Elo Touch Solutions

Touchscreen Component Compatibility: Five-Wire Resistive

The use of touchscreens and touch systems is growing at a tremendous rate. Touch applications appear in an increasing variety of applications - from the medical field, to industrial machine control, to subway ticketing machines - all fueled by a steady decrease in hardware costs. At the same time, the number of companies meeting these growing market demands has expanded, each offering a variety of technologies and solutions. In addition to the well-known market leaders, an army of smaller companies has sprung up to service niche markets - and offers a potential alternative to the market leaders.

Increased competition has always driven the technology market, leading to improved technologies at a lower price. These developments are welcome; indeed, with increased choice come also increased product overlap, and a seductive option to mix and match components from various sources in order to develop a system at a better price. The biggest incentive for mixing components is lower cost. This paper looks at the technical and commercial issues that must be considered when mixing touch components, to enable informed and balanced decision-making.

A number of different touch technologies are currently available. The resistive touchscreen market is generally split between four- and five-wire systems. The four-wire technology is geared toward hand-held consumer devices with a relatively short product lifetime; the five-wire technology, toward the industrial, point-of-sale, and medical markets, in which reliability and long lifetimes are essential requirements. This paper focuses on the five-wire resistive technology.

The Necessary Components

Three key components make up a touch system: the touchscreen, the controller, and the driver software.

Touchscreen

The touchscreen is the face of a touchsystem and the user's first contact point with the system. Its importance cannot be overstated, since it defines the quality and tactile feel of the touch system, and offers the only user interface. Key functional properties of the touchscreen are its optical transparency, its hardiness to wear and tear, and its touch accuracy. In all these areas, five-wire technology excels.

Controller

The controller - essentially the brain of the touch system - contains a microprocessor, analog-to-digital converters, and microchips to enable communication with the host PC. The controller powers the touchscreen, controls the excitation, and interprets the information received from the touchscreen. The controller filters the returning touchscreen data and converts it into raw touch coordinates, which are then sent to the PC by a digital software protocol. A good controller will also perform substantial error-checking to detect abnormal or inconsistent touches and filter them out.

Driver Software

The driver software, residing on the host PC, is required to manage the raw coordinate data coming from the controller, apply calibration algorithms, position the mouse cursor, and generate mouse clicks. Other important tasks include routines to define the video alignment parameters, and screening of incoming touch data for errors, inconsistencies, and integrity. Good driver software will also offer diagnostic information in troubleshooting situations.

Component Interaction

In evaluating the performance of a touch system, various metrics can be brought into play. Properties such as responsiveness, speed, and lightness of touch are important indicators in a successful system. These parameters all depend on strong, unambiguous relationships between the various components.

Touchscreen - Controller Issues

All resistive touchscreens have a characteristic performance, which can consist of internal resistance of the touchscreen coatings, cable resistance, touchscreen capacitance, voltage bounce, spacer-dot height, and settling times. The touchscreen is an essentially analog component, containing proprietary technology variations. Although the physical principles are straightforward to understand, in practice highly sophisticated technology and material science techniques are employed to ensure consistent and reliable performance.

Elo controllers and touchscreens have always been developed and optimized in tandem with each other. Our controllers know intimately the timing responses, bounce responses, and settling times of the AccuTouch screens. This information, unique to Elo touchscreens, is used to tune and optimize the controller to offer the best possible touch performance: fast, reliable, consistent, and durable. A third-party controller will function with an Elo touchscreen, but cannot deliver the performance required to offer the highest-quality touch experience. Inevitably, the user will notice a slower response, zingers (unexpected mouse cursor movements to unexpected places on the screen), and an inconsistent feel. It is worth taking a closer look at some of these issues:

