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I was browsing OSH park's shared circuits when I saw this: Logic Analyser Buffer.

Since a true 12v RS232 signal will exceed the Salese's voltage limits, something like this could be useful. The diode and resistor seem straight forward enough. The three pin device I assume has a zener in it that shunts current to ground if the voltage exceeds 30 volts.

The part in the picture is labeled 215, but I can't figure out what it is.

If I can figure it out I was going to make a version of this which plugs directly into my Saleae. Or perhaps one with a DB9 tap.

RyanN
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    A board design without the corresponding Bill of Materials is rather useless. The download only contains the Gerbers. You'll have to get in touch with the designer somehow. – Dave Tweed Sep 12 '14 at 01:42
  • And what is a Saleae ? – placeholder Sep 12 '14 at 02:15
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    @placeholder [Saleae](https://www.saleae.com/logic) makes a cheap ($99) logic analyzer. You won't mistake it for a Tektronix but it is better than nothing. – Spehro Pefhany Sep 12 '14 at 02:34
  • It's PC based. Here's the web page: https://www.saleae.com/. Works pretty well for Serial/SPI/I2C. – RyanN Sep 12 '14 at 05:50
  • @SpehroPefhany For anyone finding this old thread, be advised that Saleae no longer makes a cheap analyzer. They are now 5x what they used to be. They went from a "reasonable profit" pricing suitable for makers to a "what the market will bear" pricing only defense department budgets will buy. (Okay, slight exaggeration, and it is excellent gear and software. But not for a garage consultants budget. Easier to hobble along with an analyzer from aliexpress and PulseView software) – RufusVS Dec 23 '22 at 02:36
  • @RufusVS So I see, that’s a shame, as they were nicely made. I have both kinds and have been using the Chinese ones with Sigrok/Pulseview away from the office because they’re quite inexpensive and more than good enough (especially with nicer grabbers fitted). Segger’s stuff is similarly “premium” priced for whatever business reasons. If an STlink can be used instead.. – Spehro Pefhany Dec 23 '22 at 04:34

2 Answers2

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It looks to be a Fairchild BZX84-C3V6 Zener diode.

So the circuit is like this (per channel):

schematic

simulate this circuit – Schematic created using CircuitLab

The resistors appear to be 1206 types, which would be inadequate to withstand +30V continuously (about 1.5W dissipation).

Spehro Pefhany
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Depending on which Saleae Logic Analyzer you have, you may not have to do that much.

We have more information on this exact situation here: http://support.saleae.com/hc/en-us/articles/200591545-How-can-I-connect-to-a-high-voltage-TTL-signal-such-as-24V-serial-

The older products, Logic and Logic16, will require some basic protection. The new products, however, are over voltage protected to +/- 25 volts, so can connect RS-232 directly to the inputs.

The original Logic's input is a 500 ohm series resistor followed by and ESD clamp. The 500 ohm resistor limits the current a little when the input voltage goes out of range, but the ESD clamp can't take much.

Logic 16's input starts with a compensated divider, so that the ESD clamp is protected by a large resistance at DC. (north of 170 K)

For low speed signals like RS-232, I usually recommend adding a 10 K resistor in series with the input and nothing else. this limits the clamping current through the ESD clamps considerably, but at the same time creates a strong low-pass filter effect, so it won't work with signals above about 100 KHz.

Clamping, like in the other answer, is a good solution too - especially when you're working with higher speed signals.

Marcus10110
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