ModBus: Supported functions

The following table lists the ModBus functions supported in SPLat, with a brief description of the function. This is not meant as a detailed treatment of ModBus; you should refer to the published specifications online at places like modbus.org. The hotlinks in the M/S column take you to the corresponding ModBus master script command (described later).

Function	Name	M/S	Modicon ref	Description
1	Read Coil(s)	M S	0xxxx	Reads out one or more contiguous coils (external outputs or internal 1-bit variables, i.e. semaphores)
2	Read inputs	M S	1xxxx	Reads one or more contiguous inputs. In a slave SPLat accesses the debounced inputs.
3	Read holding registers	M S	4xxxx	Reads one or more contiguous Modicon type 16-bit holding registers. In a SPLat slave the addresses are mapped to RAM (data memory).
4	Read input registers	M	3xxxx	Reads one or more contiguous Modicon type 16-bit input registers. Some 3rd party products map internal values to these registers. Not supported in SPLat slave.
5	Force single coil	M S	0xxxx	Forces a single coil (Modicon: external output or internal 1-bit variable, i.e. semaphore). Force external outputs with caution! SPLat slave allows selective write protection.
6	Preset single holding register.	M S	4xxxx	Forces a value into a single Modicon type 16-bit holding register. In a SPLat slave the addresses are mapped to RAM (data memory).
15	Force multiple coils	M S	0xxxx	Forces one or more contiguous coils (external outputs or internal 1-bit variables, i.e. semaphores). Force external outputs with caution!
16	Preset multiple holding registers.	M S	4xxxx	Forces data values into one or more Modicon type 16-bit contiguous holding registers. In a SPLat slave the addresses are mapped to RAM (data memory).