Siemens COM32, Руководство по установке и эксплуатации

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COM32 INSTALLATION & OPERATION MANUAL 9
Installation and Operation
SW-2: OFF (RTS controls the RS-485 bus)
SW-2: ON (RTS is not available: the converter uses internal timing to control the
RS-485 bus)
Refer to Figure 2.2.1 for more information.
2.3 RS-485 Connections
RS-485 communications allows multiple devices to be connected on the same
bus. Up to 32 devices can be connected on a single RS-485 bus, which consists
of a shielded twisted pair cable. The overall length of the RS-485 cable connecting
all devices cannot exceed 4000 ft. (1219 m).
A COM32 is required in order to connect an RS-485 communications bus to a
computer or other RS-232 equipped device.
General Bus Wiring Considerations
Devices connected on the bus, including the COM32 and other instrumentation,
must be wired as follows:
1. 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.
2. Ensure that the polarity is correct when connecting to the RS-485 port (+) and
(-) terminals of each device.
3. The shield of each segment of the RS-485 cable must be connected to ground
at one end only.
C
AUTION
Do not connect ground to the shield at both ends of a segment. Doing so allows ground loop currents
to flow in the shield, inducing noise in the communications cable.
4. Cables should be isolated as much as possible from sources of electrical noise.
Recommended Topologies
Devices on an RS-485 bus are connected in a point-to-point configuration, with the
(+) and (-) terminals of each devices connected to the associated terminals on the
next device. This is illustrated in
Figure 2.3a .
While there are many topologies that can be used to connect devices on an RS-
485 communication bus, the two recommended methods are the straight-line and
loop topologies.
Straight-Line Topology
The straight-line wiring method is illustrated in Figure 2.3a .