Configurable I/O

Not too long ago, a senior engineer from one of the nearby chip companies was holding a Soft-I/O module in his hand and studying it. He looked up and said, “This is a magic box! It can do anything.”

Of course, this made us feel really good. He got it. He understood that—compared to any other I/O module or any PLC in the world today—Soft-I/O is unique.

One of the things that led our engineer friend to proclaim Soft-I/O the magic box is that Soft-I/O is “configurable”. There are a lot of input/output or I/O products in the world, but Soft-I/O is unique among them. Programmable Logic Controllers (PLC’s or PAC’s) are ubiquitous, but every one possesses a rigid, fixed format. Let’s look at how I/O modules work to appreciate the contrast with Soft-I/O. We offer a word of caution here: this is a pretty lengthy discussion, and you may find it a bit dense for casual reading. In fact, this is a 30,000-foot introduction to the fairly complex issue of configurability. To do it justice, we would require far more of your time. As always, we ask you for your feedback!

The Basics

First, the basics. Sensors go in to the control system, and actuators—the action of the control system—go out. That’s input/output or I/O. Electronic I/O systems have been around for more than 50 years in every factory and large building. If we look at the electrical properties of sensors and actuators, we see a very wide range. There are tens of thousands of sensors and actuators, many with unique electrical formats.  Some of these electrical formats are on/off types--which we call digital I/O--and some are continuously variable--which we call analog I/O.  Sensors that measure temperature--for example thermocouples--are generally analog whereas sensors that detect the presence of objects--for example switches--are digital.  To keep things simple in this overview, we will focus on Digital I/O.

Configurable Digital I/O

One of the most common digital sensors is a proximity sensor. In a modern semiconductor fabrication plant, you might literally count a million of them. Proximity sensors often sense the presence of metal, so they are good at verifying that doors are closed or latches sealed. They produce an on or off signal to the control system. A proximity sensor has a transistor in its output stage that can be configured in two basic ways: there is no standard since there are benefits to each configuration. We say that some of these "source" current and and some "sink" current. Well, guess what? The input system hardware needs to be different for the sourcing and sinking switch. Actuators which provide the action or output of the control system have a similar plethora of formats. A contactor to turn on a pump might require a very different interface than a valve to control pressure in a diaphragm. The list of sensors and actuators goes on and on, making connecting sensors and actuators to computers a task requiring skill and knowledge. Oh, and a lot of different hardware modules. Let’s look at some of them.

Figure 1 shows a generic digital or on/off sensor. It could be sensing anything, such as proximity of a metal latch. Or the sensor of Figure 1 might be a pressure switch that closes when a given process pressure is reached. For this discussion, we don’t care what the sensed value is. Our focus is on the interface of that sensor to the I/O system.

The sensor of Figure 1 has three terminals labeled Power+, Power- and Output. To hook this sensor up to your I/O system, you connect power and ground. Although that may sound simple, we will show that the I/O industry does not make it simple for you. Then you hook the output of the sensor to an input on your I/O system, right? Well, we say, “Maybe.” Note that the output terminal is connected to the emitter of the final stage transistor in the sensor. That means that when the sensor causes the “turn on”, the final stage transistor conducts and current is sourced to the output terminal.

No problem, you say, you merely connect the sensor of Figure 1 to the input circuit of Figure 2. What could be simpler? The circuit of Figure 2 expects that its input will source current. "No problem" becomes "Yes, problem" when we try to connect the sensor of Figure 3 to the input module of Figure 2. The sensor of Figure 3 has a so-called sinking output circuit because the output terminal is connected to the collector of the output transistor.

Hooking the sensor of Figure 3 to the input module circuit of Figure 2 will not work. Countless people over the past 50 years have made that mistake. They ordered the wrong sensor or the wrong input module and then were frustrated to see that they would not work together. That’s a fact of life, pre-Soft-I/O. Now we look at Figure 4 which is an input circuit designed to operate with a sensor configured with a sinking output stage. When we connect the sensor of Figure 3 to the input circuit of Figure 4, it works.

