The industrial input/output connector has evolved from cumbersome barrier-strip connections to the sophisticated pluggable terminal block.
By Jonathan Minnick
Over the past several decades, the input/output (I/O) connectors for industrial devices such as programmable logic controllers (PLCs), motion controllers, and other industrial-control equipment have continued to get smaller and more user-friendly.
 FIGURE 1. The barrier-strip style of terminal block was an industrial design of the 70s and early 80s. |
In the first designs of industrial controls in the 70s and early 80s, the connections to I/O boards were big, cumbersome, and had limited design options. These early barrier-strip connections required nearly an inch between pins on the printed circuit board (PCB). In addition, many of these early designs required extra time for the customer to prepare the wire and crimp the terminal to the wire before termination (see Fig. 1). If high voltages were present, the designer was required to come up with some type of touch-proof protection hardware to ensure user safety. Finally, these early I/O connectors had limited, if any, means of labeling the wiring location, resulting in the need for additional hardware and creating possible wiring errors.
Thanks, Euro
Fortunately, in the early 80s, the Euro style of terminal block was introduced to the North American market (see Fig. 2). This new style or design of I/O connection device eliminated many of the problems inherent in the barrier-strip design. The designer could now save over 50% of the required PCB space by using a terminal block with a pin spacing of 0.2". In addition, this new device provided a touch-proof design that eliminated shock. It included, in most cases, a means of labeling or marking each wiring position, thereby making it installer-friendly. These features were all much welcomed as industrial electronics continued to shrink in size, as was the trend in other electronic products.
 FIGURE 2. The fixed “Euro” terminal block of the early 80s used 50% less space than the barrier strip. |
Although most major industrial-control device manufacturers received the pluggable, Euro-style terminal block with an immediate and rapid acceptance; by the mid 80s, other requirements and features were needed. Customers wanted to wire an I/O device while plugging and unplugging the wiring. Thus, a new pluggable Euro style of I/O-connector terminal block was designed to give the customer all of the features of the fixed wiring terminal block with the addition of modularity (see Fig. 3). As manufacturers had been scurrying to design and build products that were expandable and were modular, this new type of terminal block was the answer to many design challenges.
 FIGURE 3. The pluggable, “Euro”-style terminal block added the convenience of modularity. |
The idea of providing a pluggable terminal block for I/O connections improved the installation, the serviceability, and the upgrading and replacement of all types of control equipment. While not a major innovation for the connector industry in general, the pluggable capability was unique and offered significant enhancement to industrial equipment that previously used hard, fixed, point-to-point wiring technology.
To further advance in the industrial market, these pluggable terminal blocks next had to address the special environment and wiring conditions that existed in factories, processing plants, and manufacturing equipment. In response, designers added new features such as captive screws, polarization, keying, plated metal parts, dual-contact beams, and UL 94V0 housing material to meet the harsh requirements of this industry.
Board space
Once designers widely accepted the pluggable terminal block, other issues came to the fore. Like every other connection device and piece of electronic equipment, space became more of an issue with the connections to a PCB in a controller. In some cases, industrial instrumentation devices and I/O cards required wiring to exit the connector from different positions. Because the basic pluggable terminal block had already been proven, it was now just a case of designing to meet these ever-changing demands.
Over the past decade, the pluggable terminal block has evolved to address the issues of space saving through the use of double-level headers on the PCB, which enable one terminal block to be plugged on top of another. In addition, a whole new range of smaller pluggable terminal blocks with 3.5- and 3.81-mm pin spacing provided space saving on the PCB of up to 50%.
In addition, depending on the application, customers can now order pluggable terminal blocks with wire entry on the sides, on top, and with accessories such as screw locks to hold the plug to the header. As motion controllers, programmable logic controllers (PLCs), digital panel meters, and other devices become smaller, these new pluggable terminal blocks will meet the need for flexibility and space savings.
Over the years, there have been many advancements to the quality of the Euro terminal blocks due to the demands for high quality from manufacturers of industrial computer, PLCs, and motion controllers. The machined rising clamp design is the highest quality and most common Euro terminal used today (see Fig. 4). This type of connection method gives the user a larger surface area, therefore providing a more secure connection and higher conductive electrical properties.
 FIGURE 4. Two machined rising-clamp designs, fixed (left) and pluggable (right), arose to meet demands for high quality in the industrial market. |
While connectors have become smaller to meet the demands of industrial applications, they are also being used in different applications such as thermostat controls, fire-safety controls, and high-current applications due to higher quality housing materials. With a flammability rating of UL 94 V0, this high-quality housing material will self-extinguish in less than ten seconds, allowing it to be used in applications where the connector flame-resistance is crucial.
Hazardous substances
As the world becomes more environmentally conscious, so must the connector industry. In the past, it was common to use lead material in the plating process for the metallic components of the connectors, but now, due to the initiation of the Restriction of Hazardous Substances (RoHS) directive by the European Community, the use of hazardous materials has been virtually eliminated.
Like every other connector device, the terminal block used for I/O connections has evolved significantly over the past 20 years. What was once the only choice, a barrier strip, is now largely absent from modern control equipment. Whereas the designer was faced with few options, he is now in a position to address issues of space savings, international approvals, vibration, corrosion, high current or power, labeling, keying, polarization, and more, in a sophisticated device for I/O connections, the Euro-style, pluggable terminal block.
JONATHAN MINNICK is PCB terminal blocks product manager, Automation Systems Interconnect (ASI), PO Box 1230, Carlisle, PA 17013. Tel: (877) 650-5160; Email: jminnick@asi-ez.com.
