Modbus Practical Help

The FieldServer Modbus RTU drivers offer several function moves that handle 32-bit integers and 32-bit float values. More importantly, these function moves consider all different forms of byte sequencing.

The following table shows the FieldServer function moves that copy two adjacent 16-bit registers to a 32-bit integer value.

Function Keyword	Swap Mode	Source Bytes	Target Bytes
2.i16-1.i32	N/A	[ a b ] [ c d ]	[ a b c d ]
2.i16-1.i32-s	byte and word swap	[ a b ] [ c d ]	[ d c b a ]
2.i16-1.i32-sb	byte swap	[ a b ] [ c d ]	[ b a d c ]
2.i16-1.i32-sw	word swap	[ a b ] [ c d ]	[ c d a b ]

The following table shows the FieldServer function moves that copy two adjacent 16-bit registers to a 32-bit floating point value:

Function Keyword	Swap Mode	Source Bytes	Target Bytes
2.i16-1.ifloat	N/A	[ a b ] [ c d ]	[ a b c d ]
2.i16-1.ifloat-s	byte and word swap	[ a b ] [ c d ]	[ d c b a ]
2.i16-1.ifloat-sb	byte swap	[ a b ] [ c d ]	[ b a d c ]
2.i16-1.ifloat-sw	word swap	[ a b ] [ c d ]	[ c d a b ]

The following table shows the FieldServer function moves that copy a single 32-bit floating point value to two adjacent 16-bit registers:

Function Keyword	Swap Mode	Source Bytes	Target Bytes
1.float-2.i16	N/A	[ a b c d ]	[ a b ][ c d ]
1.float-2.i16-s	byte and word	[ a b c d ]	[ d c ][ b a ]
1.float-2.i16-sb	byte swap	[ a b c d ]	[ b a ][ d c ]
1.float-2.i16-sw	word swap	[ a b c d ]	[ c d ][ a b ]

Given the vairous FieldServer function moves, the correct handling of 32-bit data is dependent on choosing the proper one. Observe the following behavior of these FieldServer function moves on the known single-precision decimal float value of 123456.00:

16-bit Values	Function Move	Result	Function Move	Result
0×2000 0x47F1	2.i16-1.float	123456.00	1.float-2.i16	0×2000 0x47F1
0xF147 0×0020	2.i16-1.float-s	123456.00	1.float-2.i16-s	0xF147 0X0020
0×0020 0xF147	2.i16-1.float-sb	123456.00	1.float-2.i16-sb	0×0020 0xF147
0x47F1 0×2000	2.i16-1.float-sw	123456.00	1.float-2.i16-sw	0x47F1 0×2000

Notice that different byte and word orderings require the use of the appropriate FieldServer function move. Once the proper function move is selected, the data can be converted in both directions.

Of the many hex-to-floating point converters and calculators that are available in the Internet, very few actually allow manipulation of the byte and word orders. One such utility is located at www.61131.com/download.htm where both Linux and Windows versions of the utilities can be downloaded.

Once installed, the utility is run as an executable with a single dialog interface. The utility presents the decimal float value of 123456.00 as follows:

One can then swap bytes and/or words to analyze what potential endianness issues may exist between Modbus RTU master and slave devices.