2-wire versus 3-wire PLCs?

I am designing a new sensor, and have been told that Japanese-made PLCs typically use 2-wire connections in their I/Os.

Now, my understanding is that PLCs from Allen-Bradley, Siemens, and others are 3-wire I/O PLCs.

What does this mean? What is the difference between these two? Any help would be most appreciated.

3 wire sensor have 3 wires. +24Vdc, 0V and a signal wire. and are PNP or NPN The diffference between 3 and two wire, besides the obvious 1 wire missing is that they are not dependant on type.
Sinking 3 wire prox's get 24 and 0 volts for power and communicate to the PLC by setting 0 volts on the signal or 24 volts. (depending on the card) Sourcing sets 24V on the signal wire. (or vice versa I can never remember)
2 wires can be set to basically whatever configuration you want depending on how you wire them. but tend to have smaller ranges than 3 wires (AFAIK).

You have been misinformed and are apparently confusing the sensor with the PLC. What matters is that a PNP or NPN sensor be properly matched with a sinking or sourcing input. Whether its two or three wires doesn't matter, the PLC doesn't care how the sensor powers itself, it only cares about the state of its input and is wholly unconcerned with how the potential is applied to the input. Besides, the majority of discrete inputs to a PLC are probably contact inputs.

Most PLC manufacturers make sinking and sourcing inputs. What differs is how the PLC manufacturers refer to the input. For example, one PLC manufacturer may call an input a sinking input because the input itself sinks current, while another manufacturer calls it a sinking input because the connected device is expected to sink current that the IO module sources (at least one Japanese maker uses this confusing terminology).

What kind of sensor are you designing?

abwiz said:

What kind of sensor are you designing?

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I'm sorry, but I have to say it.....

Three Wire!

Hmm... so your designing a sensor but you don't know how it interfaces to a plc... well I don't think many people would trust your design, it's probably already been done & the biggies could manufacture it more cost effectively Why are you bothering?

homework?

CaseyK said:

I'm sorry, but I have to say it.....

Three Wire!

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Not sure what you are saying but also not sure how someone can design a sensor and not understand two-wire or three-wire.

If you go here, may have to click on inductive proximity sensors in menu on left, you can see they offer 2, 3, 4 wire versions: http://www.sick.com/home/factory/catalogues/industrial/induktive/en.html

I can't honestly think of anything that I want to sense that there isn't a plethora of sensors on the market for. But you never know I guess, since the OP is starting from scratch I really prefer 1 wire sensors (there easier to connect and troubleshoot).

allscott said:

I can't honestly think of anything that I want to sense that there isn't a plethora of sensors on the market for. But you never know I guess, since the OP is starting from scratch I really prefer 1 wire sensors (there easier to connect and troubleshoot).

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This a joke? Please explain what a 1 wire is.

Ron,

A one-wire sensor has only a power wire, with the return signal radiating into the air at a high frequency (=1, or off =0). The PLC module has a detector that picks up the radiated signal. The big problem is that it works at the same frequency as human brain waves, so the radiated signals can cause severe headaches where there are many sensors in a small area.

The recommended method is to use the equation that calculates the maximun allowable sensors per 1000 cubic square feet, based on the number of people working in the area. Personnel with larger-than-normal heads have to be counted twice.

OMG, nevermind.

After control_rookie has a few designs under his belt, we'll have him do some 0-wire sensors, and eventually work up to a -1 wire sensor. But no-fair using Flux Capacitors, or Allowing Resident Smoke Leakage!!!


Hope all is well down south Ron.

rsdoran said:

This a joke? Please explain what a 1 wire is.

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Is my sense of humour really that bad?