Model CXT-IR
CXT-IR Instruction Manual
Rev. 2.1
Page 31 of 48
Table 2 Exception Codes
Exception
Code
Name
Meaning
01
Illegal Function
Unsupported function code. Only 03, 06 and 16 are
supported.
02
Illegal Data Address
Invalid register address detected. If any request for registers
outside of the valid register address for the CXT occurs, this
exception code will be returned.
There are other exception codes defined in Modbus™ but these are the only ones used by the CXT.
NOTE
: When using Detcon controllers, Modbus exceptions should be turned off.
4.1.2
Modbus™ Broadcast Requests
The Modbus™ broadcast request was introduced in the CXT to support commands to be executed across all
CXT transceivers simultaneously. More specifically this is used to place all CXTs in a low power state for a
specified time to conserve battery life. Only the transceivers (radio) are placed into a low power state, the CXT
sensor will continue to operate normally. If none of the CXTs are battery powered then there is no need to issue
a command to place the transceivers in low power mode.
According to the Modbus™ specification a broadcast request is defined as a request with the Address Field set
to 0 and is used for writing only. There is no response returned as with normal Modbus™ transactions since all
devices on the system are accessed at the same time. Both Function Codes 06 and 16 (writes) are supported
with a broadcast request on the CXT.
Since there is no response on the broadcast request, there is a requirement to wait for a period of time before
sending out the next request. This allows all slave devices to process the command before the next request is
received. This is defined as the “turnaround delay”. This turnaround delay should be approximately 150
milliseconds for broadcast requests sent to the CXT. Controllers in general can meet this timing because it takes
longer than this to generate the next request so it may be unnecessary to make any adjustments to meet this
timing.
NOTE:
Controllers that generate Modbus™ broadcast requests to the CXT must ensure a
turnaround delay of 150ms is met otherwise the next request can cause the broadcast request to
be overwritten and ignored.
Broadcast requests are not confined to commands to put the network in low power mode, but can be used where
any normal write command is used.
4.2
Modbus™ Register Map and Description
When the CXT is assigned a Modbus™ address, the following registers become available to the controller for
access. All CXT sensors implement this register set. Some registers are Read Only (R) and others are Read/Write
(R/W) as shown by the R/W column. This equates to specific function codes where Read is function code 03
and Write is function code 06 or 16.
NOTE
A write to a Read Only register is allowed and returns a response, but it does not change the
value of the register. In some devices this would return an exception code.
The register map (Table 3) for the CXT sensor gives a brief description for each register or register set. This
information is only meant as a reference. The registers are divided into two sections, one for the CXT sensor
and the other for the CXT transceiver. For a more detailed description of the Register Map please contact Detcon.
