User manual

Chapter Three Operation
Page 29 © January 2011
We recommend that units in the same system have the same error-check configuration, however it
is possible for users to configure the units differently. If a unit without error-check configured
receives a message which includes an error-check, then the unit will treat the additional two bytes as
data and output them to the host device. If a unit with error-check configured, receives a message
without an error-check, it will treat the last two data bytes as the error check - the error-check will
always be incorrect and no data will be output.
Error-check is strongly recommended for controlled mode operation. If error-check is not
configured, then a 905U-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. If it is incorrect, an error message
(NACK) is returned to the source address.
3.8 Serial / Radio Rates
The 905U-D 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 to 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 905U-D 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 905U-D 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 905U-D 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
905U-D will automatically calculate the override time based on the configured serial rate and
S18 (the number of bytes required to start). The override time is stored in register S19 - this
value can also be changed by the user.
3.9 Radio Interference
The 905U-D operates on the 902-928MHz license-free radio band (restricted to 915 – 928 MHz
in Australia and 921 – 928MHz in New Zealand). Devices on this radio band must use a spread
spectrum technique to allow multiple users to share the band with minimal interference.
The 905U-D uses a frequency-hopping spread spectrum technique. Each time the 905U-D
transmits, it uses a different frequency channel in the band. The 905U-D will not interfere, or
be interfered by, radio devices on other bands, such as two way radios or wireless telephones.
There can be interference from other devices on the same band. As the “hopping sequence” used
by the 905U-D is different to other devices on this band, the probability of two devices using the