The problem of upper program reading and writing plc data through modbus tcp

Question 1: Either is possible.
Problem 2: Booleans are usually read and written through the read/write coil, but they are also obtained by reading and writing input or holding registers and then retrieving them bit by bit.
Problem 3: Different series of PLCS differ greatly, the same series of PLC address definition is not different.
The above is my personal experience of the real situation, I hope you will adopt.

1. The upper computer program can read and write the input register and the hold register of PLC. The specific use of which register depends on the programming and application requirements of PLC. The input register is usually used to store the state of the PLC input signal, while the hold register is used to store the PLC internal counter, timer and other data. Different PLC brands and models may have different input register and hold register addresses, which need to be configured according to the PLC specification document or programming manual.
2. Booleans are usually read/write operations through read/write coils, not read/write input or hold registers. Coil is usually used to store the state of PLC output signal, its address can be a single bit, can also be more than one bit, and specific PLC brand and model.
3. Different brands and models of PLC register address definition does have differences. For example, Siemens S7 series PLC uses register addresses that are usually hexadecimal with DB, MB, EB, FB prefixed, etc., while Mitsubishi PLC uses word and bit addresses. Therefore, when performing PLC read and write operations, it is necessary to determine the correct register address according to the specific PLC brand and model specification documents or programming manuals.