DCCWiki, a community DCC encyclopedia.
Jump to: navigation, search

Summary: A Digitrax term refering to address management for locomotives under control by a Digitrax DCC system.

What are Slots?

Slots are a concept used in Digitrax command stations. The number of slots available determine how many locomotives you can operate.

The concept is similar to that of a "heap" used in Computer Science: It is a portion of memory set aside by the program you are running to store variables and frequently used data. In this case, locomotive addresses and consist information is stored by the command station's software for use later. Different Digitrax command stations have different amounts of memory available for slots, in the case of the Chief line, the number of slots available can be defined by changing a setting in the command station. The Advanced command stations feature Expanded Slots.

The Zephyr has ten and Zephyr Xtra features 20 slots. The Empire Builder has 22. The Chief and Super Chief have 120 slots. See the Digitrax Command Station Comparison for more info.

Slot Status

A slot can have one of four states:

  1. In Use: The slot has an assigned throttle, and the command station is sending commands to the track
  2. Common: Commands are being sent; No throttle assigned
  3. Idle: The command station is ignoring this slot; no commands are being sent related to this slot.
  4. Free: Slot is empty or has not been assigned or used. A computer running software can also clear slots.

Any slot while in use which does not receive updates from its associated throttle after a period of time may be marked as Common or Idle. If a slot's status is Common and the speed is set to Zero, the slot may be purged by the command station at a later point in time.

  • If the command station does not receive LocoNet messages in a timely manner due to corrupt messages or other activities demanding time, slots may be set to Common.

What Does it Mean?


A slot labelled Common is being refreshed by the command station, has no throttle (or other LocoNet device) assigned to it, and is unavailable for use with another multifunction decoder address. An excessive number of common status slots is often a result of operator discipline, or lack thereof. With proper discipline the number of slots with common status can be managed.

Releasing an address on a throttle sets the status of that slot to Common. If a LocoNet device takes control of that slot, its status will change to In Use.

If an operator inadvertently selects an incorrect address on a throttle, that address should be dispatched before a new address selection is attempted. If a new address is entered without dispatching the prior one, the first address will have its slot changed to common. By using Dispatch, the slot is made idle and available for use


This slot is available for re-assignment to a new address. By using Dispatch the status of the slot is changed to Idle, instead of common.

UniVersal Consists

UniVersal Consisting is the default Digitrax consisting method. Addresses in a consist remain in common status until the address(es) are removed from the consist. This ensures addresses are not removed inadvertently from a consist by reuse of their slot.


Any address which is permitted to Purge will have its slot status changed to common.

Clearing Slots

It is possible to manually clear slots using the throttle, the typical procedure with an advanced throttle is to select the locomotive address and press EXIT. This is not the same as Dispatching.

Slots = MAX Warning

This warning indicates the maximum number of slots available (or memory set aside for this) has been reached. Until they are cleared out, this message will persist. This can be done using an option switch (see your manual) or JMRI's LocoNet tools.

The main cause is locomotives which were not correctly dispatched by their operator after an operating session.

Additional Information about the Slots = Max Warning

On a DT400 or DT300, the display will show Slot=Max. Older DT100 or DT200 throttles will display FF, and the Zephyr shows FULL. All these indications mean that the number of slots available is zero, as all are in use.

Zephyr Express

The DCS52 Zephyr Express' LCD will display SLOTMAX.

Clearing the Slots

Closing Option Switch 36 clears all slots in the command station (including “UniVersal” consists) currently in the system, leaving all other option switches as set. Once accomplished, the throttle will be able to select addresses again.

The exact procedure varies depending on the command station and throttle you are using; check the section for “Option Switch Setting” in your command station and throttle manuals.

OpSw38 (Obsolete)

OpSw38 clears the Alias Address Roster, a legacy feature allowing the use extended addresses with decoders supporting only Primary addresses. The only DT100 throttles can use this feature, which is rarely used today.

