A multiplexer device for multiplexing and demultiplexing signals between a low-speed network including 28 DSX-1 signals and a relatively higher speed network including a DS-3 signal. The multiplexer device includes 7 quad DSX-1 cards and 1 spare card. The spare card is connected to the other 7 DSX-1 cards in a fashion that allows the spare card to automatically be switched in for one of the DSX-1 cards or to allow interface electronics on the spare card to replace selected ones of the interface electronics on various different DSX-1 cards simultaneously. A pair of controller cards, a primary card and a secondary card, perform the M1-3 multiplexing and demultiplexing and the DS-3 framing and transceiving. The controller cards can be selected or deselected by distributed logic in a sufficiently short time period so that alarms are not set off and so that the transition is hitless. This is accomplished by electronically enabling and disabling the transceiver. The multiplexer device is connectable to 2 different T-3 links to provide network redundancy. An array of relays provides for the connection of either controller card to either of the T-3 links. The controller cards are externally connectable to a computer network such as an Ethernet for external control thereof through SNMP or Telnet sessions. The multiplexer device is housed within a significantly reduced volume which occupies only a single rack unit on a standard 19" or 23" wide vertical rack used for telecommunications equipment. The significantly reduced volume is partially due to a dual backplane architecture in which an external backplane serves as the external connector plane while a separate internal backplane serves as the connection to the various DSX-1 cards and controller cards.