Problem with modem interfacing as it doesn send a ring

[techie]

Modem Interfacing

I have an external 56k RS-232 modem that uses Allegro Chipset (from Intel). It uses standard AT command set. But when upon an incoming call, it does NOT send a "RING" to the serial port. I opened the modem and it does have the ring detection circuit that normally modems have. Is there a command that enables sending RING to serial port. I also noticed that it responds to the auto-answer command of ATS0=1 by replying OK and turning on the LED showing AA (auto answer). But when a call comes, it does not pickup automatically.

 

[wky]

Re: Modem Interfacing

Is it possible that the ring detection circuit is not working ?
Just verify it with a scope. 8)

 

[techie]

Re: Modem Interfacing

The ring detection circuit is working perfectly. I checked the output of the opto-coupler.

I am having quite a few problems with interfacing the 56k modems to the microcontroller. In the past, I had some old 9600 baud modems that worked perfectly with the micro-controllers. Now I had to swithover to 56k modems but operate them on the same 9600baud - 2400baud. None of the modems (USR chipset, Intel Ambient chipset) are simple to use. They carry some complex superset of the AT command set.

Anyone familiar with these.

 

[glenjoy]

Modem Interfacing

Hi, try to put the Ring Indicator of your modem to an interrupt pin of the microcontroller.

Another try calling the modem and put a tester on the Ring Indicator, try to get the values.

 

[C-Man]

Re: Modem Interfacing

Maybe it helps if you change the enabling of result codes:
ATXn
n=0 Basic Result Codes
n=1 Exended Result Codes
n=2-4 Advanced Result Codes

AT&An
n=0 Disable /ARQ Result Codes
n=1 Enable /ARQ Result Codes
n=2 Enable /Modulation Codes
n=3 Enable /Extra Result Codes

I use one of these initstrings in most of my applications:
const char USR_INI_STRING[]="ATQ0V1E0L2M1&H0&I0&K0&M0S0=0";
const char STD_INI_STRING[]="ATQ0V1E0L2M1&K0%C0\\N0S0=0";

hope this helps

 

[techie]

Modem Interfacing

If i set the modem to auto-answer mode, the LED of auto-answer comes on, but it still does not reply on a ring.Could this mean a faulty hardwaere. Or is there a command to over-ride the ring detection. I amsaying this because the circuit till theopto-coupler was checked by me and is ok.

 

[C-Man]

Re: Modem Interfacing

If i set the modem to auto-answer mode, the LED of auto-answer comes on, but it still does not reply on a ring.Could this mean a faulty hardware. Or is there a command to over-ride the ring detection. I amsaying this because the circuit till theopto-coupler was checked by me and is ok.

I have not found a command a command for overriding ring detection. Maybe the modem was built for another national standard and does not recognise your ringtone. I have seen modems that have problems detecting ringtones from an internal telephone system but there should not be a problem like this with the direct phoneline.

Try to remove anything that is connected in parallel to your modem (like telephone, answering machine, fax). There is also always a possibility that you have a faulty modem.

best regards

 

[klug]

Re: Modem Interfacing

Seems modem's processor does not detect the ring signal from opto-coupler. Other way it would be sending "RING" message to COM port.

There are few possible reasons:
1. Broken input pin of processor.
2. Non standard ring signal from your telephone provider - there can be other frequency of ring tone. Standard ring frequency is 16-20 Hz.
3. As I remember, there was a method to use single line with two phones - they have different frequencies of ring tone. Check it.

Anyway, check output of your optocoupler with oscilloscope again - pay attention for frequency and stability of ring tone there.

 

[techie]

Modem Interfacing

The pin of the 4N35 optocoupler goes directly to Modem chip MD4450C (C1rrus-Amb1ent-1ntel). There is no broken connection. Upon a ring, it shows a clean, stable 25Hz sqaure wave.

What are the standards of RING in USA. Is 25Hz too high that the modem is rejecting it.

Is there AT command to configure that.

 

[klug]

Re: Modem Interfacing

It was funny error - I mean broken input of processor.

Next. That modem feature is calling "distinctive ring" - and as I found they differ not in frequency, but in ring pattern - long ring or short...

Some explanation here:
http://www.modemsite.com/56k/dring.asp

But different rings are making different string in COM, as "RING A", "RING B" ...

You have no message at all...

So, it is a big chance that your modem processor is broken.

About ring frequency - there are a lot of standards for ring frequencies - 16.7 / 20 / 25 and 50Hz .
**broken link removed**
I do not know, maybe your modem is waiting some of this frequencies.

BTW, did you try to reset modem settings to default factory settings by AT&F or AT&F1 command?

 

[techie]

Modem Interfacing

Yes i did reset the modem tto factory defaults. No change.

My only conclusion is that maybe the modem chip is faulty. Tomorrow, I will buy another similar modem and try that.

 

[dani]

Re: Modem Interfacing

hi all,
I have read these posts and i really helped me. Now i m able to interface Mobile/Cell phones and their modems too.
But there is another problem now

There came lots of CDMA telephone sets (wireless local loop complaint). With the help of AT commands I am able to dial the voice and data calls using hyperterminal or microcontroller as These CDMA phones have Serial interface . But The Ring detection is the problem as these phones donot respond +RING on ring detection, even mean while there is sound comming for ring from phone's speaker. When i set this CDMA phone in FAX mode or Modam mode then i got String +RING on terminal but no sound from the set.
Is there any way that i can detect ring in voice or normal mode from this CDAM phone.

Regards...

 

[techie]

Modem Interfacing

CDMA phones are dealing with digital data as compared to fixed line modems that are dealing with analog tones. The CDMA phones digitize voice compressing them simultaneously using special codecs designed for voice. So when when you transmit data, they detect that and use another codec. That is why you have to switch the phone to data mode when sending fax. Now this mode had to be compatible with the switch of the WLL service provider. The connection should be "data-enabled" for data transmission. YOu should check if your service provider has this enabled or not.

 

[hm_fa_da]

Re: Modem Interfacing

hi ,

there is an easier way , just make a ring detection circuit and connect it to your MCU !

regards.

 

[byjuchacko]

Re: Modem Interfacing

Hi Guys,

Any of you have some sample code for sending FAX using AT commands? Pls post it here..

Thanks,
Byju.