4.6.1 Modbus/TCP Implementation

The IO-FEP2 Modbus interface maps all parameters to 16 bit Modbus registers. Read-only parameters like states of digital inputs or switch positions are reported by 'input registers', accessible through the Modbus function 04H 'Read Input Registers'. Read-Write parameters are located in 'holding registers', may be read using Modbus function 03H 'Read Holding Registers' and set by functions 06H 'Preset Single Register' or 10H 'Preset Multiple Registers'.

Single Bit Parameters

Parameters like digital input states are encoded bitwise. Bit 0 correspond to the least significant bit in a register and addresses the lowest numbered input or output

Numeric Parameters

Numeric Parameters may be integer numbers (e.g. a delay time in milliseconds), an enumerated value (e.g. a configuration state DISABLED/ENABLED) or a floating point number. The latter ones are always encodes as scaled integers (e.g. a temperature value in 1/100 centigrades).

Character Strings

The Modbus specifies no means to transport character strings. The IO-FEP2 uses multiple, consecutive 16 bit registers for this purpose:

• Each register contains two characters, the least significant byte in the register contains the the first one.
• The character string may be either exactly two times the reserved number of registers long or shorter. If it is shorter, the string must be terminated by a 0-byte. All characters beginning with this byte up to the end are ignored.

Register Addressing

Register addresses in the following chapters all are written in decimal notation, zero-based.