105U-G Wireless Gateway
User Manual
Page
19
©
August 2003
2.5
Radio System Design
Each wireless I/O system can have up to 95 unit addresses, although up to 255 505U module
can share the same unit address (refer to 505U User Manual).
Each 105U 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 105U 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 40 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 105G can be used as a repeater module for messages between other modules.
2.5.1
Radio Signal Strength
The 105G 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 105G will not know what the radio
strength of the message from #24 to #25 is. If #25 is another 105G, then it can record this
RSSI and this register could be mapped to an I/O register in the first 105G.
These RSSI registers can be read by the host device as well as being mapped to I/O registers
in other 105G 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 105U module (except 505U modules) can be used as a repeater, however for
block read/write messages, only 105G modules can act as repeaters.
