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MAX3232E: MAX3232ECDR schematic review – DB9 and 3.5 mm stereo jack interfaces

RAYAN SAYED
Prodigy 40 points

Part Number: MAX3232E
Other Parts Discussed in Thread: MAX3232,

Hi TI Community

I am designing two separate RS-232 interfaces using two individual MAX3232ECDR devices and would appreciate a review of the attached schematic before proceeding with PCB fabrication.

  • U7 – MAX3232ECDR: RS-232 interface connected to DB9 connector J10
  • U10 – MAX3232ECDR: RS-232 interface connected to 3.5 mm stereo/TRS jack J11
  • Supply voltage: 3.3 V

1. Charge-pump connections

For both MAX3232 devices, I am using 100 nF / 50 V ceramic capacitors for the charge pump:

  • C1+ to C1−
  • C2+ to C2−
  • V+ to +3.3 V
  • V− to GND

Could you please confirm whether these connections are correct for VCC = 3.3 V, particularly the V+ capacitor connected to +3.3 V and the V− capacitor connected to GND?

Also, is 100 nF / 50 V suitable for these charge-pump capacitors?

2. DB9 interface – U7

The RS-232 signals are connected as:

  • DOUT1 → DB9 pin 2
  • RIN1 ← DB9 pin 3
  • DB9 pin 5 → GND

The other DB9 pins are unused.

Could you please confirm whether this DB9 pin assignment is correct for a standard RS-232 interface?

3. 3.5 mm stereo jack interface – U10

The second MAX3232 is connected to a 3.5 mm stereo/TRS jack

  • Tip → DOUT2
  • Ring → RIN2
  • Sleeve → GND

The intention is to use the stereo jack for RS-232 TX, RX, and GND.

Could you please confirm whether this Tip/Ring/Sleeve assignment is suitable for RS-232, and whether there are any concerns with using a 3.5 mm stereo jack for this interface?

4. Unused pins

The unused MAX3232 pins are left unconnected as shown in the schematic.

Should the unused logic inputs/outputs be left floating, or should any unused input be tied to GND or VCC?

5. General review

I would also appreciate feedback on:

  • ESD/TVS protection for the external connectors
  • Decoupling capacitor placement
  • Charge-pump capacitor placement
  • Ground/layout considerations
  • Any other schematic issues that should be addressed before PCB fabrication

The schematic for both MAX3232ECDR interfaces is attached.

Thank you in advance for your review and suggestions.image.png

image.png

  • 0
    TI__Intellectual 2550 points

    Hi Rayan,

    1. For the charge pump capacitors everything looks good in terms of their capacitance value and connection. They don't necessarily need to be rated for 50V (something like 16V should be fine if it is more convenient for you), but there's absolutely nothing wrong with the higher rating. Just make sure the charge pump and power supply caps are as close to the device pins as possible.

    2. While there's no set standard for the connection, on our EVMs we usually have pin 3 as the DOUT and pin 2 as the RIN. 

    3. I've seen that configuration for a 3.5 mm stereo jack, and also the same with the TXD and RXD pins swapped. Either way should work fine. I don't know the speed of your system or the length of your cable, but I don't see any problems as of now.

    4. DIN should be pulled up or down to VCC/GND via a resistor. The rest are fine to be left open.

    5. I'm not sure what your ESD requirements are, but note the MAX3232E has pretty good ESD protection built in:

    If you need more I'd recommend looking at TI's ESDS552 for the bus lines.

    The schematics look good as they are, just remember to put the caps as close to the pins as possible.

    -Ethan White