Abstract: A synchronous communications multiplexer for connecting a plurality of synchronous communications lines to an MP (multi-purpose) bus. In its basic form, the multiplexer is capable of handling bit oriented protocols (BOP) and is capable of being upgraded to support byte-oriented protocols. The present invention supports full- or half-duplex data rates of up to 19,200 baud. Total throughput, when operating with four ports (as an example), is in the range of 76.8 kbits/sec. A special mode of operation (unisync) allows operating a single port at 56 kbits/sec., full duplex. Other capabilities include software-selection of parameters (such as baud rate) and operation in a "transmitter queueing" mode of operation.