The FieldTalk Modbus Slave Protocol Library's server engine implements the most commonly used Modbus data functions as well as some control functions. The functions to perform PLC program download and other device specific functions are outside the scope of this library.

All Bit Access and 16 Bits Access Modbus Function Codes have been implemented. In addition the most frequently used Diagnostics Function Codes have been implemented.

The following table lists the functions supported by the slave:

Function Code	Current Terminology	Classic Terminology
Bit Access
1	Read Coils	Read Coil Status
2	Read Discrete Inputs	Read Input Status
5	Write Single Coil	Force Single Coil
15 (0F hex)	Write Multiple Coils	Force Multiple Coils
16 Bits Access
3	Read Multiple Registers	Read Holding Registers
4	Read Input Registers	Read Input Registers
6	Write Single Register	Preset Single Register
16 (10 Hex)	Write Multiple Registers	Preset Multiple Registers
22 (16 hex)	Mask Write Register	Mask Write 4X Register
23 (17 hex)	Read/Write Multiple Registers	Read/Write 4X Registers
Record Access
20 (14 hex)	Read File Record	Read General Reference
21 (15 hex)	Write File Record	Write General Reference
Diagnostics
7	Read Exception Status	Read Exception Status
17 (11 hex)	Report Slave ID	Report Slave ID
43 (2B hex) subcode 14 (0E hex)	Read Device Identification