When Is a Digital Output Not Just a Digital Output? When It’s Also a Digital Input!

July 30, 2019

Michael Jackson  By: Michael Jackson
 Principal Writer, Maxim Integrated 


When you hear about Industry 4.0, do you envision robots busy at work inside factories, picking and packing products in a warehouse or manufacturing cars? Indeed, Industry 4.0 defines the modern factory environment. It consists of dozens of networked controllers continuously monitoring inputs from hundreds or even thousands of sensors such as switches and level detectors. Simultaneously, signals are sent to a similar number of output devices such as valves, solenoids, or motor drives. Electronic marshalling has simplified the process of connecting this expanding set of field wiring back to the controller. However, problems can still occur. Let’s have a look at how these can happen and show you a quick and easy way to resolve the issues if they occur.

Automated factoryFigure 1. In automated factories like this one, electronic marshalling provides an efficient technique for signal routing.

Wired Marshalling Creates Wiring Complexities

Until recently, the standard way of connecting field I/O devices to a programmable logic controller (PLC) has been through traditional wired marshalling as illustrated in Figure 2. Multi-core cables run wires from the field devices located on the factory floor to the terminal blocks of marshalling panels, usually located in an I/O room. Here, the wiring is cross-marshalled so that each field device is connected to the I/O card of the appropriate controller channel.

Wired marshallingFigure 2. Wired marshalling

This approach has the potential to cause problems. For example, during cross-marshalling, it is difficult to keep track of where the wires are coming from and going to, leading to errors if wires are incorrectly connected or even left completely unconnected. Debugging and testing each connection can be time-consuming and laborious for technicians and engineers alike, potentially introducing costly delays to the commissioning of a new process. In theory, once debug is complete, the system should run correctly, but additional problems can emerge if unforeseen changes are required late in the project. Sometimes it may be necessary to add a new field device. For example, if a temperature switch is changed to a temperature transmitter, then a digital input will need to be changed to an analog input. An even worse situation occurs if new field devices are added to the system, but the marshalling panel does not have enough spare connections of the required type to accommodate them. In this case, the controller would need to be replaced, potentially adding additional cost and delay to the project.

Electronic Marshalling: A New Signal-Routing Approach

Wired marshalling is gradually being replaced by electronic marshalling, a new approach to signal routing in process automation (Figure 3).

Electronic marshallingFigure 3. Electronic marshalling.

The electronic marshalling technique was developed to prevent the human errors associated with the manual element of wired marshalling, namely the cross-connection of the I/O devices on the marshalling panels. As with wired marshalling, the multi-core cables from the field are routed to the right side of the terminal blocks in the marshalling cabinet by technicians on the factory floor. However, in the I/O room, there’s no longer any need to manually connect each terminal block to the appropriate controller I/O channel, as this is handled electronically within the system itself. The clear advantage of electronic marshalling is that an I/O device can be connected to a specific controller whenever necessary without physical wiring changes. If at a later stage in the project, changes are made to I/O types, or additional devices are required, then no changes are needed to existing wiring or cabinets. In addition, extra I/O capacity can be added to the marshalling cabinets and then electronically marshalled to the controllers as required. At the heart of the electronic marshalling approach is a rack of portable and replaceable modules or cards. An appropriate card type is inserted into the slot to which the wiring for an I/O field device is connected. For example, a digital input (DI) card would be placed in the slot for a temperature switch. The card would then connect to the appropriate channel of the controller. The function of each controller channel is defined by the type of card (for example, DI or digital output (DO)) placed in each slot.

Flexibility Limitations with Electronic Marshalling

While the flexibility offered by electronic marshalling is obvious, there is a not-so-obvious inherent inflexibility. Traditionally, industrial and process control engineers have used the term “digital IO” to refer to digital signals transmitted and received by PLCs. However, the term itself is something of a misnomer. There is no such thing as a “digital IO” channel on a PLC. There exists either a “digital input” channel or a “digital output” channel. Thus, if it’s necessary to change the functionality of a controller channel from a DI to a DO, or vice versa, the physical card for the channel must be changed. Also, the total number of DI and DO channels is defined by the number of each type of card in the rack. This places limitations on the flexibility of the electronically marshalled system by fixing the number of DI channels and DO channels in the rack.

Channel Configuration Flexibility

Clearly, a more desirable scenario would be to configure each channel as either a DI or a DO as needed. The great news is that this is now possible with the MAX14914 high-side switch with digital input (Figure 4). With the MAX14914, the PLC can configure each card to function as either a DI or a DO. A card does not need to be manually removed and reconfigured if the functionality of the channel changes. Control channels can truly be designated as “digital IO” channels, without limiting the number of each type of channel. The only limitation will be the number of channels the PLC itself can handle. Say goodbye to those nasty rewiring jobs!

MAX14914 block diagramFigure 4. High-side switch with settable current-limiting, push-pull driver option, and digital input configuration.

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