Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 139)

Binary Command Language
Appendix A
A-11
Communication
The scanner must supply an ACK or NAK within one character timeout
period of receiving a command block. As the slave on the link, the scanner
has until a response timeout occurs before issuing a response block.
If the scanner receives a command block correctly, it responds with an
ACK code (06 hex) and increments the command block sequence count.
The master also increments the command block sequence count and resets
the error count at this time.
If the scanner detects an error in the command block which it receives, it
responds with a NAK code (15 hex). Some typical reasons for sending a
NAK are as follows:
received invalid command block sequence count
unexpected command block
unable to handle command block at this time
character timeout value exceeded
The master stops transmitting within one-half of the character timeout
period after receiving the NAK code. The master increments its error
count and:
If the error count is: The master:
less than three retransamits the block after one character timeout period.
three stops trying to transmit the block.
If the master does not receive an ACK or NAK code within one character
timeout period after transmitting the command block, it continues as
though it received a NAK.
Handshaking
After receiving a response block from the scanner the master responds
within a character timeout period with one of these codes:
If you see: It means that:
ACK (06 hex) the command block was received correctly and that normal
communication is to continue.
NAK (15 hex) an error was detected in the response block and instructs the scanner to
retransmit the block. The scanner tries three times before terminating
communication.
SO (0E hex)
shift out
the master needs time to process the response, and instructs the scanner
to wait. The scanner must wait for the master to send a shift in character
(0F hex) before continuing communication. A shift in character indicates
that the master is ready to continue communication.