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I have so far been using some MAX3323EEPE chips for shifting between 3.3V and 5V logics on serial / UART communications. This is quite some stuff to solder on a PCB, and the chip is a bit expansive. I only do quite vanilla communications, such as up to 57600, absolute max 115200 baud rate.

Would this kind of shield:

https://www.adafruit.com/product/757

do the trick?

And this kind?

https://www.adafruit.com/product/395


Edits

  • Edit 1: typical use: I am thinking about having a project built around a 3.3V micro-controller, but I will need to be able to talk to some 5V logics stuff too. I will have a 5V step-up converter on the design, so getting 5V is not an issue. Typically, I will need to talk to a RockBLOCK 9603, which seems to be rather 5V than 3.3V ( https://www.rock7.com/products-rockblock-9603 though not completely clear to me if it would be ok to power with 5V and talk through 3.3 logics level; but other things will definitely need 5V logics levels anyways, so good to put on board).
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  • \$\begingroup\$ Are you connecting to devices that you don't control and may expect valid RS232D signaling? Or do you not need to support that signaling? \$\endgroup\$ – jonk Feb 21 at 11:28
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What are you connecting your UART/serial comms to?

The MAX3323 uses external capacitors to generate +5V and -5V, and uses those supplies to convert signals from the low-voltage side into signals which transition between +5V and -5V. It only requires one low-voltage supply to do this.

The shield devices you link to require an external supply ( +5V ) to be used, and will then convert your low-voltage signals into signals which transition between +5V and 0V

The shields will require an additional 5V power supply - and if the device you are connecting to is expecting -5V levels, they won't work.

Edit for use with RockBlock 9603:

The Serial interface on this device looks like you may be ok with 3.3V levels only - in which case no interface would be needed RockBlock 9602/9603 Serial Interface Voltage Levels

from here - https://www.rock7.com/products-rockblock-9603

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  • \$\begingroup\$ Typically on 1 side a micro controller working at 3.3V, on the other side any kind of micro controller / sensors / 'stuff' working at 5V. I will have both stable 5V and 3.3V supply available. \$\endgroup\$ – Zorglub29 Feb 21 at 11:18
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    \$\begingroup\$ As long as it's just 'stuff working at 5V' the shields should be ok. If you want to connect it to as an RS232 standard device, you will need the MAX3323 ( or similar ) as it needs a voltage swing between -3V and +3 V link \$\endgroup\$ – Yellow Yeti Feb 21 at 11:50
  • \$\begingroup\$ Yes ok, so fine with TTL which is 0-3.3V, not fine with RS232 which is +-3.3V, right? :) \$\endgroup\$ – Zorglub29 Feb 21 at 12:01
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yes, for such slow signals either kind would work.

Or you could use resistors for voltage drop and a TTL compatible buffer for the upwards shift.

but "rock7" says

There are various FTDI USB cables, the precise model used with our products is the TTL-232R-3V3, and it's available to purchase from our shop or from various electronics outlets globally.

https://docs.rockblock.rock7.com/docs/connectors

This implies that the rock block wants 3.3V signals for the UART, so you don't need any level converter, just use wires.

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