Analogue Inputs - Sink, Source, Help!

[Yabadabadoo]

I've been using PLCs for quite a while but have not managed to get my head around the various ways that analogue signals can be tied into different types (and brands) of PLC input modules. I can never be sure what types of signals a particular type of module can cope with. Can anyone offer an easy to understand explanation of exactly what is meant by "source", "sink", "positive referenced", and "negative referenced" analogue inputs, and maybe even point me in the direction of some diagrams that would make it all clear. Thanks!

[mgvol]

Sinking refers to a 2-wire analog device and sourcing refers to a 4-wire device. A-B calls this "single-ended" (2-wire) and "differential" (4-wire). In the following example, Ch. 0 thru 3 are "sinking", while Ch. 4 - 7 are "sourcing".

[mgvol]

Oops! Forgot the example:

[Yabadabadoo]

Thanks for that. So am I correct in saying that a sinking analogue input has a single wire coming in from the field and the device in the field is powered either internally or via a separate supply? And that a sourcing analogue input simply means that the AI module provides the 24V supply which goes out to the field?

[rsdoran]

This may help: http://www.patchn.com/npnpnp.htm

[mgvol]

A sinking device would typically have a 2-conductor shielded cable leaving the panel carrying 24 vdc to a transmitter's + terminal on one conductor, and the signal returning to the panel on the other conductor. Thsi "signal" conductor would land on the analog input module's + terminal, and the module's - terminal would be connected to the - side of the 24 vdc supply. A sourcing device would "power the loop" itself, and send back a + and - signal that would connect to the module's corresponding terminals.

[Yabadabadoo]

That's a useful set of diagrams that explain pretty much every flavour of digital input and output. Do you know of something similar that addresses analogue (4-20mA) I/O?

[rsdoran]

I originally placed a bunch of links but re-read the question and got a slightly confused.

If you learn how sink/source works it will not matter whether the device(s) are digital or analog. Sink/source just references the "flow" and connections associated with the type.

[Bitmore]

Sinking in sources?
If it has only 2 wires and is not powered from anything else, it may also be called "loop powered" or sinking (they act as a variable resistance to the 4-20 ma. but still get their operating power from the 4-20 ma. "loop"). The two wires coming from your control input must also include some serially connected power supply.
"Non-Loop powered devices, or sourcing" have their own power supply wires going out to them or have a power supply in them, "hence 4 wires, 2 for +/- supply and 2 for the analog signal" some are 3 wired,,, they share a common ground or return wire. Those rare devices that have their own power supplies contained in them (sourcing), ALWAYS!!!!! have more than 2 wires.
Single ended inputs are from 3/4 wire or "externally powered" devices.
Positive or negative reference has to do with "loop powered" devices ONLY as "non-loop" or externally powered devices have no such definition.
Some "loop powered/external" signal devices require a 2nd connection to the -power source or a 2nd connection to the + power source, that is what is sooooo.... confusing! Hence positive or negative reference!

ps... diagrams to follow......yeah,,wanna buy a bridge I know of up north?

[Yabadabadoo]

If you learn how sink/source works it will not matter whether the device(s) are digital or analog. Sink/source just references the "flow" and connections associated with the type. [/B]

I guess my query now moves into the internal electronics of the different types of modules. A digital module will generally be using a transistor (normally an NPN, am I correct?) whilst an analogue module will be taking the 4..20mA current into some kind of ADC. I'm still hazy on what the analogue circuit would look like, and how sink or source modules would differ internally.

[Tom Jenkins]

It's not elegant, but I use a simple trick to remember how they work.

"Sink" means that the device, be it an input or an output, lets the current sink or disappear into it, implying that the power source is external. In the case of an analog output device that means it adjusts its internal impedence until the current passing through is the desired value.

"Source" means that the device is the source of the power and the current. That means that an analog output device adjusts the current it is producing until is is the desired value.

[Yabadabadoo]

But surely a module taking an analogue input that is powered externally (not directly from the input module) can still be sink OR source, it just depends on whether the input is tied to the +ve or -ve side of the supply?

[Terry Woods]

This is the basic design of an analog input.

