WirelessHART Fieldgate SWG70
Modbus Interface
E Hauser
123
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