  • Settling Times: An electrical analog system requires a certain time period to settle down to steady values. When a touch is generated, current begins to flow and, due to characteristic impedances and capacitances, this current will settle down to a constant value only after a few milliseconds. During this settling time, current and voltage values will move around; thus, trying to calculate a touch location in this moment is futile. The Elo controller knows this, and disregards all coordinate data associated with an initial touch before a certain time. Similarly, after an un-touch, voltage values will also move around briefly - and the Elo controller knows precisely when to ignore these values. In contrast, a third-party controller will be set up to work with a variety of touchscreens, and will therefore compromise speed to ensure compatibility. This controller will wait a conservative amount of time, just to make sure steady-state values are reached. Even then, the controller will never be completely sure. This compromise inevitably leads to a slow touch response, lacking in sensitivity.
  • Oversampling: This technique can also be used to guarantee the integrity of a coordinate pair. When a touch is generated, the controller requires several consecutive identical coordinate pairs to be generated before issuing the coordinates to the host PC. This oversampling filters out coordinate changes due to brief noise and false touches. It's possible that some overvoltage could cause a spark to flow between the coversheet layer and base-plane, thus generating a measurable voltage on the controller. Without oversampling, a controller will always immediately issue a coordinate pair to the host PC, which could be a false touch. Oversampling will filter out and dismiss such a touch, since it cannot be reproduced nor will it exist for a minimum amount of time.
  • Sensitivity: Settling times and oversampling are necessary, but will affect the sensitivity of a system to a light or quick touch. Elo controllers are optimized to keep these parameters as transparent as possible. A third-party controller may well be set to a "safe" performance level that overcompensates for settling and oversampling to avoid the risk of false touches. Such a controller will have slow and insensitive touch performance, which may lead to missed touches, an irritatingly slow response, or the need to press long and hard.

Elo has conducted in-house tests to verify these issues; these demonstrate that third-party controllers do not respond to the light, fast touches that an Elo controller will invariably catch. Furthermore, those light, fast touches that are caught by a third-party controller generate stray touches, in unexpected locations, indicating a general lack of oversampling or digital filtering. Timing tests have also shown that, whereas an Elo controller typically needs 4msec to generate a touch, a third-party controller requires 7-8msec.

Data Integrity Controller Software Issues

The touchscreen driver performs the obvious task of translating raw touch coordinates into mouse movements and mouse clicks. However, other important tasks are going on too, such as error checking. A standard serial RS232 connection is generally used to transmit the digital data of bits and bytes from the touchscreen controller to the host PC. A serial connection is a very reliable data transmission option, but can never be 100-percent reliable. A variety of factors - such as cable length, external electromagnetic noise, poor plug contacts, and other environmental factors - can allow distortions, ambiguities, and errors to creep into the data stream. These errors will cause zingers, undesirable mouse-clicks (premature responses to both touches and un-touches), or no response at all. All these errors can be prevented through the use of checksum arithmetic.

The checksum technique is a well-tried and tested software tool, uniquely used by Elo drivers. It works by dividing the serial data stream into 8-byte packets, adding the bytes together, ignoring overflow, and then adding the checksum to the result to generate a defined value. This checksum is therefore a powerful descriptor of the integrity of the data. The 8-byte packet is then packed out with a predefined lead-in byte and ended with the checksum byte. When the host PC receives the 8-byte serial data (recognizable by the lead-in byte), the software driver will calculate the checksum. If this checksum is identical to the received checksum found at the end of the 8-byte packet, the data integrity is assured. If, however, an error is received within the 8-byte data packet (due to noise or dropped characters), the driver will calculate an alternative checksum. The checksums will no longer match and the data packet will be rejected.

This powerful mechanism for ensuring data integrity can be critical in medical applications, where an incorrect touch can have life or death consequences, or an industrial control application in which an undesirable touch response could have a catastrophic consequence. Elo TouchSystems understands such critical needs and offers a level of security not available from any other touch driver software. Elo's market leadership is built on such responsiveness and innovation.

Technical Support

Backward and Forward Compatibility

Touchscreen products are increasingly commonplace, but it is still a sophisticated technology. Although already offering high standards of quality and lifetime MTBFs, the technology is under continuous improvement and incremental development, to raise standards and improve product specifications. Naturally, with a single touch supplier these modifications are meticulously supervised, with checks undertaken to ensure continuous component compatibility - both forward and backward. (In most cases, customers need never know of product improvements - assuming that specifications of form, fit, and function do not change.) However, these modifications will not and cannot be quickly incorporated into third-party components; this could lead to potentially dangerous incompatibilities. A third-party component will never have full access to confidential internal design modifications, and compatibility is only as good as when the last test was performed.

