![background image](http://html1.mh-extra.com/html/elpro-technologies/wi-mod-9-d/wi-mod-9-d_user-manual_2396109029.webp)
Chapter Three
Operation
Page
29
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 WI-MOD-9-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 WI-MOD-9-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 WI-MOD-9-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 WI-MOD-9-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 WI-MOD-9-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
WI-MOD-9-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 WI-MOD-9-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 WI-MOD-9-D uses a frequency-hopping spread spectrum technique. Each time the WI-
MOD-9-D transmits, it uses a different frequency channel in the band. The WI-MOD-9-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 WI-MOD-9-D is different to other devices on this band, the