Managing the Slots

Releasing Individual Locomotives

A common reason for using all the slots in a command station is that locomotives are not properly dispatched when their operators are finished with them. To free a slot used by an individual locomotive, first make sure that the locomotive is removed from any consist that it may be part of. Then select the locomotive on a throttle, set the speed to zero, and dispatch the locomotive (see the throttle manual for instructions on how to dispatch locomotive addresses). This will set the slot to "free" and it can then be assigned to another locomotive when needed.

Continue this procedure for all locomotives that are occupying slots, but which not in use. (it may be difficult to determine which locomotive addresses are occupying slots in the command station)

Setting up the command station and Operator Discipline

  1. Purging makes a slot available for use. Purging requires the throttle be set to “0” (zero) speed and not be part of a UniVersal consist. Setting OpSw 14 to thrown and OpSw 15 to closed ensure all non-consisted individual addresses are purged.
  2. DCS 100/200 only: These command stations have 120 slots, however the factory default is 22 slots. Close OpSw 44 to make 120 slots available.
  3. Releasing or Dispatching: Ensuring each locomotive's throttle is at “0” (zero) speed and is either released or dispatched.
  4. Consists: “UniVersal” consisted locomotives (Digitrax's default method) will not be purged. Locomotives must be unlinked from their consist to allow these slots to be purged. This is an issue of Operator discipline. If you want permanent consists, consider basic or advanced consisting.
  5. Periodic OpSw 36 clear: Clearing the slots (OpSw 36) helps, especially if UniVersal consists are filling slots.
  6. If you reset OPSW 36 on your command station, all mobile decoder addresses and MU information will be cleared out of your Command Station. The manual that came with your command station will explain how to use the OPSW's (Op Switches). Note that this procedure will erase all consist information that is stored in the command station. (Universal Consists).

Advanced Slot Management

  1. Command Station Reset: Set to factory default by closing OpSw 39. CAUTION: This will clear the “Route Table’' (DCS 100/200) if in use. It will also clear the Alias Locomotive Roster (DCS 100/200). Alias Addressing requires a DT-100 throttle and is rarely used today.
  2. DCS100/DT200 Losing OpSw Settings: These command stations use a CR2032 coin cell to maintain their Random Access Memory which stores these and other parameters. If the cell is dead, all settings will be lost when the command station is shut down. Instructions for changing the cell are in the command station manual. The CR2032 cell and equivalents are commonly available. The command station requires one cell, there may be two cell holders, the empty one allows you to install a new cell before removing the old one, thus maintaining your settings. Do not install two cells as it is unnecessary.
Many erroneously refer to the memory in the command station as CMOS, and say they replaced the CMOS battery.

Expanded Slots

The DCS240 command station offers an additional range of slots, called Expanded Slots. These slots are only accessible to the DT402R2 throttle. Legacy throttles can only access up to 120 slots. The DCS240 handles this automatically by identifying the throttle type. The DCS240 offers up to 400 slots in this manner, instead of the typical 120 slots offered by other command stations.

The expanded slots are useful if a lot of consists are in use. When building a consist using the expanded DT402 throttle the consist will be stored in the expanded slots.

When controlling a consist, the top loco address controls the consist. If the consist is selected by a legacy throttle, the DCS240 will move the top loco address to the standard slot range, making a link to the other loco addresses stored in the expanded slot range. This will leave as many of the standard slots available as possible.

Computer Interface and Software


If you have a computer interface for your DCC system there are software products that allow you to see a list of all the slots on your command station, and indicate the status of each slot. Some also give you the capability to free slots with the click of a mouse button.

The free JMRI software (JMRI website) contains the Slot Monitor which provides this functionality.

Purging Made Easy

From an email written by Don Crano.

Changes in Slot Status

If the slot status changes, or is purged by the system to be used by another decoder address, it does not mean all the data is cleared from the slot, only the status flag has changed, the data stays there to either be re-used by another LocoNet device (COMMON), or purged so new data can overwrite the existing data (IDLE).

Purge Timer

Each slot has a purge timer, but that timer is not always active, this will only happen when certain criteria is met.

