Communication with CP340 between two S7-300

I work on siemens PLC(S7-300),and I want to connect two of them with CP340(RS232C).I use the transmission and receive function block which are provided by siemens,but I have a problem.When I trigger the transmission,the error and status word isnot stable to check and all the time they are zero (in monitor mode)and when I assign an output to error flag it's blink.In addition the SF led on CP340 is off all the time.I want to know what's my problem and how can I check the status word of CP340?

CP-340

in the line following the P_SEND move the status word to a safe place on the condistion of an ERROR or a DONE.

O ERROR
O DONE
JZ END
L STATUS
T SAFE
END: NOP