PLC(I/0) System
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| CPU 1211C |
- The role of the PLC's input/output (I/O) system is to translate the signals received from input devices into low-voltage DC signals that the CPU can use and to perform the reverse operation when the CPU sends signals to output devices.
- In addition, the I/O system protects internal circuits from harmful voltage and current variations.
- A small PLC may have some or all of its I/O channels in the same housing with the CPU.
- For example, the accompanying graphic shows a CPU 1211C with six on-board connections for inputs and four on-board connections for outputs.
- This S7-1200 CPU model also permits a small signal board to be added internally for some I/O expansion but does not permit the addition of external signal modules.
- Most Siemens PLC'S however including the ones shown in the accompanying graphic allow signal modules to be added.
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| Siemens PLC'S |
- Some or all of these modules may be close to the CPU but signal modules can also be remote from the CPU.
In addition for large or complex applications the number and types of signal modules is typically greater because the number and types of I/O devices required are greater.
Digital Inputs
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| Input Devices |
- Examples of Input devices used with digital signal modules are (push buttons, limit switches, temperature switches, pressure switches, level switches, flow switches, photoelectric switches, and proximity switches).
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| AC and DC Current |
- Some input devices use direct current (DC) and other input devices use Alternating current (AC).
- The specifications for each device type also identify the level of voltage and current required by the device.
- An additional consideration when matching a digital input signal module to a DC input device is the polarity of current flow.
- Some DC devices are electromechanical so the polarity of current flow does not matter but other devices have built- in electronics that require a specific polarity of connection.
- For example, consider the S7-1200 PLC signal module shown in the accompanying graphic.
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| Simatic S7-1200 |
- This signal module is designed to work with 24V DC inputs and has connections for eight DC input channels four on top and four on the bottom.
- The accompanying graphic for simplicity shows only the top connections for this signal module.
- Note that this module allows the load power supply which powers the input devices and circuits to be connected with either polarity.
- The connection diagram shown on the left has Sinking connections and the connection diagram on the right has Sourcing connections.
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| Sinking and Sourcing Connection |
- The terms sinking and sourcing are designed to identify the direction of current flow but because these terms are confusing it is best to follow connection diagrams provide with PLC documentation.
- Examples of output devices used with digital signal modules are ( lights, Relays, Solenoids, Motor Starters, Valves, and Heating elements). Some output devices use DC and other output devices use AC.
Digital output
- Three categories of digital output signal modules are AC modules, DC modules, and relay modules.
- AC output signal modules typically operate with a load power supply of 120 or 230 VAC or may be capable of operating with either source.
- These modules use a solid state switching device, usually a triac, to control each output.
- This type of output works reliably with many AC output devices, but because a Triac allows a small leakage current to flow when the output is off, it is not suitable for some AC devices.
- The accompanying graphic shows a DC output signal module designed to work with 24V DC outputs.
- This signal module has connections for eight DC output channels, four on top and four on the bottom, and it is important to observe proper power supply polarity when making connections.
- The connection diagram shows the correct connections for the top terminals of the module.
- You may see the terms source and sink as it relates to DC output signal modules, but because these terms can be confusing, it is best to refer to the connection diagrams.
- Relay output signal modules offer the advantage of being able to work with either AC or DC devices as long as the devices operate within the module's specifications and the switching speed required is not too fast.
- Each output on a relay output module is a set of relay contacts that close when the output turns on and open when the output turns off.
- The accompanying graphic shows a relay output signal module with eight output channels, three in top and five on the bottom.
- The connection diagram is for the top connections on the module.
- The markings above and below the load power supply symbol identify the connections required.
- For a single-phase AC supply, L stands for line and N stands for neutral.The + and connections apply if a DC supply is used.
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