A bus on a model railway is similar to the ring main in a building: it is usually a trunk cable, capable of handling heavy loads and to which all devices are wired in paralel.
On a model railway, a Bus is usually run in parallel to the rails all the way around a layout and droppers are used to provide feeds to the track at various places:
A bus is usually run in parallel to the rails and droppers are used to connect the two together:
On a DC layout, there can be a bus for each controller. 'Ashprington Road' had three such buses when it was wired for DC.
On a DCC layout, this is not necessary and all track buses are combined into one cable. Mains cable is normally used for DCC buses so that the increased currents supported by DCC systems can be handled.
A bus need not be a track supply: it can be an ordinary power supply just run a round a layout, indeed, the 'Ashprington Road' layout has a 24VDC bus which provides power to MERG PMD and PMR modules for each turnout and to relays which provide mechanical motion for semaphore signals.
Should/can a bus be connected into a ring ?
This is optional. On 'Ashprington Road' we have done this on all buses except the XPressNet bus.
Some people say that it should not be done with a DCC track bus.
DCC command station manufacturers have told us that it is optional and there are no negative effects to doing it. Experience on 'Ashprington Road' has found that it has not caused a problem and has benefited with the entire bus being equally loaded throughout such that we don't have any areas with voltage drops.
If creating a ring causes a problem on a layout, then it is likely that the layout has other electrical problems which should be investigated.
Lenz advise that a ring MUST NOT be made of an XPressNet bus.
If a bus is not a ring, should terminators be fitted ?
A terminator (usually a capacitor/resistor combination) serves no useful purpose on a DC layout other than perhaps, suppression.
A terminator is not necessary on a DCC layout. If such devices are necessary on a DCC layout, then it is likely that the layout has other electrical problems which should be investigated prior to fitting a terminator.
Most decoders have in-built suppression which should not be supplemented by other suppression devices including capacitors fitted to locos.
Graham Plowman (Updated 26/05/2019)