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Chapter 3
Module Tag Definitions
Table 14 - Transmit Data Format — Short_String Data Type
Byte 1
Byte 2
Next Transmit Record Number
Transmit Record Number
(Handshake Mode Only)
Table 15 - Transmit Data Format — String Type
Byte 1
Byte 2
Next Transmit Record Number
Transmit Record Number
(Handshake Mode Only)
32
Byte 3
Reserved
Byte 3
Length (LSB)
Transmit Handshake or Transmit Immediate Mode
This option defines when the ASCII module transmits data out of the serial
port. Transmit Handshake mode is used primarily in Polled I/O to help prevent
the ASCII module from repeatedly sending its data buffer to the serial device
each time an I/O command is received.
With Transmit Handshake active (default), the master changes the Transmit
Record Number value in the message header whenever it wants the ASCII
module to send a serial string to the target serial device. The module monitors
the second byte (Transmit Record Number) of the Poll command or explicit
message to determine whether the consumed data from the controller is to be
transmitted out of the serial port.
The module does not transmit the command data until it receives a new
Transmit Record Number value. Upon sensing a change in value of the
Transmit Record Number, the module transmits the data. It initiates one serial
transmission for each change in the Transmit Record Number value,
regardless of the data string value.
In Transmit Immediate mode, the ASCII module ignores the Transmit Record
Number value and initiates a new serial port transmission with each message
that is received from the controller. This technique can be useful when the
target ASCII device is not affected by receiving the same data multiple times
and operates quickly enough so that message overflows do not occur. This
option is most useful with explicit messages.
A transmission of the serial data from the ASCII module to your ASCII device
is initiated in two ways. If an I/O connection is used, serial data is sent from
the controller to the ASCII module based on the RPI rate set.
•
In Transmit Handshake Mode, changing the transmit record number
always initiates a transmission on the network. Even if you do not set
new data into the data string, the old data is transmitted.
•
In Transmit Immediate Mode, the ASCII module transmits data out of
its serial port every time it receives an I/O command or explicit message
to its transmit buffer or changes the transmit record number.
Rockwell Automation Publication 1734-UM009C-EN-P - August 2022
Byte 4
Byte 5-X
ASCII Data
Length
(max 128 bytes)
Byte 4
Byte 5-X
ASCII Data
Length (MSB)
(max 128 bytes)
Byte X+1
(Max = 132)
End of String Delimiter or
Last Data Byte
Byte X+1
(Max = 132)
End of String Delimiter or
Last Data Byte
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