S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05
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Communication processor
12
12.1
Using the RS232 and RS485 communication interfaces
Three communication modules (CMs) and one communication board (CB) provide the
interface for PtP communications:
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You can connect up to three CMs (of any type) plus a CB for a total of four communication
interfaces. Install the CM to the left of the CPU or another CM. Install the CB on the front of
the CPU. Refer to the "Installation" chapter (Page 48) for detailed instructions on module
installation and removal.
The RS232 and RS485 communication interfaces have the following characteristics:
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Have an isolated port
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Support Point-to-Point protocols
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Are configured and programmed through extended instructions and library functions
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Display transmit and receive activity by means of LEDs
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Display a diagnostic LED (CMs only)
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Are powered by the CPU: No external power connection is needed.
Refer to the technical specifications for communication interfaces (Page 743).
LED indicators
The communication modules have three LED indicators:
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Diagnostic LED (DIAG): This LED flashes red until it is addressed by the CPU. After the
CPU powers up, it checks for a CB or CMs and addresses them. The diagnostic LED
begins to flash green. This means that the CPU has addressed the CM or CB, but has
not yet provided the configuration to it. The CPU downloads the configuration to the
configured CMs and CB when the program is downloaded to the CPU. After a download
to the CPU, the diagnostic LED on the communication module or communication board
should be a steady green.
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Transmit LED (Tx): The transmit LED illuminates when data is being transmitted out the
communication port.
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Receive LED (Rx): This LED illuminates when data is being received by the
communication port.
The communication board provides transmit (TxD) and receive (RxD) LEDs. It has no
diagnostic LED.