http://www.modbus.org/specs.php

01 (0x01) Read Coils

Request:  01 bbbb NNNN
Response: 01 nn <data>
Error:    81 ee

02 (0x02) Read Discrete Inputs

Request:  02 bbbb NNNN
Response: 02 nn <data>
Error:    82 ee

03 (0x03) Read Holding Registers

Request:  03 bbbb NNNN
Response: 03 nn <data>
Error:    83 ee

04 (0x04) Read Input Registers

Request:  04 bbbb NNNN
Response: 04 nn <data>
Error:    84 ee

05 (0x05) Write Single Coil

Request:  05 aaaa vvvv (0000 or FF00)
Response: 05 aaaa vvvv (0000 or FFFF)
Error:    85 ee

06 (0x06) Write Single Register

Request:  06 aaaa vvvv 
Response: 06 aaaa vvvv 
Error:    86 ee

07 (0x07) Read Exception Status (Serial Line only)

Request:  07  
Response: 07 vv
Error:    87 ee

08 (0x08) Diagnostics (Serial Line only)

Request:  08 .......
Response: 08 .......
Error:    87 ee

15 (0x0F) Write Multiple Coils

Request:  0F bbbb NNNN nn <data>
Response: 0F bbbb NNNN
Error:    8F ee

16 (0x10) Write Multiple registers

Request:  0F bbbb NNNN nn <data>
Response: 0F bbbb NNNN
Error:    8F ee

1. coils: r-1, w-15/5
2. discrete inputs: r-2
3. holding registers: r-3, w-16/6
4. input registers: r-4

1. max quantity of HOLDING registers in a reading request
2. max quantity of INPUT registers in a reading request
3. don't use WRITE_SINGLE_COIL command 0x05
4. don't use WRITE_SINGLE_REGISTER command 0x06
5. max size of gap in a reading request

1. region:
   • COILS
   • DISCRETE_INPUTS
   • INPUT_REGISTERS
   • HOLDING_REGISTERS
2. address
3. data type
4. access:
   • read
   • write
   • rw
5. byte rearrangement
6. forced write with command 6

tag.reg1:
  name:     current.status
  type:     bool
            int     1 / 2 / 4
            [uint]    1 / 2 / 4
            float
            double
            string  1..~240
  size:     1 / [2] /  ... n
  addr:     2001
  enable:   on
  access:   r / w / [rw]
  read:     1 / 2 / 3 / [4] 
  write:    5 / [6]
  writegr:  F / [10]
  
tag.reg1_b1b0:
  name:     current.error
  type:     bits
  bit:      0 .. 15
  size:     [1] .. 16