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03 October 2000

How do Interfaces Work?

by Arnold Offner

In the fields of motion control and industrial control systems, "interface" describes the device(s) that provide the necessary connection and communication between the controller and the I/O apparatus (the latter include all input and output devices and all sensory and actuating mechanisms).

Consider a signal from a proximity sensor providing status on the presence of a conveyor line pallet. The sensor is powered by a 24-VDC signal; however, the controller only accepts signals with a transistor/transistor logic (TTL) 5-VDC level. Converting the 24-VDC signal to a TTL level requires some form of voltage adaptation to make the circuit function properly.

Another example is requiring a PC-based controller to switch an AC motor on or off. Since PCs operate at a DC level and the motor operates on an AC level, we require a device that will operate with a DC input and switch an AC load on and off. A PC won't switch a heavy-duty AC motor with its low-level voltages; hence, the system requires signal amplification.

In industrial applications, "interface" describes both the device and the action responsible for transmitting information from one device to another. In effect, four broad categories of interfaces exist:

• Wiring
• Digital
• Analog
• Serial communication

Depending on the application, these are listed in increasing level of complexity.

Wiring Interfaces

Wiring interfaces are the simplest to understand since they're used to join devices with high-density connectors to discrete, individually prepared termination points. Depending on the industry, programmable logic controllers (PLCs) will come supplied with D-Sub connectors, ribbon cable headers, or another type of high-density connector. A technician connecting the controller to the "outside world" of sensors or output devices would use a single discrete wire connection. You can also elect to use a single module incorporating the same type of high-density connection at one end, and the corresponding discrete termination points at the other. The first method can be not only time consuming but error-prone as well, since each connection requires detailed attention. By way of contrast, the second technique is faster and more reliable, since each conductor is repetitively positioned at the same point.

Sensors and actuators are now available with high-density plug or cord sets, allowing service technicians with little or no knowledge of the actual circuit diagram to install and replace sensors, actuators, and cords. The cord sets come in standardized layouts; no polarity check or circuit tracing is required.

While discrete terminal blocks are presently the de facto standard, many PLC and controller manufacturers also offer wiring interface solutions for their systems, since these result in a faster turnaround time and reduced system cost. To the system integrator, wiring interfaces mean less skilled labor, higher installation efficiency, and fewer wiring errors.

Wiring interfaces are best suited to high density, low voltage I/O cards; they allow the wiring interface itself to incorporate fusing, interposing relays (electromechanical and solid-state), signal redundancy, and other components. A plain PLC or controller connection is accomplished with a wiring interface, and any additional functionality is best described by a digital interface for any digital (or discrete) signals.

Digital Interfaces

Any on/off signal that's either input to or output from a controller invariably must adapt in some way or operate at a different voltage. Imagine switching a three-phase, 480-VAC motor on or off with a 5-VDC signal. This could be done with a solid state relay, an electromechanical relay, or a power contactor. Since the controller's simple on/off signal doesn't have the power required to turn the motor on, a digital interface adapts the controller to both actuate the motor and also protect and isolate it from the controller. PLCs have digital I/O cards that are only capable of operating certain load levels. Consequently, higher density interfaces (with a preference for 24 VDC over 120 VAC) have become more important in boosting PLC performance.

Using either a single- or multi-channel configuration, you can incorporate a digital interface on a PLC or controller. This will maximize both the signal density from each slot of a PLC, controller, or I/O card, and the flexibility external to the controller in the control cabinet.

Interface manufacturers incorporate individual channel pluggability into their products, which is a further enhancement over the fixed (and unpluggable) layout of an I/O card. Each channel can be modified or repaired quickly, while the rest of the I/O points remain untouched. The digital interface thus allows the user to reduce maintenance costs by repairing or modifying only the required channels, rather than replacing and repairing an entire I/O card. It also allows the I/O card to be standardized to 24 VDC, and creates an interface level to the field that incorporates all other signal levels. Standardizing to 24 VDC provides maximum flexibility, increased functionality, and reduced inventory levels of the various I/O cards that would otherwise be in use. With an increasingly wider range of available 24-VDC devices, the customer also benefits from reduced hardware costs and maximum PLC rack utilization.

The de facto standard in many varying control applications is 24 VDC. However, there are still applications where other signal levels are still used, such as 5 VDC (or TTL) on PC-based systems, 12 VDC (solar powered systems), 125 VDC (utility applications), and of course, the very entrenched 120 VAC. Some systems also switch or monitor 480-VAC signals or loads. Standardizing on a 24-VDC signal level in the control system means reducing various different power supplies, and more importantly, a safe operating voltage for service technicians, assembly personnel, and operators.

Digital interfaces are really on/off devices and are available in either electromechanical or solid state versions as relays or power contactors.

If a varying signal of a 0–100% range is required in a control system, we rely on analog interfaces to provide the solution.

Analog Interfaces

Temperature, current, voltage, power, frequency, pressure, flow, power factor, speed, and load-cell outputs are all signals that require higher resolution than simply on or off. We regard all analog signals as those that normally vary between 0–100%. Most control loops are commonly converted to a standard 4–20 mA, and although other signal levels are currently in use, 4–20 mA represents a de facto standard in many industries that rely on more exact measurement.

Due to the complexity of analog processing circuitry, you'll find that while a PLC I/O card allows 32 digital signals, only eight 4–20 mA loops are possible in the same size package. If the analog card is designed to process frequency, its maximum handling ability is limited to four channels.

The analog interface allows differing signals to be converted to a standard 4–20 mA loop signal that can be connected to a 4–20 mA input card. Since these signals can include temperature, pressure, power, and frequency and are application-dependent, the various component signals represent the final signal with a 16-mA range (4 mA is the zero level; 20 mA equals the 100% level; 0 mA indicates an open circuit or line break).

Serial Communication Interfaces

Almost all controllers are now required to communicate with higher-level control systems. Many interface suppliers simply look to another type of module to assist in converting standard RS-232 signals to a fiber-optic link, or allow a device with an RS-485 communications port to communicate to an RS-232 device.

The standard communication protocols of RS-232, RS-485, RS-422, V.22, and V.24—irrespective of the serial signal's own protocol—allow the user to convert the serial language between two devices. It's therefore of no relevance whether the signal is Modbus, DeviceNet, Interbus, SERCOS, ControlNet, or Profibus. They're all based on standard serial interface communication and can very easily be converted to fiber-optic, or allow communication in two- or four-wire RS-485. Serial communication interfaces also allow RS-232 devices to be bussed together in a master-slave function.

All interfaces used in an industrial application should be suited for the application, preferably DIN-Rail mountable, and conform to international and national standards. They're required to work in hot, humid areas with interference and should offer industrial robustness and reliability. MC

Additional Information

Figures and Graphics

• Figure 1. Conventional wiring practices.
• Figure 2. System cabling approach—wiring interface.
• Figure 3. Interposing relays—digital interfaces.
• Figure 4. Signal conditioning—analog interfaces.
• Figure 5. Serial communication—serial communication interfaces.

Author Information
Arnold Offner is product manager for interface products at Phoenix Contact, Inc., and can be contacted at (800) 888-7388.