Cab Bus

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The Cab Bus is what NCE calls their throttle network.

Cab Bus

The Cab Bus ties together various components of a NCE Digital Command Control system.

Systems Employing the Cab Bus


The controller bus connects up to 63 Hand-Held Cab Controllers to the Command Station. Data on the bus runs at 9600 bps with eight data bits, no parity, two stop bits and meets RS-485 specifications.

Controllers are continually in receive unless specifically addressed by the command station at which time the command station relinquishes the bus to allow the controller to respond. The Cab Bus system is a polled system. The cabs are polled, and when addressed respond by transmitting any change in status.

Cabs are not polled in numerical order. If a cab does not respond to a ping after several attempts, the command station will stop pinging that address. It will ping that address on a random basis, to speed up operations when more than 30 cabs are on the bus.

The maximum length of the bus is 500 feet and should be a continuous daisy chain without "branches" of any significant length (longer than 8 feet).


PIN 1 +12 Volts (nominal) PIN 2 Ground PIN 3 "A" lead of RS-485 signal PIN 4 "B" lead of RS-485 signal

Three Pin Plug

Tip +12 Volts (7.5 to 16 Volts DC is OK) Ring RS-485 "A" lead Sleeve Ground

Additional Info

More info is available from the MERG website: Protocol.Zip file.

The Cab Bus is a polled network, which is different from a CSMA/CD network such as Digitrax's LocoNet. As the network expands, throughput decreases as more devices have to be polled. With a polled network, the device must wait silently until addressed, whether or not it has a message to send. A CSMA/CD network does the opposite, a device sends a message without being asked for a message, and will keep sending the message until it determines that it was successful.