Some Examples

The purpose of this note is to contrast the old way with the new way. In order to show the benefit of configurability, we will hook up one each of these sensors to a conventional I/O system such as the one you would find on any PLC or PAC. We will then hook up the same sensors and actuators to Soft-I/O and show the benefits of the configurable connector of Soft-I/O. The result is a dramatic reduction in labor, panel space and hardware parts. In order to make the complexity more manageable, we will do the old-style I/O system in pieces. Otherwise it will be too complex for us to describe. We begin with Figure 5 where we hook up one sourcing sensor and one sinking sensor to on old-style I/O system.

We need two input modules, one for sinking sensors and one for sourcing sensors. If you were thinking that you just “hooked the sensors up” to the input modules, you would be sorely disappointed. Instead, you have to add a second power supply which we call the Device Power Supply. "Why do I need a second power supply?" you ask. Well, it’s a long story, but if you don’t believe us, just search online for I/O modules and see sample hookup examples. Note that they won’t show the detail that we show here because it’s so ugly.  They will only show you small examples without all the real connections.

Back to Figure 5. We hook up the Device Power Supply to the two sensors and then to the input modules because we need to reference the input modules to the power supply. Finally, we hook up the one wire from the sensor to the input module, being careful to hook the sourcing sensor to the sourcing module. In Figure 6, we hook up two actuators to two output modules. One actuator is designed to be driven by a sourcing output module while the other is designed to be driven by a sinking output module. So, the drill is the same for the outputs as the inputs. We hook up power, reference the modules and then hook up the actuators to the proper output module. When we put the old style system together, we have Figure 7.

The old-style I/O system of Figure 7 has four I/O modules, a Device Power Supply and a number of terminal blocks. There are no connectors used because everything is custom wired. Before we leave Figure 7, we ask that you look at I/O module utilization. What percentage of the I/O modules channels are used? The answer is 12.5%. Believe it or not, this low utilization is a major reason that old-style I/O systems tend to be centralized with long wire runs of large cable bundles. In the modern world, we think that centralized I/O is backwards. You will see that Soft-I/O encourages a distributed architecture.

The Soft-I/O Solution

Now, let’s look at Figure 8 in which we have hooked up a single Soft-I/O module to all the previous devices. And, oh, yeah, we threw in a thermocouple just to show how much flexibility we have ready. (Figure 8 shows only 15 pins whereas Soft-I/O has 25 pins or 11 extra for you to work with!)  Please note a few things about configurability. We just start with pin number one and hook up a wire. For a three-wire sensor, that’s three wires and three Soft-I/O pins. Notice that we did not distinguish sourcing from sinking. Soft-I/O can handle either. You just configure Soft-I/O. Now, where’s the Device Power Supply that we had with the old-style PLC I/O? Soft-I/O provides the device power. Soft-I/O can supply up to 100 Watts of power on its pins. Any pin can produce ½ amp of 24VDC. Or ½ amp of 5VDC. So, there is no second power supply and no tedious wiring. Note how simply the devices hook up to Soft-I/O. We put the first three-wire sensor on pins 1, 2 and 3. We could have chosen pins 5, 10 and 15. It does not matter because all pins are created equal.

With our sourcing sensor hooked up, we proceed to our sinking sensor.  Just pick out three more pins and then configure the Soft-I/O Soft-Device for a three-wire sinking sensor. Done. Next, we go to the actuators and hook them up. Sourcing and sinking are simply hooked up to more pins. Finally, we gently rub it in to the PLC guys. Many PLC’s cannot handle a thermocouple. Those that do will have a module that you can purchase for perhaps $500. You will need an empty slot in your PLC backplane and then install the thermocouple module. With Soft-I/O you simply pick two pins and hook up the thermocouple. Soft-I/O supports all popular thermocouple types including J, K, N, T, S, R, E and B.  (Oh, and it gives you the actual temperature--not some hex code--in degrees C, F or Kelvin.  But that's another story for another time.)

Note that we used 12 Soft-I/O pins out of 25 pins to hook up all four devices that the old-style I/O struggled with. We used two more for the thermocouple. We have 11 more for any other sensors or actuators. In summary, configurability is something that Soft-I/O invented. Old-style input/output systems are fixed configuration requiring that your wiring conform to its rigid structure. Power is not available at the input/output module. When we compare configurable Soft-I/O to the fixed-configuration old-style I/O, this is why we say that XiO reinvented Input/Output.