MAX3232 circuit problem

[pashok84]

Hi All,

Currently i am facing a problem with the Max3232 serial port circuit.

I have designed the max3232 circuit same as maxim specfied one in datasheet.

In this i can observe that character is comming from RS232 teriminal. but terminal is not receiving any characters.

When connected rx and tx of TTL side. then also no results. I monitor the rx ttl pin in oscilloscope. it is working. Tx is not working.

Can any one please help me in this regard.

Thanks and Regards,
Ashok Kumar P

 

[alexxx]

When connected rx and tx of TTL side. then also no results. I monitor the rx ttl pin in oscilloscope. it is working. Tx is not working.

Did you verify that the tx pin from the MCU side is shorted with the corresponding MAX232 pin? Are pulses produced on MCU tx pin? If not, then it sounds like there is a software problem. Or the tx line is shorted with something else on the board. To check this out you could use the tx pin as GPIO pin. Produce pulses from software. If pulses are actually produced, then there is a bug in the UART code. If not, then possibly there is a short circuit somewhere.

 

[pashok84]

For testing, i attached tx and Rx pins, expecting a loopback. pins are not connected to MCU.

TX T2OUT pin has 0V when measured at idle time and R2IN has -6v. is it correct. i am using MAX3232 with 3.3v power supply.

 

[alexxx]

i am using MAX3232 with 3.3v power supply.

So it looks like you didn't follow exactly the circuit provided in the datasheet after all. Because the schematic uses 5V supply. Also it is given in the datahseet that supply must be 5V (4.5V to 5.5V). I don't know if this is the problem, but in any case you have violated specifications.

**broken link removed**

 

[Shanthakumarji]

see this

 

[pashok84]

Alexx and Shanthakumarji, I am using MAX 3232, in which 3.0 - 5.0v is accepted as supply voltage. All 5 Cap values are 0.1uF.

 

[ud23]

your circuit looks like this make sure you connecting TX and RX pin of your DB 9 connector is straight.(I mean your DB 9 female and DB 9 male have same RX and TX pin)

 

[pashok84]

This is circuit i used. I didn't connect any mcu to my serial adapter board.

I am transmitting 'A' from terminal and i am getting the same in R2OUT pin. when i connect a jumper for loopback, it is not working. Not able to receive the character back in terminal.

 

[ud23]

so you want to do transmit and receive only using hyper terminal if i am getting you right

 

[pashok84]

Yes. In one end it is Hyperterminal and another end it is mcu.

I have made this board as a standalone board, so that i can use it for any of my prototypes.

For testing the adapter board i didn't use the mcu board, to remove complexity in debugging the board.

Hope you got my test setup and idea behind it.

 

[ud23]

when i connect a jumper for loopback,

where are you using jumper for loop back which pins for receiving data?
and if you getting pulse on TTL Rx pin then your board is working already.

 

[pashok84]

I am getting the pulse on RX TTL pin (R2OUT). i am able to see in scope.

But when i connect a jumper between RX ttl and Tx TTL pin, i am expecting a loopback. but nothing come in terminal.

 

[:pH]

are you connecting a jumper directly in serial port of PC or in the cable ?

 

[pashok84]

i am connecting after level conversion. R2OUT and T2IN of MAX 3232

 

[ud23]

do you get any pulse on your R2IN pin?

 

[pashok84]

are you asking T2OUT?

R2IN is connected to RS232 cable and i am getting pulse in R2OUT.

 

[ud23]

ya after your loop back you getting pulse on T2out?

 

[:pH]

first check if echo is working in your pc or not , connect the serial cable to pc serial port , connect jumper between Rx and Tx and check for echo in terminal program

 

[pashok84]

I have not connected my scope in it. When monitor the line with multimeter, i can observe 0v when in idle. when will be the max and min voltage in T2OUT pin? is it safe to connect the scope. (i am using small usb scope).

 

[FoxyRick]

The datasheet says that Tout is +/- 13.2V, about standard for RS-232.