Operating instructions

A Appendix: Modbus Interface WirelessHART Fieldgate SWG70

98 Endress+Hauser

A.3.4 Read Discrete Inputs

HART Specification 285 describes the requirements for discrete and hybrid field devices. A discrete

device will return discrete values only. A hybrid device may return both analog and discrete values.

Discrete inputs are read by means of HART Command 64,386, which returns a block of discrete

variables (values and status). The length of the data string depends upon the device polled. For

further details, please consult the device manual.

Request Data Bytes

Byte Format Description

0-1 Unsigned16 Index of First Discrete Variable to read

2 Unsigned8 Number of Discrete Variables to read (n)

Response Data Bytes

Byte Format Description

0-1 Unsigned16 Index of first Discrete Variable Returned

2 Unsigned8 Number of Discrete Variables returned (n)

3 Bits-8 Extended Device Status

4-7 Time Time stamp for most recent change to Actual Value of first Discrete Variable Value

8-9 Unsigned16 First Discrete Variable Value

10 Bits-8 First Discrete Variable Status

• 0x01: 1 = Discrete Variable in Simulation or Local Override

• 0x02: 1 = Discrete Variable in Fault Mode

• 0x02-0x04: Reserved, bit set to zero

11-12 Unsigned16 Second Discrete Variable Value

13 Bits-8 Second Discrete Variable Status

.....

3n+8-

3n+9

Unsigned16 Last Discrete Variable Value

3n+10 Bits-8 Last Discrete Variable Status

Command-Specific Response Codes

Code Class Description

0 Success No Command-Specific Errors

1 Undefined

2 Error Invalid Selection

3-4 Undefined

5 Error Too Few data Bytes Received

6 Error Device-Specific Command Error

7 Undefined

8 Warning Set to nearest value

9-15 Undefined

16 Error Access Restricted

17-127 Undefined