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ATTENTION: An I/O message is repeatedly sent and generates internal
writes to Class 71, Instance 1, Attribute 3. This causes you to send large
amounts of data to your device and can cause TX Buffer Overflows. You
may experience corrupted data and improper information that is sent to
your serial device when using data only mode with I/O messaging.
ATTENTION: You should take care when activating Transmit Immediate
mode while I/O Polling is active. In this mode of operation, the module
continuously transmits serial data to the target serial device. If the data
string represents a command message to your target device, these
actions could result in unintended mis-operation of your device or system.
Transmit Serial Data
The length of the string that is sent determines the use of a delimiter in
transmitting data to a serial device from the ASCII module.
•
If the string length is zero (as dynamically defined by the third and
fourth bytes in the message header), or if the data type is Array:
The ASCII module receives data that is sent from the controller and uses
the delimiter to determine how much data is sent to the serial device. The
ASCII module computes the length and then stores this as the new length
in the string attribute. This does not show up if the data type is Array, you
only see the string that is truncated, and the length is in the background.
•
If the string length > 0 or the data type is String or Short_String:
The ASCII module receives data that is sent from the controller ignoring
any embedded terminator. It stores the number of characters that are
defined in Max_Number_of_Transmit_Chars, or the total that is sent by
the master, whichever is less.
The ASCII module sends the information immediately in Transmit Immediate
mode. You can always transmit the data by changing the Transmit Record
Number.
Produce Immediate or Master/Slave Handshake Mode
Selecting the mode option defines when the ASCII module sends new data to
the controller.
In Produce Immediate mode (default), there is no gating by the ASCII module
of data that is sent from the ASCII module to the controller. The module sends
its most current serial port data to the master in response to each Poll
command or explicit message, or in response to a COS or Cyclic event. It is the
master's responsibility to be ready to accept and process new data strings as
they are received.
In Master/Slave Handshake mode, the ASCII module refrains from updating
the new ASCII data until the master requests it. This technique is useful when
the master must ensure that some specific actions take place before receiving
the new serial data.
Rockwell Automation Publication 1734-UM009C-EN-P - August 2022
Chapter 3
Module Tag Definitions
33
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