                       -------+

                              |             +----------------------+

                              +---------->  |                      |

                              |             |     ADC              |

                            +---+           |                      |

                            |   |           |                      |

         4-20 mA            |   | Precision |   Voltage            |

         Signal             |   | Resistor  |   Discriminator      |

         Controlled         |   |           |   Where the Voltage  |

         by some            |   |           |   is converted to a  |

         device in          |   |           |   numerical value    |

         the field          +---+           |                      |

                              |             |                      |

                              +---------->  |                      |

                              |             +----------------------+

                       -------+

.
When speaking of current, we are speaking of "conventional current", not "electron current".

A current, controlled by a device in the field, is applied to a precision resistor. The ADC reads the voltage across that resistor and converts it to a numerical value.

The idea of using the terms sourcing and sinking in relation to an analog input module is problematic.

The power source for the 4-20 loop might be located at the field device or at the analog input module.

Regardless of where the power source is, the current leaves the power source, and then enters either the controlling device followed by the precision resistor, or, the precision resistor followed by the controlling device. In either case, the current then returns to the power source.

The power source is "the source" and "the sink"... it's a loop.

"Sourcing" and "Sinking" are RELATIVE terms. And bear in mind, a "Sourcing Device" is NOT the SOURCE. Nor is a "Sinking Device" the SINK. The Power Supply is the SOURCE and the SINK.

If a device connects an incoming current to the (-) terminal on the Power Source then the device is called "sinking".

If a device accepts an incoming current from the (+) terminal on the Power Source then the device is called "sourcing".

If the current moves from the power source to the field device and then to the precision resistor...

SOURCE                       SINK

(-)Power Source(+)---------> (+)Field Device(-)

(+)Field Device(-)---------> (+)Precision Resistor(-)

(+)Precision Resistor(-)---> (-)Power Source(+)

.
If the current moves from the power source to the precision resistor and then to the field device...

SOURCE                       SINK

(-)Power Source(+)----------> (+)Precision Resistor(-)

(+)Precision Resistor(-)----> (+)Field Device(-)

(+)Field Device(-)----------> (-)Power Source(+)

.
In the first list, the Precision Resistor is connecting the incoming current to the (-) terminal on the Power Source. The Precision Resistor is said to be "sinking" to (-).

In the second list, the Field Device is connecting the incoming current to the (-) terminal on the Power Source. The Field Device is said to be "sinking" to (-).

It might be the case that the power source is physically located at the analog input module.

If one side of the Precision Resistor is connected to the (+) terminal of the Power Source and the other side is connected to the Field Device, then the Field Device will be "sinking" to the (-) terminal on the Power Source. In this case, the Precision Resistor (Analog Input Module) is "sourcing".

On the other hand...
If one side of the Precision Resistor is connected to the (-) terminal of the Power Source and the other side is connected to the Field Device, then the Precision Resistor (Analog Input Module) will be "sinking" to the (-) terminal on the Power Source. In this case, the Field Device is "sourcing".

It might be the case that the power source is physically located at the Field Device.

If one side of the Field Device is connected to the (+) terminal of the Power Source and the other side is connected to the Precision Resistor, then the Precision Resistor will be "sinking" to the (-) terminal on the Power Source. In this case, the Precision Resistor (Analog Input Module) is "sinking".

On the other hand...
If one side of the Field Device is connected to the (-) terminal of the Power Source and the other side is connected to the Precision Resistor, then the Field Device will be "sinking" to the (-) terminal on the Power Source. In this case, the Precision Resistor (Analog Input Module) is "sourcing".

[Bitmore]

Yes, in a way that you are thinking, you must have been taught electronics! A sinking device in the field is like an open collector output device so it needs a collector load and +Vcc hence 3 wire sinking positive reference. (Sinking devices can also be open emmiter, 3 wire negative reference)
A sourcing device is an H biased transister circuit that has it's own internal load and power supply (Vcc/Vee ie. 4 wire) and the signal is "a produced" output whether you connect a load (I/O card) or not!
Now back to the positive or negative references...
Nothing more than open collector for positive reference and open emmiter for negative reference. The device still needs one of the reference voltages (+ or-)to complete the field devices' internal circuitry.
The analog current input cares nothing about the field devices current signal except that it must flow through the terminals of the input card.
Now on to single ended or differential inputs...
Are you ready yaba?

[Yabadabadoo]

*needs more time for the last two posts to sink in before taking any more*



Thanks guys, for all that. I'm away to tally up what you've said with specific drawings that I'm struggling with here. Hopefully it'll all make a bit more sense now...