Chapter Two
Operation
MAN_905G_1.16
Page 24
Each 905U module can have up to 31 x 105S modules connected to it. These modules are
addressed 96 - 127. More than one 105S module can have the same address, provided they
are not connected to the same 905U module - that is, #100 via #16 is identified as a different
module to #100 via #65.
A constraint that needs to be considered is the capacity of the radio channel. If there is too
much traffic on the radio channel, then the system quickly becomes unreliable. The
recommended maximum average traffic density is 100 messages per minute provided all
radio paths are reliable. If there are marginal radio paths, resulting in re-tries of transmitted
messages, then the maximum traffic density is reduced considerably. Each block read/write
messages should be counted as two messages because of the length of these messages.
A 905G can be used as a repeater module for messages between other modules.
2.5.1
Radio Signal Strength
The 905G records the radio signal strength of remote modules that communicate directly
(that is, not via repeaters). There are 95 database registers (4401 – 4495) which store the
radio strengths – corresponding to remote addresses #1 - #95. The radio strength (RSSI) is
measured in dBm (relative to 1mW of RF power). The RSSI value is stored in the 8 least
significant bits of each register - a value of –84 dBm would be stored as decimal 84.
These database registers will hold the strength of the last message received from the address.
If a message is received from a remote module via a repeater, then the measurement is
recorded in the address of the last repeater. For example, if a message is received from #24
directly, then the RSSI will be recorded in register 4424. If a message is received from #24
via #25, then the RSSI is recorded in register 4425. The 905G will not know what the radio
strength of the message from #24 to #25 is. If #25 is another 905G, then it can record this
RSSI and this register could be mapped to an I/O register in the first 905G.
The RSSI registers can be read by the host device, or mapped to I/O registers in other 905G
modules.
The first half of the register (8 most significant bits) will be decimal 0 (hex 00) if the remote
module has active communications. If a comms fail status to this address occurs, the most
significant bit will be set. For example, if the last message received from #38 is –99dBm,
then the 16 bit value of register 4438 will be decimal 99 or hex 0063. If the “comms fail”
status for #38 is set, the 16 bit value of register 4438 will become decimal 32,867 (32768 +
99) or hex 8063.
2.5.2
Repeaters
Radio paths may be extended by using intermediate modules as repeaters. A repeater will
receive and re-transmit the radio message. Up to five repeater addresses can be configured -
that is, a radio message can pass through five intermediate modules. For normal I/O
messages, any 905U module (except 905U-K modules) can be used as a repeater, however
for block read/write messages, only 905G modules can act as repeaters.
2.6
Radio Comms Failure
The 905G has an internal "communications failure" (comms fail) status for each I/O point in
its database. There is also a comms fail status for each module with direct communications -
see 2.5.1 above.
