Marine Radio - Marine CB Radio User Manual

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

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

Return Query Data

Restart Communications Option

Return Diagnostics Register

Change ASC-II Input Delimiter (NOT SUPPORTED)

Force Listen Only Mode

5-9 NOT USED

Clear Counters & Diagnostics Register

Return Bus Message Count