Modbus - loss of floating point precision

[EricLarsen]

I'm trying to read some data from a Modbus instrument using the I/O server in DSC over TCP. The instrument writes a 32 bit floating point value into 2 16 bit registers. If I read the two 16 registers separately, then combine and typecast them I get the correct value. However, if I rely on the I/O server to read the floating point value directly, it gets a slightly incorrect result.

For example, my instrument wrote the value 34.80 into the two registers 30123 and 30124. If I read each register (300123,300124) separately and combine the values, I get the correct 34.80 result . However, if I read the register F300124 I get the result of 34.85. This is consistent; I always get a value that is about 0.05 to 0.10 off. At first I thought that was the incorrect floating point address, so I tried reading F300123, but that returns a result on the order of 6.7e-6.

I'm never tried to use DSC to interface to modbus, so I'm probably missing something obvious, but I'm stumped. Any ideas?