Elo is committed to its long-term customers; for example, our current 2210 AccuTouch controller offers emulation support for a long-redundant controller, the E271-60, which has been out of production for 12 years.

Technical Support

In nearly every OEM project, questions arise or a minor technical problem develops.

The kinds of problems described in "Component Interaction" are more likely to occur when multiple touch-component suppliers are involved. These problems can quickly escalate, since responsibility for problem diagnosis and resolution is unclear. A basic technical problem such as "the touch doesn't work" could be due to a software, controller, touchscreen, or cabling problem. But who will diagnose this - and who will take responsibility for solving it? When multiple suppliers are involved, finger-pointing, denial, and shuffling of responsibility are all too frequent - leaving the customer with a higher phone bill, frustration, and an inadequate system. With a single touch-component supplier, the issue can be easily cleared up through a telephone call or a site visit from an engineer.

A straightforward case of touchscreen nonlinearity can easily be due to controller and touchscreen incompatibility. A noisy voltage input from the controller could be the cause. In this case, both the controller and the touchscreen could well be operating within specs - but, due to poor device optimization and compatibility, the two working together generate an unacceptable response. Who is responsible? Who will offer a solution? A single component supplier is clearly responsible for ensuring that its components function correctly with one another.

Using multiple touch-component suppliers can also lead to warranty headaches. Building a system from multiple suppliers, or using nonstandard components can invalidate or impair a warranty. If things go wrong, the customer is left in a very tricky situation.

Specifications may change; for example, the touchscreen resistive characteristic could be significantly modified as a product improvement. Of course, a single supplier will thoroughly test such changes with native controllers and software. However, because such tests are not conducted with third-party components or software, there is a high probability that the total system will not operate as expected. Who is responsible in such a case?

When it comes to technical service and quality management, a touchscreen user will only get guaranteed, inclusive service from a single touch-component supplier. Mixing components leads to poor technical service, uncertain warranties, and questionable touch performance.

Commercial and Logistical Considerations

Elo TouchSystems has been manufacturing and selling touch components for nearly 40 years. We've stood the test of time, and it's no coincidence that Elo enjoys a leadership role in most of the vertical touch markets today. We've been able to achieve this through the winning combination of quality, reliability, and availability.

Many industrial and medical products have product life cycles of up to 10 years. During that time an OEM expects a consistent product supply and, of course, consistent product quality. Many of the new touch companies springing up have only been around a short time, are composed of a handful of people, and are very vulnerable to unforeseen market events. These companies enjoy the flexibility and efficiency of a small number of personnel, but this very advantage exposes them to potential disaster. Indeed, the impact of a single lawsuit could put such a company out of business. Even the loss of a single individual could mean the loss of all technical support, quality management, or product development. An OEM production line could be forced to shut down when a small company's quality manager takes a summer break. In contrast, Elo has a well-established management structure, and a staff of 500 employees, backed by the financial and engineering resources of a major international corporation, Tyco.

Smaller companies also have little clout when it comes to negotiating with their suppliers. Elo's size and negotiating power enable us to get the best prices and, more importantly, ensure the best terms of delivery from our suppliers. Should a component or raw material ever be in short supply, Elo will get the first delivery. Smaller companies are farther down on the priority list and have little leverage in a tight market situation. 

Summary

A touch system sourced from a single supplier is the only system that can guarantee excellent performance, future reliability, and value for money. Elo offers its customers a breadth of product knowledge, technical excellence, and maturity beyond that of our competitors. Examination of our product return data has enabled us to offer warranties that define our market leadership. We're looking for long-term partnerships with our customers and suppliers. Our commitment to quality, reliability, and availability has contributed to our past success and ensures our future customer loyalty.