Communications happen on a different time scale. In other words the PLC doesn't stop the OB1 scan while it is sending ASCII. S_Send can only send one message at a time. you can't cache them up. wait for Done to happen for Qing the next message. due to baud rate and number bits per byte the time to send each message is a variable and handled by a separate IRQ.

incidentally, the HMI doesn't happen synchronously with OB1 either. so if you are writing and, the HMI is writing a tag - things can get confusing....  Last one wins...