A while back, I fried some chips on a SBC (actually, I've done it more than once) because I was stupid and didn't closely investigate that the SBC didn't have a voltage regulator. So I put 12V into a 5V circuit before I realized there was no protection.

I want to start learning about good design practices when it comes to protection. Mainly to protect from idiots like me.

So, is it good practice to put some diodes between the internal MAX238 chip and the external serial port? For example, to only allow "traffic" to flow through Rx/Tx in the correct direction? What about the other pins?

Looking in the datasheet, I don't see any example circuits. Only test circuits and functional diagrams.

Thanks!

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Cat; the other white meat.

My solution was to have 100 Ohm resistors in the RS232 inputs\outputs that go outside the PCB,
also 100 Ohm resistors in the I\O pins of the 6522 chips (because in that time frame I had problems with buying 6522 chips),
a TVS diode between +5V and GND (transient voltage supressor diode, that SMD part below the voltage regulator),
a +5V voltage regulator soldered to the PCB (the "heatsink" had turned out to be too small when plugging some more PCBs to the bus),

...and to use a 3 pin screw terminal for the power supply with +9V at the pin in the middle and GND at the outer pins
(what makes reversing the power supply voltage by mixing up +9V and GND wires by accident a bit more difficult).

On the MAXIM RS232 drivers, RS232 inputs can handle +/- 25V or better and both the inputs and outputs can handle continuous sort circuits. They are pretty robust and I would guess they don't need further protection unless you foresee stresses beyond what I've just mentioned.

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Bill

Diodes won't block traffic in a given direction, they'd only block transitions in a given direction. You could use some kind of clamp diode idea to prevent over-voltage, but as Bill notes, a driver chip might well already have good protection.

Ah, good point. Those chips are quite robust indeed.

Thanks for the tips!

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Cat; the other white meat.

Short answer is not necessary.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Diodes when used for protection are most useful if used as clamp circuits.
But you do need a in-line resistor in series with the input or output.

The diodes then are wired from the signal line (on the internal circuit side of the resistor). One to the relevant positive supply rail. The other to the relevant negative or 0V or ground supply rail.

The diodes should be wired so that they are normally reversed biased. Fast response diodes are recommended (1N400x or other diodes intended for mains rectification are too slow for semiconductor production), and they should be rated so that they can handle the maximum expected over voltage and the current that flows at this over voltage. For circuits where the in-line resistor can be a fairly high and the protection voltage is not too high, signal diodes like 1N4148 can sometimes be used.

Protection of DC supply inputs from over voltage is far more tricky. The best protection is by using different plugs and sockets for different voltages. But for existing equipment this is of course difficult or not practical...

And most people who do electronic tinkering at some stage have killed something! At work, we used to be responsible for some telecommunications equipment. It came in both 12V DC and 50V DC versions, with the only difference being the part number. Both used the same DIN connector. Guess who connected a 12V unit to a 50V system....!

Inside it used 4000 series logic chips. One was blown apart, and none of the rest worked. I had to replace all of them.

Mark

I would recommend sockets - just in case