12.4.12.2
Return Data Format for the Read Bytes Function
The return data consists of the number of characters actually read, the number of
characters still available in the input buffer after the read is complete (if any), and the
actual input characters.
Address
Number of characters actually read
A 1
Number of characters still available in the input
buffer, if any
A 2
first two characters (first character is in the low byte)
A 3
third and fourth characters (third character is in the
low byte)
A n
subsequent characters
12.4.12.3
Operating Note
If the input data memory type parameter is specified to be a word memory type, and if an
odd number of bytes are actually received, then the high byte of the last word to be
written with the received data is set to zero.
As data is received from the serial port it is placed in the internal input buffer. If the
buffer becomes full, then any additional data received from the serial port is discarded
and the Overflow Error bit in the Port Status word (See Read Port Status Function) is set.
12.4.13
Read String Function (4403)
This function causes characters to be read from the specified port until a specified
terminating character is received. The characters are read from the internal input buffer
and placed in the specified input data area.
The function returns both the number of characters retrieved and the number of
unprocessed characters still in the input buffer. If zero characters of input are requested,
only the number of unprocessed characters in the input buffer is returned.
If the terminating character is not in the input buffer, the status of the operation is not
complete until either the terminating character has been received or the time-out interval
expires. In either of those conditions, the port status indicates the reason for completion of
the read operation.
If the time-out interval is set to zero, the COMMREQ remains pending until it has
received the requested string, terminated by the specified end character
If this COMMREQ fails for any reason, no data is returned to the buffer. Any data that
was already in the buffer remains, and can be retrieved with a subsequent read request.
Serial I/O / SNP / RTU Protocols
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