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Max232 + radio module not working correctly

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mik3ca

Member
I can't seem to make my radio module work right when I convert from the cheap transistor equivalent of the circuit to the recommended MAX232 version.

This is the max232 circuit I made.

circuit.png

The part on the top-right is the same circuit I used for the cheap transistor version

I also tried removing the 10K pull-up resistors (except for the one next to the jumper) and still no luck. The capacitors I used due to availability are 4.7uF each and the capacitors next to the regulators are 100uF each. Regulator at the top is LM1117 (5V to 3V regulator).

When I measured the output transmit pin voltage (that module connects to) and tried sending data to the unit, the voltage changes by only 0.01. When I measured pins 7 and 8, the voltage did change a lot which suggests to me things are working from the PC side.

Voltage changed by about 1 when I measured pins 9 and 10.

I also measured pins 2 and 6 and got about -7v and 7v

I made sure I had no short circuits anywhere.

I attached my PCB layout with all the components included and the grid is 0.635mm
circuit2.png

and for reference, this is my older schematic for the adapter that did work for small data but not for large streams (especially of the same)

circuit1.png and its PCB:
circuit1b.png


What could I be doing wrong?
 

dknguyen

Well-Known Member
Most Helpful Member
I don't think pin 2 is connected properly. Seems like that cap should connect to GND, and it's polarity reversed accordingly.
 
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JonSea

Well-Known Member
Most Helpful Member
One thing to check with RS232 issues - make sure TxD --> RxD and RxD --> TxD. Some boards and schematics try to be helpful and label what a pin should go to rather than what it is which only adds to the confusion.
 

mik3ca

Member
I even tried it with the radio module hooked up. When data is meant to be transmitted the radio red light goes on. In the new circuit with the max232, the red light never goes on so obviously something is wrong in the circuit between the PC connection and the radio module. Do I need a pull-up or pull-down resistor somewhere?
 

mik3ca

Member
I don't think pin 2 is connected properly. Seems like that cap should connect to GND, and it's polarity reversed accordingly.
I'm confused now. Other max232 circuits I've done with transistors I always connected this to VCC and it works. And there are sources on the net which show it being connected to vcc.

Heres a nice link to the confusion. One circuit shows it grounded, and the other shows it to vcc:

https://arduinodiy.wordpress.com/2012/03/19/serial-connection-for-your-arduino-atmega/
 

dknguyen

Well-Known Member
Most Helpful Member
I'm confused now. Other max232 circuits I've done with transistors I always connected this to VCC and it works. And there are sources on the net which show it being connected to vcc.

Heres a nice link to the confusion. One circuit shows it grounded, and the other shows it to vcc:

https://arduinodiy.wordpress.com/2012/03/19/serial-connection-for-your-arduino-atmega/
It looks like there's two different ways to hook up that pin. Some connect pin 2 through a capacitor to ground and others do what you did (except all those circuits reverse the polarity of the capacitor since the voltage at pin 2 should be higher than Vcc).

Are your caps actually polarized or is that just the schematic? You should at least reverse the polarity of that cap.
 

mik3ca

Member
I wonder would my 5V voltage regulator be the issue? I'm using a standard 7805 and I powered it with 5V from my USB and on the meter I get about a 3.8V output from VCC to ground of the max232 but pins 2 and 6 show 7.16V. I was thinking this question because the voltage is under 4.5 which is the minimum "suggested" voltage.

I do have an LM2940 which might work better but I have no 22uF caps. just 100uF.

What do you think?
 

dknguyen

Well-Known Member
Most Helpful Member
I wonder would my 5V voltage regulator be the issue? I'm using a standard 7805 and I powered it with 5V from my USB and on the meter I get about a 3.8V output from VCC to ground of the max232 but pins 2 and 6 show 7.16V. I was thinking this question because the voltage is under 4.5 which is the minimum "suggested" voltage.

I do have an LM2940 which might work better but I have no 22uF caps. just 100uF.

What do you think?
Well, your regulator doesn't have an output decoupling cap to ground. That could definitely be it.
 

Pommie

Well-Known Member
Most Helpful Member
The lable FROM UC appears to be on the connections that should be to the PC.

Mike.
Edit, why are you using a 5v regulator on 5v?
 

mik3ca

Member
Well, your regulator doesn't have an output decoupling cap to ground. That could definitely be it.
I'm sorry I did not make it obvious at first glance but there is a decoupling cap between VCC (5V) and ground. Its located next to the 3V regulator that the regulated 5V supply feeds.
 

mik3ca

Member
why are you using a 5v regulator on 5v?
I wanted to force the circuit to no higher than 5V in case I use a power supply thats too large by accident. The 5V that the regulator produces runs through the second regulator to provide 3V.
 

mik3ca

Member
ok it turned out pins 6 and 7 were shorted together by an ultra tiny trace (2mil width?) so I fixed that and replaced the regulator with LM2940 and now things are better
 
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