Westermo RM-455U-D - Manual (Page 19)

455U-D Radio Modem User Manual
Page 20 © May 2010
1.9.2 Error Check ?
Error-checking may be configured in both Unacknowledged and Acknowledged mode. When the
error-check is configured, a 16-bit CRC (Cyclic Redundancy Check) attached to the end of each
message. This check is used to detect any corruption of the data when it is received at another
455U-D unit.
Error checking is individually selected for the RS232 port and the RS485 port.
When a unit receives a radio message with error-check, it will not output data until it has received
the whole message and ensured that the error-check is correct. If the unit does not have error-
check configured, then it will output data as it is received, streaming from the radio out the serial
port. Hence operation of the units is faster if error-check is not configured.
Usually units in the same system will have the same error-check configuration; however it is
possible for users to configure the units differently. Each message sent indicates within the
message header whether it uses error-checking or not, so a single modem can receive messages
with and without error checking without requiring changes to the configuration.
Error-check is strongly recommended for Acknowledged mode operation. If error-check is not
configured, then a 455U-D unit will transmit an acknowledgment message (ACK) whenever it
receives a radio message, without checking for errors. If error-check is configured, the unit will
only transmit an ACK message if the error-check is correct.
When using OPR with Modbus RTU or DNP3 protocols, error check should be disabled as these
incorporate their own error check within the data frame.
When using OPR with Modbus ASCII protocol, error check should be enabled as this protocol
only incorporates a weak LRC error check.
When using OPR with DF1 protocol, Error check must be enabled if the DF1 protocol is
configured for Block Character Check (BCC). Error check must be disabled if the DF1 protocol is
configured for Cyclic Redundancy Check (CRC). Best performance is found when DF1 is
configured to use CRC, and Error check is disabled on the 455U-D.