PRO AIR TPK 21 - Function Code 1 - Read Coils; Function Code 17 - Report Slave ID; Function Code 3 - Read Holding Registers; Function Code 5 - Write Single Coil

13.2.1. Function code 3 - Read Holding Registers
13.2.2. Function code 17 - Report Slave ID
13.2.3. Function code 1 - Read coils
13.2.4. Function code 5 - Write single coil
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Reading from “holes” in the holding registers memory map will always return 0x0000.!
Requesting less than 1 or more than 20 values will result in Exception 3 (Illegal data value).!
Requesting data above/crossing the maximum address for holding registers (0xFFFF) will result
in Exception 2 (Illegal data address).
One 8-bit value will be returned, followed by the run status of the device (0x00 = not running;
0xFF = running).!
There are no exceptions to this function.
Coils (bool values / flags) can be read or written or both (see mapping table for details on each
coil). A coil that can be only written will return illegal data address if a read is attempted, this
will also happen if a write is attempted on coil that can only be read.
Supporting only one coil (bool value) per poll, reading from “holes” in the coils memory
mapping will always return no register error (illegal data address).
Coils (bool values / flags) can be read or written or both (see mapping table for details on each
coil). A coil that can be only written will return illegal data address if a read is attempted, this
will also happen if a write is attempted on coil that can only be read.
Attempting to write “holes” in the coils memory mapping will always return no register error
(illegal data address).