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:


do the trick?

And this kind?



  • 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).
  • \$\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, 2020 at 11:28

2 Answers 2


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

  • \$\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, 2020 at 11:18
  • 1
    \$\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\$ Feb 21, 2020 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, 2020 at 12:01

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.


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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