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Actual Max Protected Voltage Input for Digital Discovery


Tony V

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I'm considering purchasing the Digital Discovery.  I will primarily use it to decode RS-232 and RS-485 signals.  However, the specs on the device say it is designed for CMOS 0-3.3V but will tolerate 0-5V.  Obviously the RS-232 and 485 voltages are way outside this range.  All the other logic analyzers out there (Saleae, DSLogic, etc) totally and safely support the 232 and 485 voltages.  Will those signals fry a Digital Explorer?  If not, can someone point me to something in writing that states the Digital Explorer will handle those signals?  Thanks!

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Any signal input to and FPGA that goes below 0V, like true RS-232 is incompatible with FPGA pin DC specifications. If you add an interface that converts these to on the the 3.3V single-ended IOSTANDARD logic compatible logic then you are good to go. Don't drive FPGA pins below ground. There protections diodes in the FPGA device but these are not meant to counter widely out of specification input voltage levels. Read the datasheet for the Artix family to see what the DC specifications are. I haven't looked at the Digital Discovery so I don't know what design information is available. 0-3.3V would seem to be a safe bet for inputs to the product as overshoot is likely for some signal sources.

I'm not sure that the Digital Discovery product was design to compete with the other products that you mention. None of these product are meant to replace those clunky expensive logic analyzers from the big instrument vendors.

If you really want something to analyze RS-232 or RS-485 traffic there are likely cheap products to do that. Really, you could turn any cheap FPGA board with external memory into such an analyzer. If the board has an FT232H or similar device and supports synchronous 245 FIFO mode then even better. You'd still have to make an adapter to convert those signals into FPGA friendly logic levels. You then have an instrument that you can tweak to provide just about any analysis you can ever want. I did this a long time ago so I know that it's possible and doesn't require any exotic programmable logic design expertise. Also a good educational project.

If you just want a ready made inexpensive instrument the AD1 or AD2 is hard to beat. You still need to condition signal inputs to appropriate levels. Edited by zygot
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Hi @Tony V,

As you noted, the Digital Discovery Input pins can handle up to 5 V (described further in sections 2 and 3 of it's Reference Manual, https://digilent.com/reference/test-and-measurement/digital-discovery/reference-manual) but it will only output between 1.2 V and 3.3 V logic on pins DIO 24..39.
It's absolute maximum ratings based on the Help tab -> Browse -> Digital Discovery (picture attached of the page I am looking at) state up to +/- 20 V can be applied, but of course these are absolute maximum ratings, not recommended operating conditions. So I would not use it with straight RS-232 or RS-485 voltage levels.

But like @zygot indicated, there are a number of existing adapters that will safely read the traffic for you and then convert the data into a more FPGA/microprocessor friendly voltage standard. (Or other logic analyzers like a Saleae or DSLogic that already has the appropriate protection and adapters implemented; I'm not going to tell you how to spend your money).

Thanks,
JColvin

 

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