Rockwell Automation 1799-DASCII DeviceNet Serial Interface - Manual (Page 25)

DeviceNet Serial Interface Module 25
Publication 1799-IN012B-EN-P - July 2008
Set Up and Use the Swap Bytes Mode (parameter 13)
This option may be helpful if the 1799-DASCII module is connected to a DeviceNet
scanner that organizes the data string characters into data type elements that are
larger than 1 byte each. An example is the Allen-Bradley SLC 500 module. In such
cases, the bytes of the data in the master's memory organization can be reversed
from the order in which they are sent or received on the DeviceNet network and
the serial link to the ASCII device. This may cause anomalies in some cases.
The message received or desired "ABCDEFGH" string may appear in memory as
"BADCFEHG" for 2-byte word organization, and "DCBAHGFE" for 4-byte word
organization.
Transmit Byte Swapping
By setting parameter 23, the bytes from the master are swapped by the
1799-DASCII module before transmitting the string to the ASCII device.
Rules for Usage
The rules for usage are the following:
•
This feature is set for transmit and receive independently.
•
Byte swapping works better if the string length is an even multiple of the byte-swap size.
•
If a delimiter is received, then all characters up to and including the defined delimiter are sent to the
DeviceNet master.
If Pad mode = 1, then the 1799-DASCII module fills the Poll Response data
with the Pad Char up to the defined size.
If Pad mode = 0, then the 1799-DASCII module sends only the data up to
and including the delimiter.
•
If no delimiter is received, then the 1799-DASCII module receives up to
Max_Number_of_Receive_Chars, and sends this string to the DeviceNet network with an overflow
error.
It continues to receive and send strings of size
Max_Number_of_Receive_Chars, along with the overflow error, until a
delimiter is received. This could continue indefinitely if the I/O device does
not transmit the specified delimiter.
Master-Slave Handshake versus Immediate Mode (parameter 14)
If DeviceNet Master-Slave Handshake mode is selected, the DeviceNet master can
inhibit the 1799-DASCII module from sending new ASCII data until the master is
ready to receive and process the new data.