Async/Sync – How do I connect RS232 to a CML modem?
A. The previous response gives information on the synchronous/asynchronous conversion required. The other consideration is level translation. RS232 typically uses +/- 12V for its signalling levels, these must be translated to 5V and 0V for CML modems. Further, the idle state on RS232 is a continuous mark (or binary 1) represented by -12V. The corresponding logic level for a mark is 5V (high), RS232 therefore, requires inverting. The most common device used for this translation is the Maxim MAX232. There are many other devices in the family but this device provides all the level translation and inversion for the data line and a corresponding hardwired handshake. It also includes a 5V to +/- 12V switched capacitor PSU. Further information may be found in the Application Note “Using an FX469 FFSK Synchronous Modem with an Asynchronous Data I/O”. Note: The Micronas device mentioned in the Application Note “Using an FX469 FFSK Synchronous Modem with an Asynchronous Data I/O” is no longer av
