Max3232 Dual Channel Problem

[rmrps]

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

 

[hobbyckts]

Not at all, It will not give you the problem if you are using both the channel. recheck your circuit once or else post here for our verfication.

 

[KlausST]

Hi,

If you use both channels to transmit and receive serial data.. All in one chip, then I assume you don't need a RS232 converter at all.

Show your schematic...to avoid guessing

Klaus

 

[rmrps]

Thanks for the reply i am attached my circuit connection

 

[hobbyckts]

Wrong Connections.... What is your exact application? The method which you are trying to implemented here will not work out.. Explain clearly we will help you to connect properly

 

[rmrps]

Wrong Connections.... What is your exact application? The method which you are trying to implemented here will not work out.. Explain clearly we will help you to connect properly

I want to communicate sim900a module to send sms to the customers this is my application

 

[FvM]

I don't see a wrong connection.

But MAX232 is just useless in this application, you can directly connect PIC32 and SIM900 3.3V logic level signals to operate the UART connection.

 

[rmrps]

I don't see a wrong connection.

But MAX232 is just useless in this application, you can directly connect PIC32 and SIM900 3.3V logic level signals to operate the UART connection.

I can't connect sim900a direct with pic32 because sim900a is 2.8v and pic32 is 3.3v so it need some level converter

 

[FvM]

You don't need a level converter between 2.8V and 3.3V, at best series resistors for protection.

 

[KlausST]

Hi,

check if SIM900a can handle 3.3V levels. This is very likely.
if not, then a Si diode and a resistor is OK.
A true level converter, like 74AHC1Gxxx is better solution.

Back fom SIM900to pic is no problem.

Klaus

 

[rmrps]

Hi,

check if SIM900a can handle 3.3V levels. This is very likely.
if not, then a Si diode and a resistor is OK.
A true level converter, like 74AHC1Gxxx is better solution.


Back fom SIM900to pic is no problem.

Klaus

Thanks for the reply can you please post the circuit connection with diode and resistor and in my circuit connection the capacitor value is 10uf/20v it is correct or 0.1uf is correct

 

[FvM]

I read the SIM900 hardware design manual in the way that the digital inputs may be overdriven with current limiting, e.g. 1k series resistors, like many digital circuits.

If you're not sure about, you may want to make a simple voltage divider towards SIM900. Or use schottky clamping. Silicon diodes won't prevent forward bias of the SIM900 internal substrate/clamping diodes.

 

[KlausST]

Hi

Silicon diodes won't prevent forward bias of the SIM900 internal substrate/clamping diodes.

I meant the silicon diode in series with the data line to use the voltage drop from 3.3V down to 2..8V. Not for clamping to 2.8V VCC.

Connection: PIC_output --> anode_Diode_cathode --> SIM900 input. Additional resistor 10k from SIM900 input to GND.

****
FvM´s method with schottky clamping to VCC is also OK.

Klaus