Rockwell Automation Allen-Bradley 1734-232ASC Original Instructions Manual page 34

Chapter 3 Module Tag Definitions
Table 16 - ASCII Modules Consume Assembly Message with Handshake Mode (Short_String)
Byte 1
Next Transmit Record Number
(Handshake Mode Only)
34
IMPORTANT
When Master/Slave Handshake mode is active, two data fields in the message
headers are used to initiate and complete the transaction:
• The New Data Available Flag is set in the Serial Status byte of the ASCII
Poll response message.
This informs the master that a new data string is received and the
module is waiting for the OK from the master to send it.
• The next Transmit Record Number is updated by the master to the ASCII
module that it is now ready to receive the new data string.
Byte 2 Byte 3
Transmit Record Number Reserved
The master monitors the new data flag and when the master is ready to receive
new serial data, it sets a new number in the new Receive Record Number of the
I/O message. This applies only to data being sent from the ASCII module to the
master.
The operation proceeds as follows:
• The ASCII module receives a new data string.
• The ASCII module sets the New Data Flag in the Status byte of its next
produce message.
• When the master is ready to receive the new data string, it changes the
next Transmit Record Number to any value different than what it had
been sending.
• The ASCII module sends the new data when the next Transmit Record
Number changes. The ASCII module resets the new data available bit.
• If the next Transmit Record Number changes, and the module has no
new data, it sets the Handshake Error bit in its Produce Status byte.
You can set the next Transmit Record Number to any value. It may reflect the
Receive Record Number that is sent from the module, if desired, or any other
meaningful number. The ASCII module only looks for a change from the value
since it set the New Data Flag.
Rockwell Automation Publication 1734-UM009C-EN-P - August 2022
Master/Slave Handshake mode is only available if the Transmit
Immediate mode is also active.
Byte 4 Byte 5-X
Length ASCII Data (max 127 bytes)
Byte X+1 (Max = 132)
End of String Delimiter or Last Data Byte