Hello guys. I have a question.
I am trying to interface my microcontroller to a GSM modem.
The GSM module am using is SIMCOM908.
do I still need the max232 in between the microcontroller and the GSM module ?

I took me 10 seconds to find this. So what do you think?

 

Check page 30 of the datasheet I've attached. It shows that one needs to convert the module's serial signal to RS232 if you want to connect it to a PC through its RS232 port. Therefore, the module's UART output works at low voltage level, and it tells me that you do not need an RS232 converter to interface it directly to an MCU as long as both the MCU and the module are working at the same voltage, which normally is 5V.

Thanks alot. the modem has a serial to a usb converter cable.
The serial end is to go the modem while the usb end is to go to the PC.
I connected the serial end of the cable directly to the modem and the usb end of the cable directly to the PC without any MAX232 interface and it worked.

 

This is the image of the GSM modem.



This is the image of the usb to serial converter cable.

 

Thanks alot. the modem has a serial to a usb converter cable.
The serial end is to go the modem while the usb end is to go to the PC.
I connected the serial end of the cable directly to the modem and the usb end of the cable directly to the PC without any MAX232 interface and it worked.

This is a full module. You will need the max unless you can figure out how to hack in behind the R3232 interface.

 

If am to connect the GSM module directly to the microcontroller, how can I identify the TX and RX on the USB side of the of the cable ?

 

I connected the serial end of the cable directly to the modem and the usb end of the cable directly to the PC without any MAX232 interface and it worked.

That is very strange... it means that your USB to Serial cable is accepting 0-5V level signals... and not the RS232 standard ±12V

 

This is a full module. You will need the max unless you can figure out how to hack in behind the R3232 interface.

I agree... if he's plugging the USB to Serial converter directly to a DB9 connector, it means that the module that he has should be outputting RS232, and not 0-5V

 

If am to connect the GSM module directly to the microcontroller, how can I identify the TX and RX on the USB side of the of the cable ?

Does your MCU have a USB interface? What MCU brand and model are you using?

 

This is a full module. You will need the max unless you can figure out how to hack in behind the R3232 interface.

Meaning the GSM module is connected to the TTL side of the MAX232 while the PC is connected to the RS232 side of MAX232. Am I right ?

 

Does your MCU have a USB interface? What MCU brand and model are you using?

Am using PIC16F877A

 

That is very strange... it means that your USB to Serial cable is accepting 0-5V level signals... and not the RS232 standard ±12V

Some of these convertors have charge pump idea and up it to RS232, all in the back shell.
I made one years ago for a PLC system.
Max.

 

That is very strange... it means that your USB to Serial cable is accepting 0-5V level signals... and not the RS232 standard ±12V

I guess as far as it is a cable , it can accept any logic level. its just the MAX232 that does the logic conversion. am i right ?

 

I guess as far as it is a cable , it can accept any logic level. its just the MAX232 that does the logic conversion. am i right ?

If you look at the translucent back shell cover you can make out a circuit board for RS232 conversion.
ebay 172314477359.
Max.

 

If am to connect the GSM module directly to the microcontroller, how can I identify the TX and RX on the USB side of the of the cable ?

How can I get over this ?

 

Some of these convertors have charge pump idea and up it to RS232, all in the back shell.
I made one years ago for a PLC system.
Max.

That is very interesting... gonna have to try that myself... see if it works. Thanks!

 

If am to connect the microcontroller directly to the GSM module, do I still need the serial to usb converter cable? if yes, how do i conncet the usb side of the converter cable to the microcontroller ?

 

If am to connect the microcontroller directly to the GSM module, do I still need the serial to usb converter cable? if yes, how do i conncet the usb side of the converter cable to the microcontroller ?

There are quite a few posts out there on interfacing a GSM module and a Pic.
http://microcontrollerslab.com/send-sms-using-gsm-module-pic-microcontroller/ for one.

That is very interesting... gonna have to try that myself... see if it works. Thanks!

I have made a few different versions over the years, this was one for the Mitsubishi PLC which required a little larger housing.
I have used PVC pipe caps in a few down to 1/2"
Max.

 

Meaning the GSM module is connected to the TTL side of the MAX232 while the PC is connected to the RS232 side of MAX232. Am I right ?

Assuming the GSM uses a MAX232 yes. What they are using is irellevent. The DB9 tells you you will need Rs232 to interface.


You won't need to worry about the PC because (I assume) you are going to let the Pic take the place of the PC.

 

You won't need to worry about the PC because (I assume) you are going to let the Pic take the place of the PC.

What he needs to worry about is that the DB9 connector on the module is indeed delivering 0-5v signals... otherwise he's going to fry his MCU if it's delivering RS232 ±12V

 

What he needs to worry about is that the DB9 connector on the module is indeed delivering 0-5v signals... otherwise he's going to fry his MCU if it's delivering RS232 ±12V

OP said it was connecting to the PC, so dollars to donuts it is RS232 levels but you are right, it is best to be prudent and measure the levels.

Actually if the OP assumes RS232 from the DB9 then that is a good thing. The worst that will happen is that the Max won't work.

And I never meant to hook the device up direct to the MCU. I meant via a MAX232.