Siemens COM32, Руководство по установке и эксплуатации

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Installation and Operation
10 COM32 INSTALLATION & OPERATION MANUAL
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 converter 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 2.3a . The COM32 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.
Connection Methods to Avoid
Any device connection that causes a branch in the main RS-485 bus should be
avoided This includes star and tee (T) methods. Refer to
Figure 2.3b for examples.
These wiring methods cause signal reflections that may cause interference.
N
OTE
At any connection point on the RS-485 bus, no more than two (2) cables should be connected. This
includes connection points on instruments, converters, and terminal strips. Following this guideline en-
sures that star and tee connections are avoided.