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195

Reference Manual

00809-0100-4811, Rev DA

Appendix C: Rosemount 3300 Series with HART

to Modbus

Converter

January 2015

Rosemount 3300 Series with HART

to Modbus

Converter

C.9.2 Use status information to evaluate measurement validity

The transmitter updates status information about the current measurement, and this status

information is available as a bitfield register through Modbus communication.

By monitoring the status information it is possible to determine if the current measurement

output value is valid. See “Common Modbus host configuration” on page 195 for details about

the individual status bits.

C.9.3 Use Heartbeat to detect errors

By reading and evaluating the Heartbeat value from the device, it is possible to verify that the

communication link between the transmitter, HMC, RTU and even the control system

communicating with the RTU is working.

Assign Heartbeat to one of the transmitter variables (SV, TV, or QV). Heartbeat is increased by

one for each measurement cycle in the device (until it eventually starts over at zero again).

In case this value is not updated, it means that the communication link is broken.

C.10 Common Modbus host configuration

When using Modbus RTU or Modbus ASCII, the registers to receive status and variables must be

configured in the host system.

The transmission of single-precision (4 bytes) IEEE 754 floating point numbers can be

rearranged in different byte orders specified by the Floating Point Format Code. The format

code information, stated for each Remote Terminal Unit (RTU) respectively, specifies which

registers to poll from the Rosemount 3300 Series Transmitter in order for the RTU to correctly

interpret floating point numbers. The byte transmission order for each format code is

demonstrated in Ta bl e C -11 below.

Table C-11. Byte Transmission Order is specified by the Floating Point Format Code

Note

Some Modbus hosts cannot read the information described here using Input Registers (Modbus

function code 4). The Input Register information can also be read using Holding Register

(Function code 3). In this case, Input Register number + 5000 is used as Holding Register

number.

Between host system and device, it is recommended to use 60 seconds or less between polls,

and three retries.

Format code Byte transmission order Description

0 [AB] [CD] Straight word order, most significant byte first

1 [CD] [AB] Inverse word order, most significant byte first

2 [DC] [BA] Inverse word order, least significant byte first

3 [BA] [DC] Straight word order, least significant byte first

1 2 ... 200 201 202 203 204 205 206 207 208 209 210 ... 223 224

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