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Data message format
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5 Data message format
The following data message format is used for all interfaces. The ControlLogix interface
uses SINT (8-bit bytes) and the MicroLogix/SLC/PLC-5 interface uses 16-bit words for the
data arrays. All data is sent to and received from the PLC in little endian format.
5.1 Standard message
5.1.1 Receive data message
The receive data message format contains a sequence number, a length, and a data array.
Receive messages have the following characteristics:
– The produced data sequence number is incremented when the data is updated.
– The data length field indicates the number of valid bytes contained in the message.
– The message received from the PLC determines the actual length of the message
returned to the PLC. (This is often greater than the length of the actual receive data
message.)
– All unused bytes in a message returned to the PLC are filled with zeros.
– The GW EIP/ASCII... supports serial packets of up 1518 bytes and socket packets up
to 2048 bytes in “Write-To-Tag/File transfer to PLC” mode.
– For large received data packets, see Section 4.7.2, “EtherNet/IP configuration” and
Section 4.8.2, “EtherNet/IP configuration” objects or MicroLogix/SLC/PLC-5 interface.
5.1.2 Transmit data message
The transmit data message format contains a sequence number, a length, and a data array.
Table 5-1 Receive message data (GW EIP/ASCII... to PLC)
Name Data type Data value Access rule
Produced data
sequence number
UINT 0-65535 (FFFF
hex
) Read only
Data length (in
bytes)
UINT 0-(MSG payload-4) Read only
Data array Array of SINT 0-255 Read only
Table 5-2 Transmit message data (PLC to GW EIP/ASCII...)
Name Data type Data value Access rule
Produced data
sequence number
UINT 0-65535 (FFFF
hex
) Read/Write
Data length (in
bytes)
UINT 0-(MSG payload-4) Read/Write
Data array Array of SINT 0-255 Read/Write