A Guide to the Protocab system


A rechargeable Protocab lithium ion battery is installed in the locomotive and connected to the motor via the Protocab Locomotive Control Unit (LCU) which regulates the flow of electricity from the battery to the motor. A plug and socket arrangement on most Protocab products avoids the possibility of connecting the wrong components together. It also reduces the requirement for soldering, simply requiring you to apply the leads from the LCU to the locomotive's motor terminals.

The battery can be switched on either by selecting the loco by pressing a button on the controller, or by using the Acc+Ess Protocab Locoswitch which is included with the LCU.

The Locoswitch is a copper pad which is affixed inside the locomotive and which responds to your finger's energy. You don't actually have to touch the loco, the Locoswitch will respond when your finger is a few millimetres away from the pad.

Locoswitch demo
Showing that you don't need to touch the loco to switch it on, but touching the loco is also ok. You'd normally get an indication on your controller when you switch the loco on, but for this demo, we've switched on the headcode light.

The LCU includes a microcomputer and a radio frequency (RF) transceiver which both sends and receives control messages over the radio link. These messages contain encoded data that identify the LCU that the message is intended for and information on how the LCU is to respond. The messages for the LCU are generated by a similar RF transceiver in the Protocab controller which also contains a microcomputer.

Protocab: Much more than just radio control

We use the term 'wireless control' rather than radio control because the Protocab system has been designed around computer network principles and uses international network standards in handling the transmission of messages between the controller and the locomotive. The standards (laid down by the IEEE) are closely related to those used by the wi-fi router that you may have at home or in the office. The reason for going down this route when we designed Protocab, is that we want to do much more than simply controlling speed and direction of the locomotives. Apart from the wide possibility of operating functions this provides, there are many internal security and safety functions that we have built into the system.

Protocab has been designed for scalability. This means that the same system can be used by a modeller operating a small layout solo, or a layout where many operators are controlling many locomotives at the same time. For this reason, the Protocab protocols for message transmission and handling have been developed specifically for the requirements of model railway operation. Any operator can enjoy the additional functions which will be introduced over time to enhance the modelling experience. These functions (in plan) include simulations of the cab layout automatically presented to the operator for the selected locomotive, multiple loco working, and a host of auxiliary functions, such as lights, sound etc. Because not every Protocab user will want specific functions, the system has been designed that some of these functions can be added through Auxiliary Control Units (ACUs) attached to the LCU, with other functions being options on the controller.

Security within the Protocab system ensures that messages only get to and are acted upon the loco that they are intended for, as well as measures that ensure that locos cannot be controlled accidentally by other controllers. This is especially important where two or more layouts are within range of each other or on a layout where several operators are controlling several locomotives at the same time. Thus Protocab has been designed for every size of layout from the very smallest, one-person operation through to club layouts and large exhibitions where several Protocab systems are in operation.

The Protocab system makes it easier for the user to define how the system is to operate and handles much of the security automatically. Although comprehensive, these security features built into the Protocab system are simple to set up and manage.

Both the LCUs and the controller's microcomputers include programs ("firmware") that create not only control messages that pass between them, but also run a range of internal routines that are invisible to the user and which maintain the integrity of the whole system including the transmissions between all the elements of a Protocab "domain". In the case, for example, that communications are broken, both the controller and LCU are programmed to carry out immediate action processes, such as stopping the locomotive and alerting the control operator of the fact. (Some of the internal functions may not be available on the simpler Protocab controllers.)

Domains explained

The concept of the domain makes Protocab different from other wireless control systems. What makes Protocab special is that the domain is created automatically for you, so this description tells you what it is all about rather than you having to do anything to create or manage it.

With Acc+Ess Protocab, a controlling device is at the centre of the wireless network and keeps track of all the activities on the "assets" that it is controlling.

The controlling device can be the controller itself , or, for more complex layouts (or to take advantage of the additional functions provided), the Protocab "Concentrator" will enable several controllers to be operating at the same time.

Assets include LCUs and, in due course, will be extended to other devices such as signalling systems, and Auxiliary Control Units.

All these assets are "adopted" by the controlling device and added to its internal database of adopted (i.e. authorised by the operator) assets. This whole family of assets and the controlling device is the "domain" and is conceptually a "fortress" which protects all assets inside the domain (and all those outside from being accessed by assets inside the domain).

An LCU can only be adopted into one domain at any time, and the owning domain is registered in the LCU's database. This is a fundamental property that makes Protocab a very secure system and helps to ensure that unauthorised users cannot take control of your LCUs.

Similarly, the LCU's id is registered in the domain controller's database. The domain can be changed but only by the authorised owner. In the case of the handheld controller , the owner is considered to be the operator. More security will be available through the Protocab Concentrators , when the adoption process requires that a "Supervisor" enters a password before any changes can be made.

This diagram helps to explain the domain. Controlling devices and LCUs outside the domain cannot detect and therefore cannot control controllers and LCUs inside the domain. Similarly, controlling devices and LCUs inside the domain cannot detect the controllers and LCUs outside the domain.

For the more complex layout: The Protocab Concentrator

The Protocab Concentrator will enable several controllers and LCUs to be operated at the same time. It acts as a multiplexer for all messages passing throughout the domain and provides the control of the domain, including the adoption of assets. The Concentrator will also extend the network capability by providing access for different transmission standards, including Wi-Fi. This means that users who want to get the benefits of using smartphones or tablet PCs as controllers can access the same LCUs as the operator with a simple handheld controller. The Concentrator will provide automatic protocol conversion between, e.g. Wi-Fi and the network standard used by the LCU.

The wireless network

The usual network medium between the various Protocab assets is wirelessly over the air, with some devices using a physical medium such as wire. Data transmitted over the air uses the 2.4GHz unlicensed spectrum available throughout the world. The Acc+Ess RF transceivers included on the LCUs and controlling devices (controllers, Concentrators) handle aspects such as radio frequency selection automatically, choosing the most appropriate channel for data transmission of a particular packet of data. The organisation of data passing through air between the assets in a Protocab system complies with the international IEEE 802.15.4 standards for Wireless Personal Area Networks (WPANs) over which the Acc+Ess protocols have been superimposed for the specific application of this standard to model railway control.

The RF transceivers accept this IEEE 802.15.4 standardised data (which includes the Acc+Ess data payload) and, having made security checks on the integrity of the data, strips off the transmission "envelope" and passes the data up the network stack to the Acc+Ess Protocab application layer for further processing and actioning on the devices (LCUs, controllers and Concentrators).

Constant two way communications

There are many different types of data message, including, for example, speed settings, battery charge status, and a regular transmission of a small data packet that demonstrates that the asset sending it is still awake and receiving data. If this packet is not received by the asset at the other end of the transmission within a certain time, or a number of regular packets are not received, an alert routine is run that initiates an immediate action. Since both the assets at each end of the transmission are waiting for the regular "heartbeat" packet, not receiving the packet initiates an automatic immediate action that might include an emergency stop (if actioned by the LCU) or an alert to the controller that transmission has failed. These packets are sent at sub-second intervals, so a hard transmission failure would bring a loco to a rapid stop.

The Acc+Ess protocols are capable of virtually unlimited enhancement to cover a wide range of application requirements.

Page last updated: 1 May 2016