Detcon CXT-IR User Manual download pdf (Page 32)

Model CXT-IR

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. 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 only for writing. 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, also known 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.

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 Directions

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

CXT-IR Instruction Manual Rev. 1.6 Page 28 of 44

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Detcon CXT-IR User Manual Page 32

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