Siemens COM32, Installation & Operation Manual

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COM32 INSTALLATION & OPERATION MANUAL 5
COM32 Features
1
COM32 Features
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1200, 2400, 4800 or 9600 baud operation.
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Full electrical isolation between RS-485 and RS-232 to 500V.
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Works directly with Siemens SCADA software or with other proprietary soft-
ware.
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Compact, attractive package.
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May be used with direct computer connection or with modems — switch
allows enabling of the receive by carrier detect option.
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RS-485 transmit enable may be controlled automatically or by the RS-232 RTS
line.
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Powered from AC adapter (included).
1.1 Introduction
When an RS-232 communications line is connected to an RS-485 communications
bus, there are several basic problems:
1. RS-232 is fundamentally a duplex standard. Separate signals can be
transmitted and received simultaneously. RS-485 is strictly simplex. Signals
can only be transmitted in one direction at a time. When interfacing between
an RS-232 device and an RS-485 device, there must be some method of
controlling whether the RS-232 device listens or transmits onto the bus. The
COM32 provides two ways of doing this. The first is the standard approach of
using the RS-232 RTS signal to provide directional control for the COM32
module. This works fine provided that the RTS signal is available, and the
computer software being run on the PC supports this method of operation. In
many cases, these conditions are not met.
The COM32 is unique in that it also supports an “automatic” mode which
allows it to detect the change of RS-232 devices from a listen mode to
transmit mode. All the timing functions required to do this are handled
automatically for rates of 1200 to 9600 baud.
2. RS-485 loops are generally quite long, and they often run through electrically
noisy environments. The risk of exposure to transient voltage surges and
spikes on an RS-485 line is substantially higher than on an RS-232 line. The
COM32 module uses isolation between the RS-232 and RS-485 sections to
protect the computer equipment from damage.