MODBUS TCP OVERVIEW

In principle, any data structure which can be ‘cast’ to an array of 16-bit words can be transported, and will arrive unchanged on a machine with the same data representation.

984 16-bit Unsigned Integer

Natural meaning: bit 15 - 0 of integer = bit 15 - 0 of register

984 16-bit Signed Integer

Natural meaning: bit 15 - 0 of integer = bit 15 - 0 of register

984 ASCII

Although PLC’s had no text manipulation capabilities as such, the original ladder language editors allowed registers to be displayed as 2 ASCII characters each. The first character displayed was the UPPER byte (bits 15 - 8) and the second character displayed was the LOWER byte (bits 7- 0). Note in particular that this is the reverse of any use of a character array in C or other high level languages on modern PLC’s.

984 Floating point

Intel single precision real
First register contains bits 15 - 0 of 32-bit number (bits 15 - 0 of significand)
Second register contains bits 31 - 16 of 32-bit number (exponent and bits 23 - 16 of significand)

984 Single precision unsigned decimal

Although the range of values is limited at 0 - 9999, the data representation is the same as a 16-bit unsigned integer

984 Double precision unsigned decimal

This data format is now little-used, except to drive old-style 4-digit decade displays.
The range of values is 0 to 99999999. The first register contains the MOST significant 4 digits, the second register contains the LEAST significant 4 digits, each expressed as binary values in the range 0-9999.