Relay logic

Each rung would have a unique identifying reference number and the individual wires on that rung would have wire numbers as a derivative of the rung number. Thus, if a rung was labelled as 105, the first independent wire would be 1051, the second as 1052, and so forth. A wire would be named for the top most rung to which it connected, even if it branched to lower rungs. When designing a system, it was common practice to skip numbers for the rungs to allow later additions as required.

When the rack was manufactured, as a wire was installed, each end would be marked with wire labels (a.k.a. wire markers). This also applied for pulling wire into the factory through conduit or in trays where each wire would have corresponding numbers. Wire labels were typically pieces of white tape with numbers or letters printed onto them and collected in small, pocket sized booklets. A number strip would be peeled out and wrapped around the wire near the end. Wire numbers were made up of a series of the number strips so wire 1051 would be four strips. There are also pocket sized printers that print onto an adhesive backed label that can be wrapped around the wire.

1. The two vertical lines that connect all devices on the relay logic diagram are labeled L1 and L2. The space between L1 and L2 represents the voltage of the control circuit.

2. Output devices are always connected to L2. Any electrical overloads that are to be included must be shown between the output device and L2; otherwise, the output device must be the last component before L2.

3. Control devices are always shown between L1 and the output device. Control devices may be connected either in series or in parallel with each other.

4. Devices which perform a STOP function are usually connected in series, while devices that perform a START function are connected in parallel.

Figure 1 shows a typical relay logic diagram. In this circuit, a STOP/START station is used to control two pilot lights. When the START button is pressed, the control relay energizes and its associated contacts change state. The green pilot light is now ON and the red lamp is OFF. When the STOP button is pressed, the contacts return to their resting state, the red pilot light is ON, and the green switches OFF.

In many cases, it is possible to design a relay logic diagram directly from the narrative description of a control event sequence. In general, the following suggestions apply to designing a relay logic diagram:

2. Draw a sketch of the operation process. Make sure all the components of the system are present in the drawing.

3. Determine the sequence of operations to be performed. List the sequence of operational steps in as much detail as possible. Write out the sequence in sentences, or put them in table form.