Hi,
you needn't 2 connections for reading and writing. You need only 1 connection. With the runtime parameter WRITE_READ you can control, whether the current order should read or write. With WRITE_READ = FALSE and ENQ_ENR = TRUE you will read registers and with WRITE_READ = TRUE and ENQ_ENR = TRUE you will write registers.
The initialization parameters you have to set in OB100. This is the register parameterization, with modbus registers are "mapped" in the DB.
E.g.:
DB_1 = 1
START_1 = 0
END_1 = 300
=> The DBW0 in DB1 is the modbus register 0, DBW2 is modbus register 1, DBW4 is modbus register 3 etc.

With the runtime parameters START_ADDRESS and LENGTH you set the current modbus registers, which should be read or write.
E.g.:
START_ADDRESS = 5
LENGTH = 10
WRITE_READ = FALSE
=> You read 10 registers from the communication partner and the first registerist the 5th holding register (Holding Registers 5 - 14).

You are right, you can start the next order not untilif the previous order was finished with DONE_NDR = TRUE or ERROR = TRUE. Normally 1 telegram needs 3 cpu cycles.

If the order was finished, then you can change START_ADDRESS, LENGTH and WRITE_READ to new values and then start the order with ENQ_ENR = TRUE.

Wüstenräuber