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Multidrop Bus (MDB) is a more exotic RS232/RS485 application, used for example in vending machine controllers, which requires a 9 bit compliant UART. The 9th data bit is used for selecting between an ADDRESS and a DATA mode.

 

A way to monitor and simulate such communication links using standard 8-bit UARTs, i.e. standard RS232-to-USB converters, is to use temporary parity changes.

 

See also Wikipedia on MDB and the original MDB 3.0 specification for more information and details.