Series 2000 Communications Handbook - Soliton

Modbus and EIBisynch Advanced TopicsSeries 2000 Communications Handbook7.4. FLOATING POINT PARAMETERSThese use the IEEE format for floating point numbers, which is a 32 bit quantity. This isstored in consecutive Modbus addresses. When reading and writing to floats, it is necessaryto read or write both words in a single block read or write. It is not possible, for example, tocombine the results of two single word reads.This format is used by most high level programming languages such as ‘C’ and BASIC, andmany SCADA and instrumentation systems allow numbers stored in this format to bedecoded automatically. The format is as follows:BIT 31 30 23 22 0Sign 2 7 2 0 2 -1 2 -2 2 -23{--- -EXPONENT--- -}{------ -- - -- ------FRACTION---- ---- -- -----}where value = (-1) Sign x 1.F x 2 E-127Note that in practice, when using C, IEEE floats may usually be decoded by placing thevalues returned over comms into memory and ‘casting’ the region as a float, although somecompilers may require that the area be byte swapped high to low before casting. Details ofthis operation are beyond the scope of this manual.The format used to transfer the IEEE number is as followsLower Modbus AddressHigher Modbus AddressMSB LSB MSB LSBBits 31 - 24 Bits 16 - 23 Bits 15 - 8 Bits 7 - 0For example, to transfer the value 1.001, the following values are transmitted (hexadecimal).Lower Modbus AddressHigher Modbus AddressMSB LSB MSB LSB3F 80 20 C57-4 Series 2000 Communications Handbook Part No HA026230 Issue 2.0 Feb-00