MAXREFDES176#: IO-Link 16-channel digital input hub

Advanced factory automation solutions (i.e., Industry 4.0) require an increasing number of smart sensors, which are typically controlled using IO-Link point-to-point serial communication between the sensor and controller (master). As a leading provider of IO-Link sensor transceiver and master transceiver ICs, Maxim Integrated® also provides complete reference design solutions to help our customers improve their time to market. These proven designs cover all of the hardware and software requirements needed for compliance with the IO-Link standard.

Programmable logic controllers (PLCs) and distributed control systems (DCSs) use more digital inputs than any other input signal type combined. Running wires in parallel back to the PLC for industrial digital inputs is not only expensive, but also creates a wiring rat's nest that is difficult to maintain. Field buses improved the situation considerably by moving the I/O interface from the PLC to a remote I/O hub mounted close to the field sensors. However, the lack of a single, universally accepted field bus has also created confusion, training challenges, high costs, and compatibility issues among equipment.

IO-Link is the first open, field bus agnostic, low-cost, point-to-point serial communication protocol used for communicating with sensors and actuators that has been adopted as an international standard (IEC 61131-9). IO-Link finally standardizes interoperability of industrial equipment from all over the world. IO-Link can function directly from the PLC or be integrated into all standard field buses, quickly making it the defacto standard for universally communicating with smart devices like the MAXREFDES176#.

Maxim Integrated and TMG TE collaborated to design the MAXREFDES176# reference design that is compliant with the IO-Link version 1.1/1.0 standard. The MAXREFDES176# design consists of an industry-standard MAX22515 IO-Link device transceiver, an Atmel ATSAM low-power microcontroller that uses the TMG TE IO-Link device stack, and a MAX22192 isolated octal digital input serializer daisy-chained with a MAX22190 octal digital input serializer to provide a total of 16 digital inputs that are IEC 61131-2 compliant. The complete reference design fits on a 72mm x 54mm printed circuit board (PCB).The MAXREFDES176# IO-Link digital input hub consumes minimal power, space, and cost, making it a complete solution for the many digital input sensors found in various industrial control and automation applications.

The MAX22515 IO-Link device transceiver is compliant with the IO-Link version 1.1/1.0 physical layer specification. It integrates the high-voltage functions commonly found in industrial sensors, including drivers, and two linear regulators, all in a tiny 2.5mm x 2.0mm WLP. The MAX22515 features extensive integrated protection to ensure robust communication in harsh industrial environments. All four I/O pins (V24, C/Q, DO/DI, and GND) are reverse-voltage and short-circuit protected and feature integrated ±1kV/500Ω surge protection. This enables a very small PCB area with no required external protection components, such as TVS diodes. The low on-resistance driver (C/Q) further reduces power dissipation so that this reference design consumes minimal power with very low thermal dissipation. Operation is specified for normal 24V supply voltages up to 36V. Transient protection is simplified due to high voltage tolerance (i.e., 65V absolute maximum rating for the I/O pins without the integrated TVS diodes) in addition to the integrated surge protection.

The two integrated LDO regulators in the MAX22515 generate 3.3V and 5V, reducing the number of additional external components and the required space.

The MAX22515 features a flexible control interface through either an SPI or I2C interface. In this design, we use I2C to reduce the number of pins required by the microcontroller. The I2C (or SPI) interface provides extensive diagnostics for the MAX22515, and a 3-wire UART interface is provided for IO-Link communication.

The MAXREFDES176# does not require external protection devices such as varistors or TVS diodes due to the integrated surge protection in the MAX22515 at the IO-Link interface. This reference design meets both IEC 61000-4-2 for electrostatic discharge (ESD) up to ±4kV and IEC 61000-4-4 for electrical fast transient (EFT) ±4kV standards. It is designed to meet a surge capability (2A at t = 1.2/50μs) up to ±1.0kV.

The MAXREFDES176# uses an industry-standard M12 connector, which allows a 4-wire cable to be used.

The MAX22192 is an IEC 61131-2 compliant industrial digital input device with integrated digital isolation. The MAX22192 translates eight 24V, current-sinking industrial inputs to an isolated, serialized SPI-compatible output that interfaces with a 1.71V to 5.5V logic voltage. A current-setting resistor allows the MAX22192 to be configured for Type 1/Type 3 or Type 2 inputs. The MAX22192 has an isolation rating of 600VRMS for 60 seconds and is available in a 70-pin GQFN package with 2.3mm clearance and creepage.

The MAX22190 is also an IEC 61131-2 compliant industrial digital input device, but without integrated isolation. The MAX22190 translates eight 24V, current-sinking industrial inputs to a serialized SPI-compatible output that is daisy-chained with the MAX22192. A current setting resistor allows the MAX22190 to be configured for Type 1/Type 3 or Type 2 inputs.

A LATCH signal on each digital input chip allows for synchronizing input data across both devices.

The Atmel microcontroller limits the reference design operating temperature range to -40°C to +105°C. However, the MAX22515 IO-Link transceiver, MAX22192, and MAX22190 can all operate over the -40°C to +125°C temperature range.

The MAXREFDES176# consumes 30mA (typ). A green LED indicates the 3.3V alive signal from the MAX22515, and 16 green LEDs are used to indicate the status of each digital input.

For detailed information on the digital input devices please refer to MAX22190 and MAX22192 data sheets.
Non-Isolated vs. Fully Isolated Mode of Operation

The MAXREFDES176# can be operated in two different isolation modes depending upon whether RI1 and RI2 (both 0Ω resistors) are installed (non-isolated) or not (fully isolated). In non-isolated mode, power (+24V) is provided from the IO-Link master through pin 1 of the X1 connector. In fully isolated mode, two power domains and two power sources are required. For the "IO-Link Side" (as seen in Figure 1), the IO-Link master provides power (+24V) through pin 1 of the X1 connector. For the "Field Side," an external 24V supply is required, which is connected through terminal T7.

The MAX17621C current limiter is used to provide overvoltage (OV) and reverse-voltage protection for the external supply, blocking any current flowing in the reverse direction and providing protection for the circuit. In this reference design, a SMAJ33CA TVS was included for surge protection.
Detailed Description of Firmware

The MAXREFDES176# ships preprogrammed as a working IO-Link device ready to connect to an IO-Link master. The firmware targets an Atmel ATSAM microcontroller and follows the simple flowchart shown in Figure 2. The firmware utilizes the TMG TE IO-Link device stack. After plug-in, the MAXREFDES176# waits for a wake-up signal from the IO-Link master. Once the wake-up signal is received, the MAXREFDES176# synchronizes to the IO-Link master using the 230.4kbps (COM3) baud rate, and communication parameters are exchanged. The IO-Link master then starts a cyclic data exchange by requesting the sensor process data. If the MAXREFDES176# is removed, the IO-Link master detects a missing sensor.