User manual

105G Wirless Gateway USer Manual
Elpro_man_105g_1.18.doc Page 24
2.5 Radio System Design
Each wireless I/O system can have up to 95 unit addresses, although up to 255 505K module can
share the same unit address (refer to 505K User Manual).
Each 105U module can have up to 31 x 105S or 10 x 115S expansion I/O 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 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 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.
The RSSI registers can be read by the host device, or 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 505K modules) can be used as a repeater, however for block read/write
messages, only 105G modules can act as repeaters.