A USB to serial converter board, where the host is galvanically isolated from the target

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Taking a page from my isolated USB null modem project, I've decided to design a USB to serial board with full isolation, including power for the target.

This project is effectively a combination of my TTL USB UART board, itself based around a CY7C65213 USB UART chip, and half of the isolated USB null modem project.

The isolators of choice are the SI8035 and SI8055. They're the same, except that one has 3 channels and the other 5. That's because there are 3 transmit channels (TXD, CTS and DTR) and 5 receive channels (RXD, RTS, DSR, DCD and RI). In addition to this, 5 volts from the USB bus is passed to the target side with an isolated DC-to-DC converter module. The 5 volt output is run through an LDO to make 3.3 volts and both the 5 volts and 3.3 volts are selected with a solder jumper and the selected voltage becomes both the supply voltages for the isolators (which also makes them the voltage level for the resulting signals), and is passed through an AP2331 protector to the V+ pin of the target connector.

The 5 volt input to the DC-DC converter is switched on and off with a P-MOSFET and the power enable signal from the UART chip.

As with the isolated null modem, the ground planes on the primary and secondary sides are kept as far apart as possible. The DC-DC converter chip has the primary and secondary side pins closer together than that, so to improve the creepage distance there, a slot is cut between them.


Adobe Portable Document Format - 44.55 kB - 12/11/2021 at 04:53



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  • RS-232

    Nick Sayer01/02/2022 at 07:05 0 comments

    I designed an RS-232 adapter and posted it over at the original non-isolated variant page. It also works on this isolated version, and the net result is a galvanically isolated USB to RS-232 adapter.

  • First prototype

    Nick Sayer12/21/2021 at 22:44 0 comments

    The first prototype works flawlessly, so far as I can tell (with the proviso that DCD and RI are not connected because I don't have any SI8055s and there won't be any until August).

  • Supply chain issues

    Nick Sayer12/20/2021 at 02:39 0 comments


    I have some SI8035s left over from my development of the USB Isolated null modem.

    But I don't have any SI8055s on hand... and now they're backordered until August. None of the SI80xx line are on hand anywhere. We can't have nice things.

    For prototyping purposes, I can use a 35 to replace the 55. Doing so means that DCD and RI will fail to work, but it's a start.

    EDIT: I found a few of these on AliExpress and have enough on hand for now.

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