How it all works:

When a LocoNet device (Throttle) is maintaining a SLOT in the refresh stack it will be required to check that the SLOT status matches its internal state before reusing any SLOT. If a device disconnects from the LocoNet and does not access a slot within the system's PURGE time, the Master (Command Station) will force the SLOT to "COMMON" status so other system devices can use that SLOT.
Typical purge time of a Master is from 200 to 600 seconds. Slot update activity is about every 100 seconds, i.e. if a user makes no change to a throttle/slot within 100 seconds, the throttle/device should automatically send a speed update at the current speed setting to reset the Purge timeout for that Slot.
Note that with a LocoNet wireless throttle, (IR/Radio) you can see the ping or update being sent while the throttle is in Power Save mode, it will wake up, send, then go back to PS approximately every 60 seconds. This prevents the system from releasing that address and making it available to another throttle.
Before an address will be purged, the following criteria must be meet.
  1. The decoder's speed data must be at '00' or the Master (Command Station) must be set to force '00' speed upon purge.
  2. Any slot with the UP-CONSIST flag set will never be purged. This indicates the slot/decoder address is part of a consist. Prevents a locomotive from being purged while in operation as part of a consist.

Operator Discipline

Before removing a throttle from the layout:

  1. Un-Link consists, that clears the UP=CONSIST flag.
  2. Make sure all addresses associated with the throttle are at speed '00' (stopped with the throttle closed) and/or have the Master set to reset speed to zero when the slot is purged.
  3. Release or Dispatch any addresses associated with the throttle.

The above steps go a long way to keeping the FF or Slot=Max status from showing up.

The term UniVersal is a Digitrax registered trade mark. It is used to name or describe their command station assisted consisting. Digitrax actually uses what is called Unified Consisting, the system allows three modes of consisting, Advanced, Basic, and UniVersal, and may be mixed, matched, and even nested inside other consists. It is Unified because it can be done automatically without user intervention as they set up the consist.

Option Switch Notes

Consult the manuals for your system to determine the correct option switches for your command station.

OPSW 13 changes purge time from 600 to 200 seconds. Good for faster, more rapid purges, bad for tethered throttles, as when an operator unplugs a non-wireless throttle from the LocoNet they now have 200 seconds to plug in elsewhere. OPSW 14 can completely stop purging from happening.

OPSW 15 controls the speed value, can be set to force speed value to 00 upon purge. This prevents runaways from happening should an address be purged while a train is in motion.

Slot Following

Slot Following refers to operation of a stolen locomotive. When this happens the throttle will click every time the remote throttle initiates a speed change. The display will also change to indicate what is happening.

This feature allows two DT throttles to control a single address. Both throttles will show the speed and direction on their displays. This can be useful for training operators, allowing the mentor to take control without having to take the throttle physically from the operator.

Another application is computer control: A computer running CTC and routing control programs on LocoNet can provide some automatic control over the train. It can change speed or stop the engine, while letting the engineer holding the throttle know what is happening.

Operating a Stolen Address on a DT40X Throttle

When the command station detects that a loco address that is in-use on one of its throttles is being changed by another throttle or computer, it will cause the throttle to “click” every time it sees a remote throttle change its locomotive settings. If that locomotive is in the active throttle, its speed display will also show the changes. This is called slot following.

This allows two DT throttles to run a single locomotive address with both throttles able to send commands to the loco. Both throttles will show the current speed and direction of the locomotive in their displays.

Slot following is useful for training new operators. The supervisor can “steal” a locomotive that is selected on a trainee’s throttle & be able to “look over the trainee’s shoulder” and closely supervise that locomotive’s control. The supervisor can gain instant override control without having to physically “grab” the trainee’s throttle. This lets you have unskilled visitors participating and enjoying operations without too much anxiety for either party.

Slot following also allows a computer on LocoNet to run CTC & routing control programs with automated control over locomotives. The computer can control speed and stop engines automatically while letting the engineer with the throttle in his hand know what is happening.

Ghost Throttle

A ghost throttle is one that you are not aware of. It was used to steal control of the locomotive and is still active. Erratic operation is often the result, as the command station overrides your throttle using the ghost throttle settings.


Information on resetting command stations and other additional information is from the 'Slot=Max' document found in the Digitrax Yahoo Groups file area. Author unknown.

Email written by the late Don Crano in 2004 on purging.