I want my RS232 and RS422 to be galvanically isolated and ESD protected. I have two options to implement this requirement.


Use MAX3232 for RS232 driver with ESD diodes near the connector and lines connecting to MCU I will isolate using optocoupler ACPL-217. Next, for power supply I will have to use transformer driver say MAX253CSA or SN6501 with transformers.

Use similar type of set-up for RS422 with MAX489, optocoupler and transformer driver.


Use MXL1535E for RS422, with external transformer only(transformer driver is inbuilt) to implement the design. No ESD diodes also, since it is +15KV ESD protected.

Use MAX3250 for RS232 interface, with ESD diodes also.

Looking into space constraint option 2 looks better with fewer components, but please suggest your views which option is better for the good and robust design?

  • 1
    \$\begingroup\$ If space is an issue, maybe check out Analog Devices isolators. Some, like ADM2687E, have integrated DC-DC converters. \$\endgroup\$
    – user28910
    Commented Oct 2, 2013 at 13:52

1 Answer 1


A design I have used for a while now and that seems to work very well, is to use TI ISO7221A for UART (TTL) isolation, and then a driver on the nonisolated side, i.e. MCU->ISO7221->driver. I use this with an FTDI FT231 virtual serial driver, but if you prefer bare RS232 you can just use a MAX3232 instead.

The advantage of this design is that it is a fair bit cheaper than using one of your isolated drivers, while still getting well-defined performance (contrary to using a bare photoisolator).

For isolated power I default to those SIP 5V to 5V isolated supplies, e.g. XP Power or TracoPower TMA0505S. Make sure you filter these puppies really well - at least a big (>100µF) bulk electrolytic capacitor, but preferably a full common-mode filter (CM choke + 2 capacitors). Unfiltered these little power supplies kick out horrible amounts of EMI.

As ESD protection I usually just stick with simple BAT54S clamping to the ground and Vcc: cheap and cheerful. Make sure to put a nice big zener on the power lines as well, as close to possible ESD sources and clamping diodes as possible, with nice fat traces.

  • \$\begingroup\$ And what power supplies can be from XPPOWER? \$\endgroup\$
    – AKR
    Commented Oct 2, 2013 at 14:52
  • \$\begingroup\$ @AKR: for instance IB0505S, but you can look at the manufacturer catalogue or do a selection on one of the popular distributors (Farnell/MCM/Newark/Element14, Digikey, RS Components, Mouser) to see which fits your purpose best. \$\endgroup\$
    – user36129
    Commented Oct 3, 2013 at 17:23

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