sinking and sourcing concepts - buffalo state...

1 - 8 0 0 - 6 3 3 - 0 4 0 5e35-2 Appendix

When choosing the type of input or outputmodule for your system (orDL05/DL06/DL105 I/O type), it is veryimportant to have a solid understanding ofsinking and sourcing concepts. Use ofthese terms occurs frequently in discussionof input or output circuits. It is the goal ofthis section to make these concepts easy tounderstand, so you can make the rightchoice the first time when selecting the typeof I/O points for your application. Thissection provides short definitions, followedby general example circuits.

First you will notice that the diagrams onthis page are associated with only DCcircuits and not AC, because of the refer-ence to (+) and (-) polarities. Therefore,sinking and sourcing terminology appliesonly to DC input and output circuits. Inputand output points that are sinking orsourcing can conduct current in one direc-tion only. This means it is possible toconnect the external supply and fielddevice to the I/O point, with current tryingto flow in the wrong direction, and thecircuit will not operate. However, thesupply and field device can be connectedevery time based on an understanding ofsourcing and sinking.

The figure below depicts a sinking input.To properly connect the external supply, itmust be connected so the input provides apath to supply common(-). So, start at thePLC input terminal, follow through theinput sensing circuit, exit at the commonterminal, and connect the supply (-) to thecommon terminal. By adding the switchbetween the supply (+) and the input, thecircuit is completed. Current flows in thedirection of the arrow when the switch isclosed.

By applying the circuit principles to the fourpossible combinations of input/outputsinking/sourcing types, there are fourcircuits, as shown above. The commonterminal is the terminal that serves as thecommon return path for all I/O points inthe bank.

Sink/source I/O circuits combine sinkingand sourcing capabilities. This means thatthe I/O circuitry in the PLC will allowcurrent to flow in either direction, as shownat the right. The common terminalconnects to one polarity, and the I/O pointconnects to the other polarity (through thefield device). This provides flexibility inmaking connections to your field powersupply. Please note:

• Wire all I/O points with a shared common as either sinking or sourcing.

• Do not use an AC power supply on a DC sink/source I/O point.

+

--

InputSensing

PLCInput

Common

(s inking)

+

--

InputSensing

Load

S inking Input(IE C: pos itive logic)

S inking Output(IE C: negative logic)

S ourcing Input(IE C: negative logic)

S ourcing Output(IE C: pos itive logic)

PLC

Input

Common

+

--

OutputSwitch

PLC

Output

Common

+

--

InputSensing

LoadPLC

Input

Common

+

--

OutputSwitch

PLC

Output

Common

PNP

NPN

S ink/S ource Input(IE C: pos ./neg. logic)

S ink/S ource Output(IE C: pos ./neg. logic)

+

--InputSensing

Load

PLC

Input

Common

OutputSwitch

PLC

Output

Common

+

--OR

+

-- +

--ORPNP/NPN

Sinking and Sourcing ConceptsSinking = provides a path to supply common (-)

Sourcing = provides a path to supply source (+)

24VDC

+

–

Output Sensor

Common

Input

OpticalIsolator

Current SourcingConfiguration

(NPN) Current SinkingField Device

DC Sourcing Input Module+ –

NPN (Sinking)Field Device Example

24VDC

+

–

Output Sensor

Common

Input

OpticalIsolator

Current SinkingConfiguration

(PNP) Current SourcingField Device

DC Sinking Input Module+ –

PNP (Sourcing)Field Device Example

Field device examples - 3 wire connections

Volume 13

w w w . a u t o m a t i o n d i r e c t . c o m Appendix e35-3

CompanyInformation

SystemsOverview

ProgrammableControllers

Field I/O

Software

C-more & other HMI

Drives

SoftStarters

Motors &Gearbox

Steppers/Servos

Motor Controls

ProximitySensors

Photo Sensors

Limit Switches

Encoders

CurrentSensors

PressureSensors

TemperatureSensors

Pushbuttons/Lights

Process

Relays/Timers

Comm.

TerminalBlocks & Wiring

Power

CircuitProtection

Enclosures

Tools

Pneumatics

Appendix

ProductIndex

Part #Index

Volume 13

Sinking and Sourcing ConceptsCommon terminals and how to use themIn order for a PLC I/O circuit to operate,current must enter at one terminal andexit at another. This means at least twoterminals are associated with every I/Opoint. In the figure at the right, the inputor output terminal is the main path for thecurrent. One additional terminal mustprovide the return path to the powersupply. Together, the main path and thereturn path create a loop, or a completecircuit for current to flow.

If there was unlimited space and budgetfor I/O terminals, then every I/O pointcould have two dedicated terminals.However, providing this level of flexibilityis not practical or even necessary for mostapplications. So, most input or outputpoints on PLCs are in groups that sharethe return path (called commons). Thefigure at the right shows a group (orbank) of four input points that share acommon return path. In this way, the fourinputs require only five terminals insteadof eight.

NOTE: Assuming all input circuits have a similarresistance, the current at the common terminal isfour times greater than the current at any one of theinputs. This effect is especially important to note foroutput circuits, where the current through a commonterminal can reach several amperes. You will needto decide whether to fuse each output point individu-ally, or to put a fuse in the common terminal path.

Wiring labels and how to interpret themDL205, DL305, DL405 - Most DL205,DL305 and DL405 input and outputmodules group their I/O points intobanks that share a common return path.The best indication of I/O commongrouping is on the wiring label, such asthe one shown at the right. The miniatureschematic shows two circuit banks witheight input points in each. The commonterminals are labeled “CA” and “CB,”respectively.

In the wiring label example, the positiveterminal of a DC supply connects to thecommon terminals. Some of the symbolsyou will see on wiring labels and theirmeanings are shown at the right.

DL05/DL06/DL105 — Most DL05, DL06and DL105 input and output circuits are

grouped into banks that share a commonreturn path. The best indication of I/Ocommon grouping is on the wiring label.Sample DL05, DL06 and DL105 wiringlabels and their meanings are shownbelow.

+

--

I/OCircuit

PLC

(I/O Point)

Return Path

FieldDevice

Main Path

+

--

InputSensing

Input 4

Common

Input 3

Input 2

Input 1

I/O Common Grouping Bar (DL105)

Two banks of four inputs and one bank of two (DL105)

Two banks of four inputs and two banks of three outputs (DL05)

Input Bank (DL05)

0123

4567

ATB

12--24VDC

D4--16ND2

0123

4567

B

INPUT

CA0

4

15

26

37

CB

40

51

62

73

10.2-- 26.4VDC4-- 12mA

L

AC supply

AC or DC supply

Input S witch

Output Load

DC supply+--

DL405 inputmodule shown