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I need to make serial connection between regular PCs and a 3.3v TTL device, I using the Maxim 233 which has internal capacitors.

I have the PCs incoming TD connected to R1_IN and RD T1_OUT. The ground pin5 is connected to the frame ground as well and its connected to the maxim233s GND.

What is unique in my setup that I running this chip from 3.3V instead of 5V because if I run it from 5V it will give out 5V on its low ttl part and can hurt the micro.

The outgoing data from the device to the PC works.

The incoming data from the PC to the device does not.

I can measure -10V if I check on the GND and the incoming wire.

Any ideas how to debug this any further? All the connections are triple checked and if it works one way there must be some minor issue. I use the same 9600 baud data rate for all tests on both ends. I have tried to replace the chip just in case that didn't solve anything.

enter image description here

https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Null_modem_DB-9_xon-xoff.svg/1920px-Null_modem_DB-9_xon-xoff.svg.png

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    \$\begingroup\$ The MAX233 and related devices are specified for Vcc = 5 V +/-10%, so may not operate correctly with a 3.3 V Vcc. \$\endgroup\$ – Peter Bennett Oct 26 '20 at 20:13
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    \$\begingroup\$ Maxim MAX3233E +3.3V, ±15kV ESD-Protected 1µA, 250kbps, Dual RS-232 Transceiver with Internal Capacitors -- that's the 3.3V version of the MAX233. (Full disclosure: I am an applications engineer at Maxim Integrated) \$\endgroup\$ – MarkU Oct 26 '20 at 23:30
  • \$\begingroup\$ Well thats nice except the MAX 3233E cost 10$ per pice without shipping from Ebay and it's not even available on LCSC to directly put it on my pcb. Also others achieved success with using the 233 and 232 from 3.3V so it's weird why exactly the incoming data does not work when the outgoing does. Can you recommend some alternative? \$\endgroup\$ – reaverx Oct 27 '20 at 12:39
  • \$\begingroup\$ Well like the datasheet says, it does not have to work below 4.5V, but it also does not quarantee it does not work, so some chips may work and some don't. Asking for recommendations what to buy is off topic and subject to opinion-based answers. Finding cheap and easily available 3.3V RS232 tranceivers is not hard, the most common ones simply need the external caps to operate. \$\endgroup\$ – Justme Oct 27 '20 at 13:49
  • \$\begingroup\$ Would it work if I would chain it with a regular 5to3V TTL converter? Would it affect the maximum data throughput/transfer speed? \$\endgroup\$ – reaverx Oct 28 '20 at 7:31
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Datasheet specifies an operating supply voltage between 4.5V and 5.5V.

So part is not rated to work at 3.3V.

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