Serial I/O, SNP & RTU Protocols 135
Section 6: Serial I/O, SNP & RTU Protocols
This Section discusses the following topics related to communications on CPU serial ports
COM1 and COM2:
â–ª Configuring Serial Ports Using COMMREQ Function 65520
â–ª Serial I/O Protocol
â–ª RTU Slave Protocol
â–ª SNP Slave Protocol
Details of the RTU and SNP protocol are described in the Series 90 PLC Serial
Communications User’s Manual, GFK-0582.
6.1 Configuring Serial Ports Using COMMREQ
Function 65520
The Serial Port Setup COMMREQ function 65520 (FFF0 hex) may be used to activate a
serial communication protocol for a serial port, overriding the protocol that was specified
in the port settings of the CPU configuration. The COMMREQ installed protocol remains
active as long as the CPU is in RUN Mode. When the CPU is STOPped, the COMMREQ
installed protocol is removed, and the protocol settings from the CPU configuration are
reactivated.
The COMMREQ requires that all its command data be placed in the correct order (in a
command block) in the CPU memory before it is executed. The COMMREQ should be
executed by a contact of a one-shot coil to prevent sending the data multiple times. For
details on the operands and command block format used by the COMMREQ function,
refer to PACSystems RX3i CPU Programmer’s Reference Manual, GFK-2950 Section 4.
The COMMREQ uses the following TASKs to specify the port for which the operation is
intended:
task 19 for COM1
task 20 for COM2
Note: Because address offsets are stored in a 16-bit word field, the full range of %W
memory type cannot be used with COMMREQs.
6.1.1 COMMREQ Function Example
In the example, when %M0021 is ON, a Command Block located starting at %R0032 is
sent to COM2 (communications task 20) of the CPU (rack 0, slot 0). If an error occurs
processing the COMMREQ, %Q0110 is set.
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