Current Loop

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Summary: A Current Loop is a technique used for signalling on a circuit. By using current in the place of voltage, data can be transmitted over a distance without voltage drop issues. A typical application is RailCom.

The Current Loop

The intention of this page is to explain the current loop, and what it is used for.

As stated in NMRA RP 9.3.1 page 1 line 29, this technique should be used to transmit bits from multifunction decoder to the receiving gadget.

During this process the booster will be disconnected from the track, while the detector and multifunction decoder are connected in series to form a loop. The multifunction decoder must be designed and built with this feature in mind. The booster must include the RailCom Cut Out Circuit for this feature to work.

Multifunction Decoder Communication via a Current Loop

The NMRA DCC Standard calls for data from the layout to be transmitted via a current loop. The multifunction decoder will modulate the track current in a specified manner which the command station or other devices can interpret. Current loops are very common in the digital world, so the parts needed to implement this technique are readily available. Tests have confirmed that error free transmission was possible at distances up to 100m.

The multifunction decoder uses its power supply, which supplies the correct voltages to the multifunction decoder circuits and motor control system, to feed the current loop. The multifunction decoder uses stored energy to provide the current source.

To transmit a Zero bit, the multifunction decoder must source 30mA of current for 4 microseconds. For a value of One, the multifunction decoder must not sink or source a current greater than 100 µA. Track voltage ranges from zero to 2.2V. The minimum current required for a "zero" value is 10mA, any current below 6mA is considered to be a value of "1".

Cut Out

During this process a device called a cutout disconnects the DCC power source (boosters) to the track, so the detector can read the data being sent by the multifunction decoder. The cutout will reconnect the booster should a DCC signal from another source appear on the track. The cutout also shorts the rails together to complete the loop. The cutout may be incorporated into the booster.

A DCC packet will initiate the process. Once the track power is disconnected, the multifunction decoder will transmit the data, and when that process is completed, track power will be reconnected. The entire data transmission will occur in less than half a second. A total of eight bytes of data can be transferred during the cutout session. The data is encoded using a 4/8 code to minimize errors. Each byte must contain four logical 1 states and four logical zero states. If not, the byte is considered to be incorrect.

RailCom addressing also allows the system to identify stationary and multifunction decoder addresses.

Standalone Cutout Device for RailCom

Produce or remove a cutout for implementing RailCom on your Model Railway layout


For additional detail consult the RailCom page.

Further Reading