Input / Output Assembly Handshaking Breakdown
ASCII to PLC
The PLC monitors the Input Sequence Number for a non-zero value that doesn’t match the Input
Handshake. This indicates new data is ready for processing.
Upon receiving new data, the PLC reads the length of the data and processes it (up to 200 bytes). The
PLC then echoes the Input Sequence Number into the Input Handshake. This is the indication to the
490NBX that the message was processed.
New messages from the 490NBX are only written to the PLC when the Input Sequence Number matches
the Input Handshake. All other messages are queued in the 490NBX until the queue is full. Messages are
then discarded based on the discard rules defined by the user.
The Sequence Number will roll over to 1 after 65535.
PLC to ASCII
If the Output Sequence Number matches the Output Handshake, the PLC can write a new message to
the 490NBX.
The PLC must write the Output Length and Output Data first, then increment the Output Sequence
Number.
Once the message is processed by the 490NBX, the Output Handshake is echoed with the Last Output
Sequence Number.
Zero is an invalid value for the Output Sequence Number.
Input / Output Assembly Diagnostics Breakdown
When I/O Messaging is used, only the following counters or statuses are used on the Diagnostics page:
• The ASCII to PLC buffer displays:
• Last message sent to PLC
• Next message stored in ASCII queue
• Current message being processed
• Send Test Message to PLC
• ASCII Event counters
• PLC to ASCII buffer display
• Last message received from PLC
• Send Test Message to ASCII
Real Time Automation, Inc. 35 1-800-249-1612
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