Until a week ago I never thought about oscilloscopes with more than four analog inputs. But suppose you need a fifth or sixth analog channel, what then?  I suppose you could borrow a scope and have four channels on one scope and a fifth channel on the other, but that could prove awkward when you must compare timing on one set of traces to the sole trace on another scope.

An advertisement for USB "scope" modules from Chronologic, an Australian company made me think about the feasibility of bench scopes with more than four channels. The company's USB-inSync CL4000 modules let engineers stack as many analog channels as they need (as many as 99) and acquire data through a USB connection. These modules offer 14-bit resolution and acquire signals at 100 Msamples/sec. The buffer memory provides a 128-Mbyte capacity. (Keep in mind that's enough memory for only 64 Msamples because you have 14-bit data.) Each unit handles only one analog channel at a cost of $US1495. That's pricey. (www.chronologic.com.au/)

The USB-inSync technology, though, might make this purchase worth the cost. The basic USB protocol includes no provision for synchronous operations or accurate timing signals among several USB devices. Chronologic's white paper on USB-inSync technology explains the new timing capabilities:

"Each USB-inSync device is equipped with a local clock that is accurately phase locked to that of every other USB-inSync device attached to a given Host PC. Furthermore, each device contains its own ‘real time’ counter that is accurately synchronized to those of every other USB-inSync device. A deterministic triggering system is also provided so that processes can be started and stopped in a precisely controlled manner. These two features extend USB beyond the realms of its original concept and allow new synchronization, control and monitoring possibilities within a distributed PC-based environment."  (For the complete paper, visit: www.chronologic.com.au/whitepapers.html.)

So the cost of the USB scope modules relates directly to the accuracy of timing between modules, which provides a module-to-module accuracy within as little as one nanosecond. Over longer distances, the timing accuracy worsens, but on a lab bench, a nanosecond should suffice. If you need more than the two or four analog channels on your present scope, the USB-inSync module deserve a look. And the modules come with dedicated stand-alone scope software and LabVIEW drivers. Chronologic also manufactures precision-timing devices, so the company engineers know how to make accurate timing work.

A separate input lets you trigger data acquisition based on an external signal or you can set standard trigger conditions such as edges, glitches, and so on, as well as use software to trigger the units. The company's information does not describe how an external trigger propagates through to other modules, so I have asked more information and will include an update as soon as I receive it from Chronologic.

To be fair, you can buy instrument cards that connect to the PXI or PCI bus and digitize five or more analog signals. National Instruments sells a PXI-5105 and a PCI-5105, both of which provide eight analog inputs and sample these inputs simultaneously at 60 Msamples/sec. The PCI version that offers 12-bit resolution and comes with 128 Mbytes of memory costs about $US 6500. The PXI version has the same specs and price, but you need a PXI mainframe and additional components to use this card. The hardware cost equals $US 813 per channel, but without software. (www.ni.com)

NI also sells an 8-bit USB-based module (NI-5133) that digitizes two channels simultaneously at 100 Msamples/sec. The version with 32 Mbytes of memory costs $1544, or $772 per channel. The comparison with the NI-5105 doesn't account for the difference in memory, though.

You can find other USB-based scope pods or modules, but as far as I know, none provide parallel triggering without running the trigger to each module.  If you want to suggest another USB-based scope module, or have some results from hands-on testing, feel free to put your information in a comment for this entry. --Jon