# Why buy protocol analyzer modules for an oscilloscope?

Scopes with logic analyzer inputs often have optional add-on modules that let them decode various serial protocols: SPI, RS-232, I²C, CAN, etc. Tek and Agilent want around US $700 per protocol family for theirs. My question is, why would I want to buy a software module more expensive than Photoshop to do this when there are many PC-based protocol analyzers on the market at a fraction of that cost, many of which decode multiple protocol families? Are Tek, Agilent, et al just soaking EEs, or do these modules have features you don't get with a US$150-400 PC-based protocol analyzer?

• Wait until you price spectrum analyzers with demodulators. – Ben Jackson Jan 4 '12 at 1:59

I think the main benefit is that you can measure analog signals directly correlated in time to decoded serial protocol traffic in an integrated display.

These types of modules will also often come with an indexed search kind of capability to let you jump to user-defined events through the common interface of the scope. Sometimes they will also identify errors or warnings in the serial traffic as well. Lots of bells and whistles beyond just decoding the serial traffic for you.

For a very large company that bills engineering hours at say $100/hour they can justify such a capital expense if the features of the module will save 7 hours of labor (debugging time) over the useful life of the scope/module, as compared with the doing that same labor without it. The calculus works out, since these companies are obviously able to sell them. Whether that kind of thing is worth$700 to you is another story though.

• Doesn't the Bitscope option (first link) do this signal correlation, too? I don't see trigger output pins on any of the others, though, so there's no way to do a lash-up that way. +1 for you, too. :) – Warren Young Jan 3 '12 at 20:26
• @WarrenYoung yes the Bitscope does appear to be a pretty powerful little device, but as others have noted it requires a PC to run its user interface.... – vicatcu Jan 3 '12 at 21:48

All of the other answers are valid. I'll just add another reason to get one with your scope:

You can put some channels of the scope on your I2C/SPI/Etc signals, and other channels on "unrelated" signals. Then trigger the scope on I2C/SPI/Etc events and see what the unrelated signals look like. In that way you can debug interfacing issues that are time-aligned with your I2C/SPI/Etc data.

Bench space?

Why have a scope and a logic analyser when the two can be in one box and save you room on your bench?

• Fair point. I'm not considering the required PC, because as far as I'm concerned, it had to be there anyway. Not all workspaces are like this of course, so +1. – Warren Young Jan 3 '12 at 20:16

Convenience, speed, and integration with other channels. Alhough the options for the more expensive scopes are rather expensive, there are flaws in the licensing implementation in several of the Big Guys' decode options which make the cost somewhat more affordable :-)