Tiny IO-link transceivers simplify industrial automation interface designs
Many of today’s sensors and actuators used in industrial automation consist of electromechanical switches whose signal status is made available to remote I/Os of system controllers that communicate via fieldbus networks. Connection to a sensor consists of a three-wire cable providing a line for the switching signal and 24V and 0V supply lines (Figure 1).
Gradually, these switches are supplemented with sensor types that use optical, inductive, capacitive, transformer and other physical effects to allow for no-contact operation and simpler adjustment. This trend is supported through the availability of miniature, low-cost microcontrollers.
Until recently, this three-wire connection technology neither supported the use of engineering tools or controllers to parameterize these flexible, high-performance sensors, nor the display of important diagnostic messages. With the introduction of IO-Link, however, the availability of UART interfaces in most microcontrollers has driven the need for a new IO-Link transceiver (Figure 2). This new transceiver enables a more intelligent design of the last meter of communication link between sensor and controller, or fieldbus node.
(*See Figure 1 & 2)
This article gives an overview of IO-Link, discusses the functional operation of an IO-Link device transceiver, and presents a solution to protect space-sensitive IO-Link designs against powerful surge transients.
IO link system
IO-Link is a smart concept for the standardized connection of sensors and actuators to the control level by means of a low-cost, point-to-point interface. The new IO-Link communication standard below the field bus level permits central diagnosis and location of faults as far as the sensor/actuator level (Figure 3). Commissioning and maintenance are drastically simplified by allowing parameter data to be modified dynamically directly from the application.
(*See Figure 3)
The increasing intelligence of field devices (sensors and actuators) and their integration into the overall automation system permit access to data right down to the lowest field level. The result is higher plant efficiency and reduced engineering cost.
An IO-Link consists of the following components:
1. the IO-Link devices, such as sensors and actuators equipped with microcontroller and transceiver,
2. an IO-Link master comprising a dedicated transceiver and a powerful host controller, and
3. a three-wire cable for positive supply, ground and signal transport, representing the actual link.
The master can have one or more ports, which, per point-to-point communication link, either connect to proper IO-Link devices, or standard I/O devices. This flexibility requires the master to be able to detect the difference between the two device types.
Communication takes place at one of the three kbaud rates: 4.8, 38.4, or 230.4. At the beginning a device is always in the standard I/O (SIO) mode. The corresponding master port, however, might have a different configuration. If this port is set to SIO mode, the master acts like a digital input. If the port is set to communication (COM) mode, the master tries waking-up the connected IO-Link device.
During wake-up the master sends a defined signal and waits for the device to respond. The master then tries addressing the device at the highest defined baud rate. If unsuccessful, the master switches to the next lower baud rate. At every baud rate, the master tries addressing the device three times. When the master receives a response, the device is woken up and communication between the two begins.
At first, they exchange the communication parameters before they start exchanging cyclic process data. The efficiency of IO-Link becomes obvious when a device is removed during operation. In this case the master detects a communication abort and reports it to the control system. It then tries to cyclically wake up the device. After another successful wake-up communication, parameters are read out again and validated, and the exchange of cyclic data starts again. When the master terminates communication, master and device return to the standard IO mode, also known as fall back.
Three types of data are exchanged: cyclic or process data, acyclic or service data, and events. In general a device only sends data when requested by the master. While acyclic data and events are explicitly requested by the master, cyclic data are sent after a master’s IDLE telegram.
Device process data are sent cyclically within one data frame, if the data width does not exceed two bytes. Otherwise the data is split into several frames and sent in multiple cycles. If the process data is invalid, the master generates a diagnostic message.
Service data include states and parametric values sent by the device as well as parametric values and commands sent by the master. Thus, service data are used to configure a device or to read its status. Process and service data are transmitted in one or more telegrams. Figure 4 shows a typical data exchange.
(*See Figure 4 & 5)
Events such as contamination, overheating, or short circuit trigger the device to set an event flag (bit 7 in the CHK/STAT byte) of a process telegram. The master detects the active flag, reads out the event, and reports it to the programmable logic controller (PLC). During an event read out, no service data can be transmitted.
Data transmission across an IO-Link packs each byte into a standard UART frame (Figure 5). This frame consists of a start bit, eight data bits, one parity bit and one stop bit. Inverse logic is applied meaning a logic 0 causes a voltage of 24V nominal on the C/Q line with respect to data link ground (L–), while a logic 1 yields 0V on C/Q with respect to L–.
