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Interfacing Siemens modem to Microcontroller

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liupengjian

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Hi, Everyone.
I encountered a problem in my project which needs to use MCU 18F452 to send SMS via Siemens modem MC35i, the MC35i is a chipset engine only, so I interface RX and TX directly to my 18F452 RX and TX pin. After power up and activate MC35i by Ignition pin (IGT). However after I sent some basic AT command, the modem chip just stay silenctly without any response.

I checked the datasheet but cannot get any clue.
Thanks for any advice from you.
(The attached is the block diagram for MC35i engine)
 

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Gday,

Have you tried to use the modem with Hyperterminal, and does it respond then? Also, if you plug YOUR app into hyperterminal, does that send what you want it to do?

Also, check you are using the right end-of line character --- modems want \r instead of \n. Also, the modem might want you to use handshaking on rs232 - look up on the web how to fake this, its easy to do, just wire 2 pins together.

--JB.
 
Thanks for your advice, I have tried to connect the engine to my hyperterminal through a transciever chip MAX232, but it seems doesnt work neither, and found that the MC35i logic level is 3.3V for all digital pins. So is it possible that the logic level has any effection on the Rx and Tx pins?
 
liupengjian said:
Thanks for your advice, I have tried to connect the engine to my hyperterminal through a transciever chip MAX232, but it seems doesnt work neither, and found that the MC35i logic level is 3.3V for all digital pins. So is it possible that the logic level has any effection on the Rx and Tx pins?

i have this question also in my mind. what is the minimum TTL input for max232 IC?

I saw in the datasheet of max232, minimum TTL Voltage Output of Max232 is 3.5V, so i presume that this is also the minimum for TTL Voltage Input. Are my assumptions right, or did i missed something in the datasheet ? :roll:
 
Yo,

The MC35i states the zif conn has a "– RS232 bi-directional bus" on it. This usu. means it uses hardware handshaking.

Your app can easily support this: wire the pic to the pins. Alternatively, you can fake this : simply wire the "CTS" pin (clear-to-send) to the "RTS" pin.

Try this, i can't seem to get full datasheet, can you send the link ? Preferably post the pinouts. The fact that there are 8 pins indicates that this is the case (the /8 beside bus).

Hope that helps. Else email me / rite back.

==JOHN
 
Yo,

The MC35i states the zif conn has a "– RS232 bi-directional bus" on it. This usu. means it uses hardware handshaking.

Your app can easily support this: wire the pic to the pins. Alternatively, you can fake this : simply wire the "CTS" pin (clear-to-send) to the "RTS" pin.

Try this, i can't seem to get full datasheet, can you send the link ? Preferably post the pinouts. The fact that there are 8 pins indicates that this is the case (the /8 beside bus).

Hope that helps. Else email me / rite back.

==JOHN
 
Thanks for all your help, please refer to the following link for data sheet
**broken link removed**


And for MAX232, yes, the logic level maybe also incompatible with 3V logic but right now I dont have any 3v to 5v logic converter in hand. I can only assume that the 3v is compatible with 18F452.
 
Yo,

The MC35i states the zif conn has a "– RS232 bi-directional bus" on it. This usu. means it uses hardware handshaking.

Your app can easily support this: wire the pic to the pins. Alternatively, you can fake this : simply wire the "CTS" pin (clear-to-send) to the "RTS" pin.

Try this, i can't seem to get full datasheet, can you send the link ? Preferably post the pinouts. The fact that there are 8 pins indicates that this is the case (the /8 beside bus).

Hope that helps. Else email me / rite back.

==JOHN
 
Oops, sorry. Seems to have posted twice. My net cut out and i didn't think it posted last night, so i posted it today... shoud have scrolled down. If there's a moderator around, please delete my messages.

CHeerZ,
J^B
 
gday... just downloaded datasheet and had a look. You state u tried a max232 in your design... why? The logic level on the mc35 is 3V =>> a max232 will step it up to twelve, possibly damaging the modem!!

"The electrical characteristics do not comply with ITU-T V.28. The
significant levels are 0V (for low data bit or ON condition) and 2.65V (for high data bit or OFF
condition). For electrical characteristics please refer to Table 22.

MC35i is designed for use as a DCE. Based on the conventions for DCE-DTE connections it
communicates with the customer application (DTE) using the following signals:
Port /TXD @ application sends data to the module’s /TXD0 signal line
Port /RXD @ application receives data from the module’s /RXD0 signal line
"


I hope 4 ur sake that you haven't damaged ur modem. The pic's pins should be wired directly to the modem (unless, of course, you are using the interface board, and not the socket itself.)

The PIC should be running on 3.3V (or about that)... if you're using 5V, you might want to use voltage dividers on the PIC's outputs to ensure that you don't stuff your modem (inputs will still work).

To wire it up : PIC's TX pin goes to the RXD0 Line and the RX pin goes to the TXD0 pin (try swapping these though... the manual is rather ambiguous). Then, you have to implement the handshaking (HW) - or you can "spoof" it.

Read here :

http://www.zytrax.com/tech/layer_1/cables/heavy.htm

I would recommend that you wire RTS to CTS to spoof that signal. (I assume know to use a common ground for the modem and the pic). Wire the DTR0 pin to DSR0 pin and the DCD0 pins (all three together). I know that's not clear, if you don't understand, post back.

Also, ensure the command you send is "AT...\r", not "AT...\n", as cr is used instead of lf.

Hope that helps.
Feel free to email or msn me : beatonj -AT- gmail.com

--jb
 
Thanks for all your help, I haven't damaged my modem yet, ha, The reason why I use MAX232 is I need to test it talking with my PC, so I need to interface it with the transciever chip.

And moreover, I have succefully solved the problem already just now :D , The reason for 18F452 failling to talk to my Siemens modem is their logic levels incompatible to each other (one is 5v logic and the other is 3v). And I use PIC16LF76 which can work with 3v logic to communicate with MC35i instead of 452, it works!

So my conclusion for this is that 18F452's RX pin is ST (Schmidt trigger) input, it need 0.8*VDD to recognise the signal as high, and this will reach to 3.36V, but on the other side, the MC35i modem will only output high at 2.73V according to the datasheet. However their Tx pins seem can compatible with each other even though they are in different logic levels.

And thanks a lot for all your help, especially who those suggested the logic level problems.
:D :D
 
Hi, I'm also doing a similar project but using picaxe as my IC. As it outputs 5 volts, would I need to have a voltage divider to lower it or will another chip be able to do it as that would be better overall. I would like to use the modem to send lines of text to a mobile phone that is pre-determined by the Picaxe IC.
 
Hi, I'm also doing a similar project but using picaxe as my IC. As it outputs 5 volts, would I need to have a voltage divider to lower it or will another chip be able to do it as that would be better overall. I would like to use the modem to send lines of text to a mobile phone that is pre-determined by the Picaxe IC.

Either run the PICAxe off the same supply as the modem (presumably you're talking about a chip, and NOT a complete boxed modem like the OP), or use a simple resistive attenuator to feed the modem chip. This only need to be one way, the RX line on the PIC is fine with the lower voltage.
 
Thanks. Would you have a circuit diagram for wiring the PICaxe to the MAX232 then onto the modem. This is such a great help as this is for my year 12 project:)
 
The PICaxe help files drop away a bit around serial data, there isn't much there. I'm using a 20X2 PIC that is going to read 8 digital inputs then for each permutation, there will be defined text that I would then like to send via text to a mobile phone. That is what I am intending to do for my project (this aspect)
 
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