User's Manual

Chapter Three Operation
Man_455U Rev 1.0 Page 27
each message. These bytes are used to detect any corruption of the data when it is received at
another 455U unit.
Error checking is configured using the AT&E command. This allows error checking to be
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. 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 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.
3.7.3 Data Encryption Option
Some applications require that the system be made secure from eavesdropping and hacking.
To provide for these applications, the modem supports AES-128 data encryption. This is
enabled by setting the the encryption option (AT Command AT#E) and by setting the
encryption key (AT Command AT#S). The modem may be configured to transmit messages
with or without encryption, accept messages without encryption, or to require that received
messages are encrypted.
3.8 Serial / Radio Rates
The 455U will operate most efficiently when the serial and radio data are configured to the
same value. If the serial data rate is less than the radio rate, there is a risk that the radio will
empty the input buffer too quickly, resulting in a single input message being broken into more
than one output messages. Many host protocols such as those used by PLC’s, will not accept
a message being broken.
To avoid this occurring, the 455U will automatically delay the radio transmission starting.
This is called “PLC Mode”. The radio will not start transmitting until a certain number of
bytes have been input into the input buffer. The 455U calculates the number of starting bytes
depending on the values of the configured serial and radio rates. The number of bytes to start
transmitting is stored in register S18 (refer section 4 “Configuration”) - when a configuration
is entered whereby the serial rate is less than the radio rate, the 455U will automatically enter
an appropriate value in S18. The user can change this value. If the serial rate is the same as
the radio rate, or more, than there is no delay.
There is an automatic protection - if a certain time has elapsed and the number of starting
bytes has not been input, then the radio will start transmitting. This is an override protection.
The 455U will automatically calculate the override time based on the configured serial rate