Everyone in the automation world has to deal with wires at some point. This seems like a natural place to begin.
Basic (and I mean basic) signal definitions:
"Discrete" or "digital" signals show true or false. In a typical VFD application, you might send out a 24VDC (or 120VAC signal) to the common contact on the relay showing motor run status (on or off). The relay's coil would energize, causing the signal to return (or breaking the signal) on operation. Another example might be a switch. Both are "dry contacts," specifically, metal-on-metal connections. They will be rated at a voltage but may work (temporarily) at a higher voltage.
Some devices are PNP or NPN, which might end up as a separate post someday. However, AutomationDirect does a pretty good job of explaining. I
Case study / real-world error: If you have a signal you are wiring, check the voltage and type. You can destroy some very expensive 24VDC gear by tying them to a 2400VDC bus. (Names will be withheld to protect the guilty.)
"Analog" wires signals a range of values, 0-100% of a signal. Often, these are 0-10VDC, 0-5VDC, 0-20 mA, 4-20mA, +/- 5VDC, +/-10VDC. The wider the range, the more resolution you will get. If you use values ranged above and below 0VDC, you can use the sign to denote a different dimension and saving on wires. Consider a drive that can run between 0-60 Hz and clockwise (+) or counter-clockwise (-). If you use a 4-20 mA signal, my favorite, you can get alarms for errors. If the wires are loose, your sensor loses power or a fuse blows, you will get 0 mA, which your PLC can be set to alarm at. If your sensor is smart enough, it may throw an error by showing 2 mA.
Other cables often mean networked devices. These are the "smart" ones, which may come in and show self-diagnostics, or allow other information to be transmitted from them. There are custom cables ("purple hose") for Profibus devices as well as standard Cat5 (ethernet) cables for others, like MODBUS TCP.
A shout-out to HART HART is a nice interface that allows you to run networked devices on a standard analog signal. If you are bored and nerdy, it's actually rather cool to see the details. If you are busy and practical, you should still look at it! Your techs can go online with systems using hand-held devices and with minimal re-wiring. Similarly, many PLCs have cards that will let you program this.
Beware: Imagine not having to drive out into the woods at 0030 to re-set a sensor, as you can have your operators handle it... Sounds great, if you have properly secured your system. If not, you could have an internet stranger take it over.
Common error: AC and DC wires not segregated
If you have AC and DC wires running in parallel, the AC wires are going to induce a voltage into the DC signal. This will "disappear" after adding a resistor, but it is best to keep these wires in separate metal conduits. Run them at 90 degree angles when possible and keep any sections where they do cross as short as possible.
Word to non-electricians: You can get away with a lot if you keep the panel clean and sweep up when you are done. I have seen a lot of engineers (and a few technicians) hang wires in panels. Do not do this. Run them in raceway and, if it cannot be in raceway, use monochrome tie-wraps and sticky-backs to hold them. Cut them off with cleavers like this, rather than this, so you do not get your hands cut when you are servicing them. Keep the copper exposed as long as necessary and short as possible
And one for electricians: Please help the non-electricians here. Treat them how you wish your journeymen treated you. You never know, you might need an engineer tell you how a unit is supposed to work, the HMI guy to draw something, a former operator to help with the mechanical bit... or the former IT gal to help with your printer.
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