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High-speed relay modules are USB plug-and-play peripherals

Posted: 17 Feb 2005 ?? ?Print Version ?Bookmark and Share

Keywords:measurement computing? national instruments? ieee-488/gpib interface? pci? isa?

Kudos to Measurement Computing Corp. for battling it out in a very competitive arena. The company's been dishing up PC-based test-and-measurement hardware and software for quite some time now, competing with the likes of National Instruments and others with low-cost data-acq products.

Today, Measurement's wares include gobs of analog and digital I/O boards, special serial interfaces and IEEE-488/GPIB interfaces for use with USB, PCI, ISA, cPCI, PCMCIA and PC/104 buses, as well as RS-232/RS-485 serial systems.

These USB-ERB24 and USB-ERB08 offerings add to, and build on, the company's rather extensive hardware and software product line-up, this time leveraging the utility of USB (Universal Serial Bus and Windows PCs. Both of these Measurement Advantage modules are supported under Windows 98 SE, Windows ME, Windows 2000 and Windows XP.

No third-party device drivers needed
Both USB-ERB modules also rely on the Microsoft-defined HID (Human Interface Device) class drivers that are part of Windows. As such, no third-party device drivers are needed. Moreover, you can connect a USB-ERB module before or after you install the software, and without having to power-down your PC. When you connect a module, as an HID it will be automatically detected by your PC, and the software will be configuredeven if operating as a hub.

In use, either module can also be mounted on DIN rails, or used on the bench. If you put them on your bench, they won't take up too much room; their aluminum enclosures measure just 8.2-by-5.2-by-1.6 inches in size.

Not mentioned in the company's release notes is that these latest USB-powered modules (more on the power requirements in a moment) are USB 2.0 compatible (that's full speed 480Mbps USB), and they're backward compatible with 11Mbps USB 1.1. The USB cables can be as long as three meters.

With drivers for popular data acquisition applications, including National Instruments' LabVIEW graphical environment, these Measurement Advantage modules should prove useful and straightforward to implement and use, however they do require the use of an external 9V power source. That is, they don't derive all their operating power from the USB port they're connected to, which makes sense when you consider that as much as 850mA can be drawn with all relays on. Measurement ships these products with an external power adapter.

Regardless of how they're powered, the Measurement Advantage unit's relays are all Form C (SPDT) types, so you can get either normally-open or normally-closed contacts at the unit's screw terminal strip (which can accommodate field wires ranging from 22AWG size to hefty 12 gauge wires).

And, these contacts can switch a hefty 6A (at 240Vac or 28Vdc), and exhibit just 100 m? of resistance (max). However, you may have to place counter-EMF clamping diodes across any highly inductive loads that you're switching. In any case, these relays are fast, pulling-in in 10ms (max) and releasing in 5ms. They're also rated for 10 million mechanical operations, so they should be worry-free for quite a while in most automated measurement applications.

In addition, communication across the USB connection, you can configure relay logic polarity and pull-up/pull-down state. For logic, it's configurable per bank using a DIP switch to invert or non-invert (default) the logic. What's more, these switch settings for polarity can be read back through the USB bus.

The pull-up/pull-down states are also user-configurable per bank using a different DIP switch. Like the logic polarity settings, pull-down or pull-up switch settings can also be read back across the USB connection, using software.

The press release mentions the daisy-chain power supply interconnection scheme for configuring wiring of larger systems. As the name implies, the daisy-chain power output option permits multiple Measurement Advantage boards to be powered from a single external power source in a daisy-chain fashion. A supplied wall-wart or brick will deliver as much as 4A to additional modules.

The only caveat in implementing this topology is that the voltage drop between the power-supply input and the daisy-chain output is on the order of a half volt, so you have to account for that IR drop to make sure that the last module in your chain will still see at least 6Vdc.

- Alex Mendelsohn

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