Async/Sync – When I connect two PCs together using CML modems and transmit data from the serial port, I get a lot of errors. Why?
A. The PCs typically use wired handshaking or Xon / Xoff for flow control. Flow control is not used by synchronous modems and is therefore ignored. The result is that data is passed to the synchronous modem at a rate governed by the PC and will not be synchronised at the modem. This will result in bits being lost if the data rate is too high or bits being inserted if the data rate is too low. A similar problem is seen at the receiving end. The extra, or missing, bits cause data and framing errors on the receiving PC. The solution is to buffer the data between the PC and the modem. The buffer will receive data from the PC and provide the handshaking required. The data will then be passed at the correct rate for the modem. Mismatches in the data rates are absorbed by ‘idling’ the synchronous connection or ‘stalling’ the transmission from the PC. A similar scheme is adopted at the receiving end. An Application Note “Using an FX469 FFSK Synchronous Modem with an Asynchronous Data I/O”, is
