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I work in a company which develops devices that need to withstand harsh environmental conditions, shocks, radiations, vibrations and whatever. Really, too many to count.

In one of those devices is a small existing board which basically takes RS-232 and RS-422 signals, runs them through some kind of protection, and transfers them forward.

Here are the two types of circuits:

RS-232 Protection RS232 Protection

RS-422 Protection RS422 Protection

World - the signal comes from the outside (or goes outside).

Internal - the signal connects internally to a connector in the device. All the internal signals are connected to some industrial board that acts as some controller.

In the board, the resistors are on one side and they are SMD. The diodes and capacitors are on the other side and are TH. The diodes used in the RS-232 (and perhaps also RS-422) protection circuits are the 1N4744A.

To be honest, We're not sure what these protection circuits do, since it was designed long ago and the engineer that designed it left the company. Perhaps lightning protection? It would be great if you could tell me what those circuits could be for.

We want to produce more of these boards but the TH components being TH is giving us problems. I know this isn't a lot of information, but is it possible for you to know if the diodes and capacitors can be SMD instead of TH?

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    \$\begingroup\$ Of course you can. Just check the datasheets for specs. \$\endgroup\$ – Tom Carpenter Oct 22 '18 at 12:16
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    \$\begingroup\$ They look like simple protections against high-frequency noise and spikes. The capacitor looks like a short at high-frequency. Looks like a 7.2MHz cutoff frequency. The zener (maybe TVS) diodes serve to clamp spikes on the line. Hardly anything survives lightning. Mabe with some MOVs and/or gas discharge tubes but none of those are here. \$\endgroup\$ – vini_i Oct 22 '18 at 12:35
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    \$\begingroup\$ This question is not really answerable at schematic level. It's near certain that a solution based on surface mount technology could be engineered to meet your needs. But it's not a given that simply transplanting the existing design would achieve the original functional goals, nor randomly slapping parts on copper pads the mechanical ones. Geometry and part details can matter. Design of mechanically sound boards requires thought. \$\endgroup\$ – Chris Stratton Oct 22 '18 at 15:02
  • \$\begingroup\$ @Eran No one has given an answer, they have given "just my opinion" hints and snark - something comments are not supposed to be used for. \$\endgroup\$ – pipe Oct 22 '18 at 15:26
  • \$\begingroup\$ @ Pipe Given we are not the original designer, "just my opinion" is an appropriate comment, as we strive to learn the requirements. \$\endgroup\$ – analogsystemsrf Oct 22 '18 at 15:30

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