Watlow CPC400 - Manual (Page 202)

Appendix A: Modbus Protocol CPC400 Series User’s Guide
188 Watlow Anafaze Doc. 0600-2900-2000
Field Format
When messages are transmitted on standard Modbus seri-
al networks, each character or byte is sent in this order (left
to right):
Least Significant Bit…Most Significant Bit
The bit sequence is as follows:
• With parity checking:
• Without parity checking
Error Checking Methods
Modbus RTU use two kinds of error checking:
• Parity checking
• Frame checking (CRC)
Parity checking can be optionally applied to each character,
while the frame checking is applied to the entire message.
Both the character check and message frame check are
generated in the master device and applied to the message
contents before transmission. The slave device checks each
character and the entire message frame during receipt.
The master is configured by the user to wait for a predeter-
mined time-out interval before aborting the transaction.
This interval is set to be long enough for any slave to re-
spond normally. If the slave detects a transmission error,
the message will not be acted upon. The slave will not con-
struct a response to the master. Thus the time-out will ex-
pire and allow the master’s program to handle the error.
Note that a message addressed to a nonexistent slave de-
vice will also cause a time-out.
Parity Checking
You can configure controllers for even, odd or no parity
checking. This will determine how the parity will be set in
each character.
If you choose either even or odd parity, the quantity of bits
that are set to 1 will be counted in the data portion of each
character (8 bits). The parity bit will then be set to a 0 or 1
to result in an even or odd total of bits set to 1.
Start 1 2 3 4 5 6 7 8 Parity Stop
Start 1 2 3 4 5 6 7 8 Stop Stop