Radio Signal Strength - Weidmuller WI-GTWY-9 User Manual

WI-GTWY-9-xxx Wireless Gateway
to the Ethernet Interface, and handles all radio communications. I/O in the Radio Interface is
linked to I/O in the Ethernet Interface in a flexible way via WI Series Configuration Software.
The Ethernet Interface provides a total of 2048 input bytes and 2048 output bytes. An Ethernet
byte can contain 8 discrete (binary) values, or two bytes can be used for a 16-bit analog or pulse
register. So the Ethernet Interface is limited to 4300 discrete inputs (the limit of the WI-GTWY-
9-xxx database) or 1024 analog inputs (the limit of the Ethernet interface) or a combination. The
same applies for outputs.
For example, an Ethernet host wants to read 500 analog inputs (1000 bytes). The remaining
input bytes (1548) could be used for 12,384 discrete inputs - but the WI-GTWY-9-xxx database
is not this big. Provided there are no outputs required, there could be 3800 discrete inputs (4300
– 500 analogs). If there are outputs required, then the number of discrete inputs available will be
further limited.
2.5 Radio System Design
Each wireless I/O system can have up to 95 unit addresses, although up to 255 WI-I/O 9-K
module can share the same unit address (refer to WI-I/O 9-K User Manual).
Each WI-I/O 9-x module can have up to 31 x WI-I/O-EX-1-S-1x modules connected to it. These
modules are addressed 96 - 127. More than one WI-I/O-EX-1-S-1x module can have the same
address, provided they are not connected to the same WI-I/O 9-x 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 WI-GTWY-9-xxx can be used as a repeater module for messages between other modules.

2.5.1 Radio Signal Strength

The WI-GTWY-9-xxx 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 WI-GTWY-9-xxx will not know what the radio strength of the
message from #24 to #25 is. If #25 is another WI-GTWY-9-xxx, then it can record this RSSI
and this register could be mapped to an I/O register in the first WI-GTWY-9-xxx.
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