S7-200 modem connection

[bjump 0]

Hey all, I am trying to interface an old modem/radio with the S7-200. Trouble is, the modem uses the RTS signal to key the transmitter. Well the S7-200 RTS signal does not hold constant during the entire data transmission. I believe that it just pulses before the data from the S7-200 is transmitted. I've also tried to use SM4.5 to trigger one of the dry contacts which is hooked to the radio's transmitter. The problem is, the SM4.5 goes low as soon as data starts transmitting. By the time the contact closes and the radio transmitter keys, most of the data has already been sent out by the PLC. Can anyone think of anyway I can key the transmitter some time before the port's transmission begins? I am communicating modbus and using the MBUS_INIT and MBUS_SLAVE function blocks.