LINK Systems OmniLink - PLC Interface - Manual (Page 28)

June 5, 2000 Man ual Rev 1 .0
5.1
Section 5 Communications Protocol
This section is for those responsible for programming the PLC or other device that will communicate
with the Link PLC Interface .
The protocol uses messages that consist of a start of message character, a data field, and an end of
message character. Note that all numbers transferred have leading zeros if necessary to fill out the
number field.
Section 5.1 Conventions
[LF] - Start of Message Character. Line Feed (decimal 10, hex 0A).
[CR] - End of Message Character. Carriage Return (decimal 13, hex 0D).
[S] - A single space character. (decimal 32, hex 20).
# - A single ASCII decimal digit (decimal 48 to 57, hex 30 to 39).
PLC - Used generically to refer to the device that the Link equipment is communicating with. This
could, for instance, be an axis controller.
Section 5.2 Acknowledgments
In order to assure that the data sent to a PLC is what it received, most messages are acknowledged by
returning the same message back to the Link equipment for verification. If the message is not the same
as what was sent, it will be retried up to 2 additional times (3 tries total). If the message is still not
accepted, the Link Operator Terminal will report a communications error.
If a PLC receives a message that is invalid for any reason (wrong length, number too big, number too
small, etc) it should reply with:
[LF]NAK[CR]
as a negative acknowledgment. This acknowledgment will be subject to retries since the message may
be invalid due to noise etc. Optionally, the PLC can simply ignore the message and send no reply.
If a message type is simply not supported by the PLC then the recipient should reply with:
[LF]NOT[CR]
as an indication to the sender that the message is being ignored on purpose. The message that generates
this response may still be retried since it may be the result of noise but no communications error will be
generated if all retries result in this reply!
Section 5.3 Numeric Value Formats
Unless otherwise noted in the particular command section, a fixed numeric format is employed in order
to keep the protocol as simple as possible for the PLC to handle. All parameter values are sent with nine
digits, zero padded as necessary. For instance, 456 would be sent as 000000456. This is referred to as