PCs meet real-world logic

Digital I/O cards interface a PC

Rick Nelson, Senior Technical Editor -- Test & Measurement World, 3/1/2001

The PC has increasingly become the platform of choice for test-and-measurement applications, and you’ll need flexible digital input and output capabilities to handle logic housekeeping chores in your PC-based test systems. You might need to generate a logic level that initiates an external actuator for connecting a DUT to a test fixture, or you might want to monitor a DUT power supply to ensure your DUT is properly powered up (Ref. 1). Because of the variety of I/O signals your application might involve, and because of the variety of PC architectures you might be working with, you can benefit from evaluating the offerings of several manufacturers of digital-I/O (DIO) cards (see Table 1).

Your choice of digital I/O card will center on four key questions:

• What type of PC are you using? If you are starting from the ground up to build a ruggedized PC-based test system for a harsh production environment, consider using the CompactPCI format with its PXI instrumentation extensions (Refs. 2 and 3). You can select from a range of PXI instruments (from PXI pioneer National Instruments and many others) that plug into a CompactPCI/PXI chassis; those instruments can handle your analog and digital electrical-measurement chores, and for DIO housekeeping, you can choose DIO cards from manufacturers listed in the CompactPCI/PXI category in Table 1.

If you’re using a standard off-the-shelf commercial PC to control a rack of IEEE 488 instruments, you can choose DIO cards that plug into a desktop computer’s PCI bus or ones that plug into a laptop’s PC Card (PCMCIA) slot. As another alternative, you can choose stand-alone DIO modules that plug into a desktop or laptop computer’s USB port. Such modules, from firms including Biodata, Intelligent Instrumentation, and IOtech, come with programs that add data-acquisition capabilities to their host computers. The modules also come with drivers that support test-related programming environments such as National Instruments’ LabView.

An additional physical embodiment of the PC’s electrical architecture is represented by the PC/104 configuration (Ref. 4). PC/104, however, targets embedded industrial applications and lacks the instrument extensions that PXI provides for CompactPCI. You shouldn’t completely rule PC/104 out as an instrumentation platform, especially if you need to develop a compact instrument that can fit into tight remote locations, but it would be a poor first choice.

• What I/O logic levels and output configuration does your application require? Many products offer standard TTL levels, where an input or output of 0 to 0.8 VDC represents a logic low, while 2 to 5 VDC represents a logic high. Alternatively, you can find products that can handle wider I/O levels. Analogic’s CPCI-DIO-96, for example, recognizes a 0-V to 1-V input level as a logic 0 and a 3-V to 30-V level as a logic 1. On the output side, the card supports voltages to 30 VDC (Figure 1). Boards such as Gespac’s PCICIO-12 provide open-drain (FET) outputs that can switch external voltages of 48 VDC and higher.
   
• Does your application require electrical isolation between the PC and the external environment? Such isolation can improve noise immunity, and it is mandatory if common-mode voltages exist between your computer’s ground and external equipment’s ground. Common-mode voltages could arise if, for example, your PC runs off a standard 120-VAC line while the external actuators it controls operate on power supplies referenced to the ground of a 460-VAC three-phase distribution system. If you anticipate grounding problems, you can employ boards such as Datel’s CPCI-562 ( Figure 2) that employ optically isolated inputs that can accept AC or DC voltages to 24 Vrms or higher. On the output side, that board provides isolation by means of 2-A, 250-VAC electromechanical relays. Pickering Interfaces offers a variety of boards with optical I/O isolation as well as a choice of electromechanical reed, power, and RF relay options.

• Figure 1. If you move beyond basic TTL or CMOS building blocks, you’ll need to understand your DIO card’s I/O output circuitry, which you can glean from your card’s data sheet. Analogic’s CPCI-DIO-96, for example, presents a modified open-drain output, with an internal pull-up network equivalent to 39 kW connected to 4 V. To accommodate higher output voltages, you can add an external pull-up resistor connected to 30 V, but you must choose R to limit iO to 150 mA (or to 1 A total per group of eight channels).

  Figure 2. A full-wave rectifier allows Datel’s CPCI-562 to accommodate AC and DC inputs. Optical isolation rejects common-mode voltages between the computer and external environment. Electromechanical relays provide isolation for output signals.

  Can your PC handle the DIO signal processing, or will it need an assist from your DIO board? Almost any DIO board supports static-I/O operations. Such boards present to the PC in real-time the logic levels they receive from external sensors, and they transmit in real-time to external actuators logic levels generated by the PC. With such a scheme, it’s incumbent on the host PC to determine when inputs are valid and when outputs are appropriate.

Slightly more complex boards support block (or strobed or latched) I/O (Ref. 5). Block I/O schemes require that a DIO board maintain valid I/O levels until receiving a strobe signal that indicates when transfers should take place. You might want to have timer/counter circuitry—such as that found on Access I/O Products’ PCI-DIO-24H/24D cards—on your DIO card to assist with simple processing chores.

At the most complex, you can choose a board such as Innovative Integration’s M62/67, a PCI card with onboard DSP and memory. It accommodates digital I/O via plug-in mezzanine cards.

Flexible son cards

Mezzanine cards such as Access I/O Products’ models can afford considerable flexibility when you are configuring digital I/O for your PC-based test applications. And while Access I/O Products’ mezzanine cards are tailored for the firm’s processor boards, you can choose from open-standard versions, most notably IndustryPak (IP) models (Ref. 6). Developed for the high-end VMEbus with its VXIbus instrument extensions, IP mezzanine modules can reside in PC environments when plugged into PCI or CompactPCI carrier boards. Acromag is one firm making both DIO IP mezannine modules as well as PC-compatible carrier cards.

Many of the companies listed in Table 1 offer analog I/O cards as well as DIO versions. On some offerings, a few DIO channels complement what are primarily analog I/O models. You might find such a card, perhaps with onboard processing, that accommodates your entire analog-measurement and digital-housekeeping chores. If you are relying on more traditional instruments to make your measurements, however, then you can find a DIO card that economically completes your PC-based test system. T&MW

References

1. Holland, Nancy, “Adapt Automated Instruments to Production Test,” Test & Measurement World, December 2000. p. 21.   

2. Bassak, Gil, “Engineers Embrace CompactPCI,” Test & Measurement World, October 15, 1999. p. 12. 

3. “The PXI Modular Instrumentation Architecture,” The PXI Systems Alliance, Austin, TX. www.pxisa.org/whatspxi.doc.

4. “What is PC/104?” PC/104 Consortium, Mt. View, CA, 415-903-8304. www.ampro.com/university/wpapers/wi104.htm.

5. Maharajh, Niranjan, “Digital I/O Applications,” Application Note 131, National Instruments, Austin, TX, November 1998. www.ni.com/pdf/instrupd/appnotes/an131.pdf.

6. Titus, Jon, “VXIbus Mezzanine Modules Maximize I/O and Function Choices,” Test & Measurement World, August 2000. p. 13. 

Rick Nelson received a BSEE degree from Penn State University. He has six years experience designing electronic industrial-control systems. A member of the IEEE, he has served as the managing editor of EDN, and he became a senior technical editor at T&MW in 1998. E-mail: rnelson@tmworld.com.