WirelessHART Fieldgate SWG70 Modbus Interface
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16.3.4 Read Digital Inputs
HART Specification 285 describes the requirements for discrete and hybrid field devices. A
digital device will return binary values only. A hybrid device may return both analog and
binary values. Digital inputs are read using HART command 64.386 which returns a block of
binary 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 Binary Variable to read
2 Unsigned8 Number of Binary Variables to read (n)
Response Data Bytes
Byte Format Description
0-1 Unsigned16 Index of first Binary Variable Returned
2 Unsigned8 Number of Binary 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 = Binary Variable in Simulation or Local Override
• 0x02: 1 = Binary Variable in Fault Mode
• 0x02-0x04: Reserved, bit set to zero
11-12 Unsigned16 Second Binary Variable Value
13 Bits-8 Second Binary Variable Status
.....
3n+8-
3n+9
Unsigned16 Last Binary Variable Value
3n+10 Bits-8 Last Binary 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
7 Undefined
8 Warning Set to nearest value
9-15 Undefined
16 Error Access Restricted
17-127 Undefined
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