Hach sc100 User Manual page 53

Address
01
A.5 Floating Point Measurements
A.5.1 Big-endian and Little-endian
A.5.2 Changing the "endian" Order
Exception Codes
An illegal request will generate a Modbus Exception Code. The most common code is
caused by reading data from an unused register. An exception response message
consists of the slave address, the function code with the high order bit set, the exception
code, and a CRC. In the following response message, the slave reported exception code
02, indicating an illegal data address.
Function
81
More Information
More information about the Modbus Protocol can be found on the Modbus-IDA website at
www.modbus-ida.org.
Floating point measurements are available from the sc100/Digital Sensor system. Each
floating point number is a 32-bit IEEE-754 value, occupying two 16-bit registers
(back-to-back); this has one sign bit, eight exponent bits and 23 mantissa bits, and is
stored in Motorola (big-endian) order.
Big-endian and little-endian are terms that describe the order in which a sequence of
bytes are stored in the computer memory. Big-endian is an order in which the "big end"
(most significant value) is stored first (lowest storage address). Little-endian is an order in
which the "little end" (least significant value in the sequence) is stored first. For example, in
a big-endian computer, the two bytes required for the hexadecimal number 4F52 would be
stored as 4F52 in storage (if 4F is stored at storage address 1000, for example, 52 will be
at address 1001). In a little-endian system, it would be stored as 524F (52 at address
1000, 4F at 1001).
The sc100/Digital Sensor registers are big-endian byte order within a register and
little-endian word order for multi-registers Floats.
The default "endian" order is little-endian word order for multi-registers Floats (SWAPPED
order). The order can be changed in the NETWORK SETUP menu to NORMAL order
(big-endian word order for multi-registers Floats).
Modbus Protocol
Data
02
Checksum
C1 91
53