Hi Friends I am using max 3232 for interfacing sim900A module and pic32mx360f512l and for interfacing pic and sim900a module i am using single max3232 ic that is i am used dual channel in max3232 one channel for pic and another channel for sim900a module the problem is when i am sending AT commands to the sim900a module the pic not receiving any response from sim900a module and sometimes its working fine and sometimes its not working. i guess this problem is due to using of both channel used at a same time in max3232 please give me your guidance to resolve this issue

 

All of the channels on a MAX3232 are independent - you can transmit on one while receiving on another. You might have a wiring problem but more likely its software.

Good luck.

 

All of the channels on a MAX3232 are independent - you can transmit on one while receiving on another. You might have a wiring problem but more likely its software.

Good luck.

Thanks for the reply JohnlnTx i think it is not software issue because when switch off and on the power means its working well and i am used 10pf/16v tandulum type capacitor for max3232 connection it is correct value capacitor or some other to use

 

OK. You can test the comms link by removing the PIC and tying the RX and TX UART lines together to make a wrap plug. Use HyperTerm or some other terminal emulator to send a text file to the serial port and see the result. If you see bad characters, you can troubleshoot the comms problem from there. If the file of text is returned with no errors, your problem is likely on the PIC end.

If you can't remove the PIC, hold it in reset by pulling MCLR/ down. That will float the UART pins and you can jumper them and do the wrap test.

Note that the wrap test won't find an incorrect baud rate, parity settings etc. since the terminal uses the same settings for TX and RX. Its still a good test. I keep wrap plugs handy for just such an occasion.

i am used 10pf/16v tandulum type capacitor for max3232 connection it is correct value capacitor or some other to use

Table 2 in the datasheet shows the required minimum capacitor values for different voltages from 0.1uF to 1.0uF. I think your cap description and values are a typo? I've never seen a 10pf tantalum. Tantalums or a good ceramic would work fine if you have the correct value.

Don't forget to decouple the power supply - the datasheet says .1uF is good for most applications.

 

What package of MAX3232 are you using? DIP or SMT?

 

What package of MAX3232 are you using? DIP or SMT?

I am used smd type max3232

 

I am used smd type max3232

Ok, that's important... there are two things that you first have to check for when using a chip like that.

• Some SMT chips are very easy to damage when soldering, and most of them should not be soldered at a temperature above 450°F, so check for that first.
• Second thing would be for you to make sure that you're using 0.1µF caps and not higher values. Old 232 chips used 10µF between pins and the newer designs have cut that value. Also make sure that your capacitor's polarities are correct, according to the datasheet.
• And third, a trick that I like to use sometimes when things just don't go my way is I place a LED in series with a resistor in such a way that it will flash when data is being sent or received, first at the TTL side of the chip and then at the RS232 side. For that, I configure the UART's speed at its lowest possible value, which is 150 or 300 bauds, depending on your hardware. At that speed, the LED will flicker if data is being sent or received. If the LED doesn't flicker, then there's something wrong with the hardware, and if it does, then there's something wrong with the software/firmware setup, or the chip is damaged and it will not translate between logic levels.

 

Ok, that's important... there are two things that you first have to check for when using a chip like that.

• Some SMT chips are very easy to damage when soldering, and most of them should not be soldered at a temperature above 450°F, so check for that first.
• Second thing would be for you to make sure that you're using 0.1µF caps and not higher values. Old 232 chips used 10µF between pins and the newer designs have cut that value. Also make sure that your capacitor's polarities are correct, according to the datasheet.
• And third, a trick that I like to use sometimes when things just don't go my way is I place a LED in series with a resistor in such a way that it will flash when data is being sent or received, first at the TTL side of the chip and then at the RS232 side. For that, I configure the UART's speed at its lowest possible value, which is 150 or 300 bauds, depending on your hardware. At that speed, the LED will flicker if data is being sent or received. If the LED doesn't flicker, then there's something wrong with the hardware, and if it does, then there's something wrong with the software/firmware setup, or the chip is damaged and it will not translate between logic levels.

Thanks for the reply your write now i am used 0.1uf capacitor and still now i am not facing any problem communications are perfect i am keep on monitoring with led flash and one more thing is previously i am doing the communication with two individual max3232 with 10uf/20v tandulam type capacitor that time i am not facing any issues and recently i changed my design to use single max3232 ic to communicating pic and sim900a so my guess is when communicating with single max3232 and 10uf/20v tandulum means that will manage the propagation delay but with single max3232 communication the propagation delay increases due to 10uf/20v tandulum capacitor