An IO-Link device comprises a transducer or physics to digital converter and the device transceiver (Figure 6). When the device is connected to an IO-Link master through the three-wire interface, the master can initiate communication and exchange data with a remote node with the IO-Link transceiver acting as a complete physical layer for the communication.
(*See Figure 6 & 7)
Figure 7 shows that the device driver output (CQ) can be used in push-pull, high-side, or low-side configurations using the enable (EN) and transmit data (TX) input pins. The internal receiver converts the 24-V IO-Link signal on the CQ line to standard logic levels on the receive data (RX) pin. A simple parallel interface is used to receive and transmit data and status information between the slave and the local controller.
IO-Link transceivers commonly implement protection features for overcurrent, overvoltage and overtemperature conditions. They also provide a current-limit setting of the driver output current using an external resistor. If a short circuit (SC) occurs, the driver outputs are internally limited, and the slave generates an error signal.
The transceiver also possesses an overtemperature shutdown feature that protects the device from high-temperature faults. A modern transceiver can operate either from an external 3.3-V or 5-V low-volt supply, or derives the low-volt supply from the IO-Link L+ voltage (24V nominal) via a linear regulator, to provide power to the local controller and sensor circuitry.
Wake up detection
The device may be in IO-Link mode or SIO mode. If the device is in SIO mode and the master node wants to initiate communication with the device node, the master drives the CQ line to the opposite of its present state. It then either sinks or sources a wake up current of up to 500mA for a duration of typically 80 microseconds.
The device transceiver detects this wake-up condition and communicates to the local microcontroller via the WAKE pin. However, for over-current conditions shorter or longer than a valid wake-up pulse, the WAKE pin remains inactive,
or an high-impedance state (Figure 8).
a) Over-current due to transient
b) Wake-up pulse from master
c) Over-current due to fault condition
(*See Figure 8)
Current limit indication, short circuit current detection
If the output current at CQ remains at the set current limit for a duration longer than a wake-up pulse, the CUR_OK pin is driven logic low, indicating an over-current condition. The CUR_OK pin turns inactive when the CQ pin is no longer in a current limit condition. The state diagram shown in Figure 9 illustrates the various states and under what conditions the device transitions from one state to another.
(*See Figure 9 & 10)
If the transceiver’s internal temperature exceeds its over-temperature threshold, the CQ driver and the voltage regulator are disabled. As soon as the temperature drops below the temperature threshold, driver and regulator are re-enabled.
CQ current-limit adjustment
The CQ driver current-limit is determined by the external resistor at the LIMADJ pin. Figure 10 shows the typical current-limit characteristics as a function of RSET.
Over-voltage and reverse polarity protection
Reverse polarity protection is included in the device. Any combination of voltages between 0 and 40V may be applied at the pins L+, CQ and L– without causing device damage. For protection against high-energy transients, such as 1 kV surge transients, external protection devices must be added (Figure 6).
The surge test is performed with a 40ohm resistor in series to the surge generator’s source impedance of 2ohm. Thus, a 1 kV test voltage yields a short-circuit output current of 1 kV – 42 = 24A. This current must be shunted to ground (L–) by a transient voltage suppressor (TVS) whose clamping voltage is less than the transceiver’s transient stand-off voltage of 50V.
This challenging task can be accomplished with 3 kW TVS diodes in SMC package. Each line L+ and C/Q receives a TVS connected to L– for diverting normal surge transients to ground. For inductive loads on C/Q, a third TVS is recommended between the L+ and the C/Q lines.
IO-Link is the first worldwide standardized IO technology (IEC 61131-9) for communicating from the controller to the lowest level of automation. The interface is a fieldbus-independent point-to-point connection that uses an industrial three-wire cable to transmit sensor signals to the controller and, conversely, relays control data to the sensor/actuator level.
In order to ensure the requirements for over-current and over-voltage protection, defined in the IO-Link specification, legacy designs of IO-Link devices required a large number of individual components.
The SN65HVD101 and SN65HVD102 are examples of device transceivers from Texas Instruments that offer designers of IO-Link-capable sensors/actuators a highly integrated and space-saving alternative by providing up to 500mA drive capability in a tiny, 4mm2, 16-pin QFN package.
* Download the SN65HVD101/2 data sheet, SLLSE84A, March 2013
* IO-Link Overview
* IO-Link Interface and System Specification V1.1.1