6.2 Serial I/O Protocol
Serial I/O protocol is a communication protocol that is driven entirely by the application
program. Serial I/O protocol is active only when the CPU is in RUN Mode, since it is driven
completely by COMMREQ functions in the application program. Those COMMREQ
functions are described in detail within this section.
When the CPU is stopped, a port configured for Serial I/O protocol will revert to a STOP
Mode protocol as specified in the port settings of the CPU configuration. If a STOP Mode
protocol was not specified, RTU slave protocol is used by default.
Serial I/O is the only protocol supported by CPE400 and CPL410. CPE400 requires
firmware version 9.40 or later.
6.2.1 Calling Serial I/O COMMREQs from the CPU Sweep
Implementing a serial protocol using Serial I/O COMMREQs may be restricted by the
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 sweep time is 5ms or longer, since timely response is not guaranteed.
6.2.2 Compatibility
The COMMREQ function blocks supported by Serial I/O are not supported by other
currently existing protocols (such as SNP slave and RTU slave). Errors are returned if they
are attempted for a port configured for one of those protocols.
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