User Manual
Chapter 12
GFK-1503F
Jan 2020
Serial I/O / SNP / RTU Protocols
210
12.3
Calling Serial I/O COMMREQs from the PLC
Sweep
Implementing a serial protocol using Serial I/O COMMREQs may be restricted by the PLC
sweep time. For example, if the protocol requires that a reply to a certain message from the
remote device be initiated within 5mS of receiving the message, this method may not be
successful if the PLC sweep time is 5mS or longer, since timely response is not guaranteed.
Since the Serial I/O is completely driven by the application program, in STOP mode a port
configured as Serial I/O automatically reverts to SNP slave, to facilitate programmer
communication. Therefore, while in Stop mode, Serial I/O protocol is not active; it is only
active when the PLC is in Run mode.
When the port reverts back to SNP Slave, the same serial communications parameters (baud
rate, parity, stop bits …) as the currently
-active Serial I/O protocol are used.
Therefore the programmer must use the same parameters for it to be recognized. If any of
the parameter values associated with the Serial I/O protocol are not supported by the SNP
Slave protocol, the programmer will not be able to communicate with the PLC via that port.
12.3.1
Compatibility
The COMMREQ function blocks supported by Serial I/O are not supported by other
currently-existing protocols (such as SNP slave, SNP master, and RTU slave). Errors are
returned if they are attempted for a port configured for one of those protocols.
12.3.2
Status Word for Serial I/O COMMREQs
A value of 1 is returned in the COMMREQ status word upon successful completion of the
COMMREQ. Any other value returned is an error code where the low byte is a major error
code and the high byte is a minor error code.
Содержание PaCSystems VersaMax Series
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