 m). The RS-485 bus may be configured in straight-line or loop
topologies.
Straight-Line Topology
Loop Topology
General Bus Wiring Considerations
Devices connected on the bus, including the meter, converter(s) and other
instrumentation, must be wired as follows:
Connect the shield of each segment of the cable to ground at one end only.
Isolate cables as much as possible from sources of electrical noise.
Use an intermediate terminal strip to connect each device to the bus. This allows
for easy removal of a device for servicing if necessary.
Install a ¼ Watt termination resistor (R T ) between the (+) and (-) terminals of the
device at each end point of a straight-line bus. The resistor should match the
nominal impedance of the RS-485 cable (typically 120 ohms – consult the
manufacturer’s documentation for the cable’s impedance value).
SHLD
RS-232 to
RS-485
Converter
Termination
Resistor
+ SHLD + SHLD +
SHLD + SHLD + SHLD
R
T
SHLD
+
R
T
-+
COM
RS-485
SH + -
+ SHLD + SHLD
SHLD + SHLD +
SHLD Termination
Block + SHLD
SHLD +
SHLD
RS-232 to
RS-485
Converter
-+
COM
RS-485
SH + -