PowerLogix COM128 RS-485 BUS Connections, Convertorconnections, General BUS Wiring Considerations

COM128 Installation and Operation Manual

4.3RS-485 BUS CONNECTIONS

4.3.1CONVERTORCONNECTIONS

The RS-485 port of the COM128 provides screw down, captured wire type terminal blocks that unplug from the main chassis to simplify wiring.

Refer to the following sections for detailed information on wiring requirements and recommended topologies.

4.3.2GENERAL BUS WIRING CONSIDERATIONS

Up to 32 devices can be connected on a single RS-485 bus. The overall length of the RS-485 cable connecting all devices cannot exceed 4000 ft. (1219 m).

Devices connected on the bus, including convertor(s) and other instrumentation, must be wired as follows:

a.Use a good quality shielded twisted pair cable for each RS-485 bus. It is recommended that AWG 22 (0.6 mm) or larger conductor size be used.

b.Ensure that the polarity is correct when connecting to the RS-485 port (+) and (-) terminals of each device.

c.The shield of each segment of the RS-485 cable must be connected to ground at one end only.

CAUTION

Do not connect ground to the shield at both ends of a segment. Doing so will allow ground loop currents to flow in the shield, inducing noise in the communications cable.

d.It is recommended that an intermediate terminal strip be used to connect each device to the bus. This will allow for easy removal of a device for servicing if necessary. The terminal strip should be connected as shown in Figure 4.3.2.

SchneiderElectric

4.3.3RECOMMENDEDTOPOLOGIES

Devices on an RS-485 bus are connected in a point-to-point configuration, with the (+) and (-) terminals of each device connected to the associated terminals on the next device. This is illustrated in Figure 4.3.3a.

While there are many topologies that can be used to connect devices on an RS-485 communication bus, the two recom- mended methods are the straight-lineand loop topologies.

STRAIGHT-LINE TOPOLOGY

The straight-line wiring method is illustrated in Figure 4.3.3a. Note that connections are shown for one RS-485 port only. The COM128 supports four RS-485 buses simultaneously. The COM128 can exist at any position on the RS-485 bus, including an end point.

Each end point of the straight-line bus must be terminated with a 1/4 watt resistor. These termination resistors reduce signal reflections which may corrupt data on the bus.

Termination resistors are connected between the (+) and (-) terminals of the device at each end of the bus. This device can include either a convertor or any other instrument. The value of the resistor should match the line impedance of the cable being used. For AWG 22 shielded twisted pair cable, values between 150 and 300 ohms are typical. Consult the cable manufacturer’s documentation for the exact impedance of your cable.

LOOP TOPOLOGY

The loop wiring method is illustrated in Figure 4.3.3a. The COM128 can exist at any position on the RS-485 bus.

One advantage of the loop topology is that a single open circuit fault condition anywhere on the loop will not result in the loss of communication between the computer station and any of the remote devices.

The loop topology does not require termination resistors at any point on the bus.

Figure 4.3.2 RS-485 Intermediate Terminal Strip Connection

RS-485	Terminal	RS-485 Cable
Device	Strip	22 gauge shielded
		twisted pair

RS-485 PORT	SHLD
SHLD
	To next
	device
	To next
	device

DISTANCE X

NOTE

Terminal strip connection

method adds 2 times

distance X to overall RS-485

cable length.

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