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Glossary of terms you will come across during your Phase 2 training.

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On a CAN bus, all nodes can transmit as well as receive messages. If two nodes attempt to access the bus at the same instant, unless precautions are taken, there is a danger of data collision, with subsequent loss of data and delays whilst data bus conditions are returned to normal.

CAN provides for non-destructive bus arbitration - no message gets lost or destroyed.


On a controller area network (CAN), any node (station) connected to the network may transmit a new frame as soon as the bus is idle. There are 4 different types of message frames:

  1. Data Frame - Sends Data: Data transfer from one sending node to one or more receiving nodes.
  2. Remote Frame - Requests Data: Any node may request data from another node. Consequently, a remote frame is followed by a data frame containing the requested data.
  3. Error Frame - Reports Error Condition: Any device on the bus, sender or receiver, may signal an error condition at any time during a data frame or a remote frame transmission.
  4. Overload Frame - Reports Node Overload. A node can request a delay between to data frames or two remote frames, meaning the overload frame can only occur between data frame or remote frame transmissions.
This is a serial communication port that is used for communication with other devices such as the EST.

Data sheetThere are very many different types of electronic components, and picking the right one for the job can be problematic. Thanks to the World Wide Web (www), sourcing the relevant technical information on a component is now just a matter of typing the number into a search engine such as google.

For example, typing the transistor number 2N3904 into a search engine produces many hits, among them will be this datasheet, which gives us all the information we need to decide if we can use this particular component in a project.

Here are some suggestions for sites with data sheets:


When the ECM senses a serious problem, it may de-rate the engine. This means that the maximum speed of the engine is restricted to 1,200rpm. This de-rate is also known as limp home mode. In other words, the ECM is aware of a serious fault, but the engine will not be damaged as a result of this fault. In order to give the operator time to get back to garage or base, the ECM will allow the pump to provide reduced fuel, which results in a maximum engine speed of 1,200rpm, or enough fuel to limp home in the vehicle.

Derates are one of 3 kinds:

  1. Reduction of rated power
  2. Reduction of rated engine speed
  3. Reduction of rated machine speed for OEM products

This is where the electricity goes from one terminal to another without passing through any load. Becasue there is no resistance, too much electricity will flow. If the circuit is protected by a fuse, the fuse will blow. Otherwise, the circuit will overheat and burn.

This is a dangerous situation, and must be avoided at all costs.

Other circuits include: short circuit, open circuit and closed circuit.


Machines or vehicles are sometimes fitted with safety devices (buttons, switches, levers or other similar mechanisms) which must be pressed, held or closed in order for the machine or vehicle to operate.

If something were to happen the driver (if he or she were injured and became unconscious and unable to control the machine or vehicle), the device would not be properly engaged and the machine or vehicle would cease to operate.

Use of the term has spread to describing, say, for instance, the safety swithces under a driver's seat, which is closed by the weight of the driver. If the driver were to fall from the vehicle, or was not properly seated in the vehicle, the engine will not run.


In its normal state, the valve allows oil through. If another device physically touches the plunger and wheel on the valve, this changes the valve by closing it. Oil will no longer get through the decelaration valve, and must pass through some other route, normally a flow control valve, to get to tank. because of the venturi in this valve, we can control the flow, and thus the speed of operation of the circuit.

Check here for an example of a decleration valve being used in a circuit to control the speed of extension of a cylinder rod.


Company contact for P and S Civil Works.


  • Landline: 049 4330 115



This is a valve screwed into the engine cylinder. They are normally used in consaws and other two-stroke engines, to aid cold-starting.

Pushing the button causes the valve to open, and compression gasses to leak out through a small hole. This relief of compression allows us to crank the engine a little faster.

The advantage of that is when the button snaps closed, the piston is moving quickly and the heat generated in compression does not have as much time to escape to the cooling jacket, so cold-starting is made easier.

Once the engine gains momentum, the extra compression closes the valve, and the engine starts.

Note: Small, four-stroke petrol engines are also often fitted with some sort of an automatic decompression device.

Flash video(Flash animation)

 Contributors to this page: Joe Duffy, Phase 2, May to September, 2011.


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Please note, if you want to contact the main Training Centre in Baldoyle, you need to ring 01-8167400.

If you want to contact the CP Fitters' Section, you can Contact Us by email,, or by telephone: 00 353 1 8167543 This number gets you through to the CP Fitters' Section, Baldoyle Satellite Unit.

Our Address is : Construction Plant Fitters' Section, Unit 116, Grange Way, Baldoyle Industrial Estate, Baldoyle, Dublin D13 CA48.
We operate in a satellite unit which is about 300 metres away from the main training centre. You can see the location of this unit here, in Google Maps. You can also Check here for directions to the Baldoyle Training Centre.

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