Cab Bus

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The Cab Bus is what NCE calls their throttle network protocol.

There are two physical forms of the Cab Bus wiring. The command bus or throttle network uses a four wire cable. Cab bus connections between the throttle and the command station or throttle network use a 6P4C or 6P6C RJ connector, which can use four or six conductors in the cable. The 6P6C is specifically for the Power Cab.

Cab Bus

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

Systems Employing the Cab Bus

Overview

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).

NCE suggests no longer than 1000' is acceptable, with branches not longer than 20'.

Controller Bus Overview

The controller bus connects up to 63 Cabs to the command station.

Data on the bus is 8N2 (eight bits, no parity, 2 stop bits) following the RS-485 specification.

The cabs are in listen mode until they a specifically addressed by the command station (polled), at which time control of the bus is passed to the cab. All data sent from the cab is required to have bit 7 set to 1. A "ping" has bit 6 equalling 0, all other commands or data will have bit 6 set to 1

Operation

The cabs are continuously polled (pinged) for any change in status. When polled, the cab will reply with either a 2 byte response indicating a button press (first byte) then the second byte with speed information, or with a five byte message indicating the device address (2 bytes), then the operation (2 bytes) followed by a one byte XOR checksum.

The cab will begin transmitting approximately 780 μS after receiving the last stop bit from the command station.

Cab Addressing

Cabs 0 and q are reserved" Cab 0 is the broadcast address of the command station. Cab 1 is reserved for future applications.

Cabs are not polled sequentially, for faster response active cabs are polled more often than inactive cabs. Inactive addresses will be polled periodically for the purpose of discovering new cabs on the bus.

Normal Operation

  • Command station pings an address
  • Cab answers
    • If the cab has nothing to send, it does not have to respond.
  • If command station has data for the cab, it is sent after the cab finishes transmitting
  • Repeat process by pinging the next address

If a cab does not respond for several pings, it may be dropped from the queue for inactivity. It will then be pinged periodically until it responds to a ping.

Response Time

Under normal circumstances, with 10 cabs in operation, a cab will be pinged about 23 times a second.

To poll all 63 addresses, the frequency per cab will be 3 to 4 pings/second, assuming 63 cabs in use and all are responding with a simple 2 byte response. Most NCE users will never see this level of performance.

Wiring

DIN Plug

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

This is optional, and can be used when wired for compatilbiity with the Lenz XpressNet

Three Pin Phone Plug

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

RJ-11

  1. Reserved
  2. +12 VDC
  3. A lead of RS-485 signal
  4. B lead of RS-485 signal
  5. Ground
  6. Reserved

Only the 4 inner wires are used. This is the preferred interface.

NCE Cab Bus Cables

There are three types of cables:

  • Control Bus Cable with RJH 4P4C. Cable connects boosters to command station, max 300'
  • Cab Bus Cable: RJ12 6P4C cable used for tethered cabs. Max. 40'
  • Cab Bus Cable with RJ12 6P6C, used for the Power Cab.

NCE Power Cab: Cab Bus Cable

The Power Cab comes with a custom RJ12 6P6C cable. Do not attempt to make your own. Replacements are available from NCE. Third party cables will not have the heavy gauge wiring needed to carry the track current.

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.

NCE DCC Cables Explained for more information on Cab Bus cables, and how to order a new one for your Power Cab.

NCE Helpdesk

There are a number of useful articles on the Cab Bus on the NCE website: