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device ID. The Base Radio uses register addresses 1 through 10 with each subsequent
group of 10 registers used by a Transmitter.
The RF online/offline status of a Transmitter can be determined through a number of
different sources. In the Base Radio holding registers, a count of the current number of
online Transmitters can be found at address 00004. Also the individual online/offline
status of each Transmitter is stored in the Base Radio holding registers at addresses
00005, 00006, 00007, and 00008. These registers are bit field variables with each bit of
the 16-bit number representing the status of a Transmitter. If the bit is set, the
corresponding Transmitter is online. If the bit is cleared, the corresponding Transmitter is
offline.
In the Transmitter holding registers, bit 1 in the Transmitter & Sensor Status holding
register holds the online/offline status of the particular device. Any Transmitter that is
currently offline will return NaN (Not a Number) when queried for its primary or secondary
measurement value. Note that if a Transmitter is online and is a single measurement
device type, NaN will also be returned for the Secondary Sensor Value, as the
measurement does not exist in these devices.
6.3 Modbus Commands
The Base Radio responds to six Modbus commands including the Read Holding
Registers (03), Diagnostic (08), Get Com Event Count (11), Get Com Event Log (12),
Report Slave ID (17), and Read Device Identification (43). The Read Holding Registers
command provides the bulk of the functionality with the remaining commands being used
for diagnostics/troubleshooting purposes. No other Modbus commands are supported
and will be responded to with an ILLEGAL FUNCTION exception (code 01). A description
of each command code is listed in the following sections.
6.3.1 Command 03-Read Holding Registers
This command is used to read the contents of a contiguous block of holding registers in a
Base Radio or Transmitter. If an invalid register address is requested, an ILLEGAL DATA
ADDRESS exception (code 02) will be returned.
Section 6.4 lists the holding registers for the Base Radio and Transmitters, as well as
how to understand the contents of the registers.
6.3.2 Command 08- Diagnostic
This command provides a number of tests for checking the communications between the
Base Radio and the Host master device.
Subfunction Code Transmitter Device Status
0
Return Query Data
1
Restart Communications Option
2
Return Diagnostics Register
3
Change ASC-II Input Delimiter (NOT SUPPORTED)
4
Force Listen Only Mode
5-9 NOT USED
10
Clear Counters & Diagnostics Register
11
Return Bus Message Count
27