Analog Infinity Doesn’t Compute

With an A/D Converter, the Analog Signal is Approximated into a Fixed Number of Slices. The More Slices, the Higher the Resolution of the Converter

By Mike Bacidore, managing editor

At THEIR basic level, I/O signals indicate the coming and going of data from sensors, switches, actuators, transmitters, controllers and other devices. “There are DIN-rail-mountable digital and analog I/O terminals that facilitate these signals,” says Skip Hansen, I/O systems product manager at Beckhoff (beckhoff.com). “A switch, because it’s an on/off device, is a digital device. A transmitter is an analog device because it can output numerous values.”

A transmitter tracks precise measurements for pressure or temperature, adds Hansen. “A core sensor needs an analog-to-digital converter to measure and output the values. The output data then is sent via PLC to the central processor for logic control, or the data could be multiplexed with other I/O signals and sent via a bus coupler to a master controller through one of the dozens of available industrial-use, fieldbus protocols.”

Fundamentally speaking, the two basic types of remote measurement and actuation signals are analog or discrete. “Analog signals are measurements or actuations that conceptually have an infinite number of values in their range,” says Mike Berryman, market development manager at Advantech (advantech.com).

For example, if a pressure transmitter has a measurement range of 0–100 psig, it could transmit any value within that range. In practice, however, the usable values can’t be infinite given the need to digitize the signals  for use in data acquisition and control, explains Berryman. “Using an A/D converter, the analog signal is approximated into a fixed number of slices.” The more slices, the higher the resolution of the converter, so the more accurate the measurement. Converter resolution is specified in bits. A 16-bit A/D converter divides an analog signal into 65,536 slices.

“Since we are talking about using a network to transmit this information, it might be necessary to send the 16 bits in smaller sections,” explains Helge Hornis, intelligent systems manager at Pepperl+Fuchs (pepperl-fuchs.com). This decision will depend on the network. Some networks send a lot of data at one time, and the analog value can be transmitted easily in one operation. “Unfortunately,” cautions Hornis, “when this same network is used to transmit the state of a single discrete sensor (1 bit), it wastes a lot of bandwidth—all the other bits in the frame are still shipped, but don’t carry any useful data. Ethernet is that kind of network.”
Some networks have very small data packets, and must use multiple packets to send the 16-bit value, explains Hornis. “This takes extra time, but if these networks are used with sensors, they operate very efficiently. AS-Interface (AS-i) is an example of that.”

In general, Hornis suggests users ask a few questions when selecting a network. Is this network mostly used for sensors or analog values? Are the analog values fluctuating rapidly? Does the network allow resolution to be tweaked so that update speed is optimized?

AS-i transmits 4 bits of data with each packet. As a result, the analog value must be transmitted over several network scans. “In this case, the designers decided not to send this data one packet after another, but interspersed data packets from the other I/O nodes on the network,” says Hornis. “The advantage of this is that digital I/O isn’t slowed down by analog data. More specifically, transmitting a 16-bit analog value can take anywhere from one full network scan to a maximum of seven scans. This is great news for the user, as it’s easily possible to pick the right device for the job. For the user, the details of how data is transmitted is unimportant. The controller understanding the profile knows how to put the data back together, so it can be used by the PLC.”

Hornis explains the fastest way to transmit a 12-bit analog value is by using three network addresses, each sending 4 bits.

“With digital I/O, you’re dealing with TTL-level logic—that is, discrete signals—between the module and the controller,” says Tom Edwards, senior technical advisor at Opto22 (opto22.com). “This is a very direct, high-speed connection.”

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