Single-chip MCUs support low-cost data acquisition

In many manufacturing environments, it is desirable to have a data acquisition (DAQ) system to monitor the network remotely for troubleshooting or control purposes. To facilitate this, many manufacturing applications tend to rely on a proprietary interface and protocol to provide a status or diagnostics port to deliver the information.

By Nate Smith October 1, 2006
A single chip Ethernet microcontroller contains all the functions to drive DAQ in one device.

In many manufacturing environments, it is desirable to have a data acquisition (DAQ) system to monitor the network remotely for troubleshooting or control purposes. To facilitate this, many manufacturing applications tend to rely on a proprietary interface and protocol to provide a status or diagnostics port to deliver the information. However, implementing a DAQ system based on a standard interface—such as the Ethernet and TCP/IP protocol—reduces both development time and cost.

Deployment of Ethernet connectivity in industry is becoming a popular trend. According to data from research company ARC, there are now three to four million industrial Ethernet-enabled devices in Europe alone, with a growth rate that exceeds the IT average. An Ethernet-based DAQ system is very convenient for remote maintenance, because it overcomes distance limitations of traditional serial interfaces. In addition, a plant using machinery from many vendors can be integrated using an Ethernet-based DAQ system into a single system for easy control.

A typical Ethernet-based DAQ system communicates over Ethernet to the central office at one end, and communicates over Universal synchronous/asynchronous receiver/transmitter (USART) and serial interfaces from sensor subsystems at the other end. For monitoring and troubleshooting in manufacturing, the non-deterministic nature of Ethernet is not an issue.

(Ethernet is constantly evolving, and efforts are underway by the IEEE standards committee to improve Ethernet determinism. With the new precision time protocol (PTP) IEEE1588 standard, it is now possible to synchronize the local clocks in sensors, actuators and other modules using the same Ethernet network that also transports the process data.)

For monitoring and troubleshooting in manufacturing applications, a 10 Mbps data rate is more than adequate. For example, discrete manufacturing uses hundreds of programmable logic controllers (PLCs) that rely on serial links based on RS-485 to exchange data at rates of only a few kbps. In a DAQ system involving the Foundation Fieldbus H1, the Ethernet communication operates at 31.25 kbps.

Three deployment methods

There are basically three methods for deploying Ethernet in DAQ system applications. First, you could use the old PCI/ISA controllers, which feature 80-plus pins interfacing to 8-bit microcontrollers (MCUs). This PC-based solution tends to be complex, occupies a large footprint and can be expensive.

Second, you could consider deploying a serial interface-based, low pin count Ethernet controller. While most Ethernet controllers come in 80-pin packages or greater, there are only a few that offer comparable features in a 28-pin package at very low cost, which simplifies the design and reduces printed circuit board space. Additionally, this class of Ethernet controllers employs the industry-standard serial peripheral interface (SPI) connection, which only requires four lines to interface to a host MCU.

Third, if you prefer a single-chip solution, you could consider the latest low-cost 8-bit MCUs which feature an on-chip medium access controller (MAC) and physical layer device (PHY) optimized for embedded Ethernet applications. Designing Ethernet-based DAQ subsystems using these MCUs is a good fit for factory applications, when you need to get a few signals (I/O) onto the Ethernet network for monitoring and control.

The DAQ system is strategically placed where it can monitor individual devices and communicate with higher level control systems.

These DAQ subsystems typically feature an Ethernet port, optically isolated digital inputs, analog inputs, relay and analog outputs. They also feature serial interfaces like RS-232 and/or RS-485 ports. Some modules may even feature a sensor port.

To be able to handle large amounts of I/O, you can daisy chain a few of these subsystems and minimize wiring costs. Using software, you can schedule tasks to run on both time- and event-triggered actions to monitor plant machinery. This approach can be quite cost-effective compared to alternative solutions such as supervisory control and data acquisition (SCADA) systems, PLCs or distributed control systems (DCSs) that require separate Ethernet controllers, I/O cards, racks, power supplies, cables and software.

Connectivity included

Most sensor subsystems that connect to a DAQ system already incorporate some type of serial connectivity, usually RS-232, RS-485, I2C or CAN, from which information about system operation can be obtained. Depending on the application, your Ethernet-based DAQ system design can be as simple as a probe based on an MCU with an on-chip Ethernet controller, which then interfaces to the sensor’s serial port and communicates with the various sub-systems to obtain status information. This data is sent over the Ethernet for further processing

In the other direction, depending on the action required, commands are sent over the Ethernet into the DAQ system for activating sensors and subsystems, using low-speed serial interfaces. Automation engineers can use this probe design to develop an Ethernet-based DAQ system, or retrofit existing plant machinery into an Ethernet-based DAQ system for monitoring and supervisory applications.

TCP/IP stack firmware

Another important consideration in the design of an embedded Ethernet device is TCP/IP stack firmware. An Ethernet module alone only supports the processing of Ethernet packets. Higher layer protocols like IP, TCP, and UDP must be supported in firmware for the device to be able to communicate with the rest of the world. Some silicon manufacturers offer the TCP/IP stack for free, while others rely on third parties that charge an upfront fee and/or royalty.

Today, many 8-bit microcontrollers are available in 64- to 100-pin packages and come with 128 kilobytes of Flash and 4 kilobytes of SRAM, which is ample code space to house a TCP/IP stack for a Web server in your application. When compared to available multi-chip Ethernet offerings in the market, an integrated single-chip MCU DAQ system solution offers a 68% space reduction and cost savings around 50%.

In the past, engineers would devote considerable effort to develop this control interface, its protocols, and the resulting software to display status information on a PC or handheld device. Instead, you can simply take advantage of a tried and tested, free TCP/IP stack from a microcontroller vendor for easy connection to the Internet. Then, using suitable application software, you can deploy an Ethernet-based DAQ system for diagnostic or monitoring purposes of manufacturing systems.

For more information :

Author Information
Nate Smith is product marketing manager for the Advanced Microcontroller Architecture Division of Microchip Technology Inc. Microchip Technology is a leading supplier of analog and microcontroller semiconductors, including the PIC18F97J60, a single chip Ethernet microcontroller.