A long but light hearted story/problem regarding a handphone

[qtommer]

hi,

I have a PIC communicating with a SE T630 Handphone via the good ol MAX232 ( I did an SMS control system)

This board worked fine when I used it about 6 months ago. Everything was going smoothly, SMS control could be initiated.

However, recently when I got back to using my setup again, the communication some how seemed to have a sort of "intermittent" problem with the handphone.

The serial connector to the handphone type is a snap-on version. So I snapped on the serial connector head to the port of the handphone but my board did not indicate communication (each successful AT configuration I indicated with a blink of an LED).
I had to adjust the snapped on head in a "sweet spot" and then, only did the board indicate communication (blink the amount of times required.) So ,I held it in it's "sweet spot" using an elastic band and everything worked fine. SMS control could be achieved in my system once again.

So I wondered what the problem was, I checked the communication of the handphone with Hyper Terminal and the communication with Hyper Terminal was flawless. AT commands were transmitted to the phone, Responses were received without any flaw. Did not require me to adjust the snapped on cable head in a sweet spot for it to communicate. Only when I connected it back to my board did I have to hold it with an elastic band.

The problem gets worse, After a few weeks, communication could no longer be established with the board no matter how I held it in with an elastic band. So I wondered if it was a phone problem.

I went to a 2nd handphone dealer and borrowed a similar SE phone model, I snapped on the cable and voila, communication was established. No need for elastic band...However, the downside was that this phone only supports PDU mode. I developed my program in Text mode. So I had to return the handphone.

I recently purchased a another SET630 2nd hand and it worked flawlessly with Hyper Terminal. I plugged it with my board and now the board is not detecting anything.

So im wondering if the SET630 handphone has a tendency to have defects where it works well with a PC but not a MCU? I know it sounds very fishy but Im sure my board is working because it worked before. And I've troubleshooted the board for sleepless nights. All the connections and required voltage levels are fine.

So I was wondering if it was the RS-232 voltage levels differing between the MAX232 output and that of the board. For as long as the voltage level is below -3V, a Logic 1 should will be registered (EIA-RS232 specs ).I measured both the output of the MAX232 and the PC Serial Port. Both gave me an approximately -6V reading. Should not be a board problem


So my question (finally) is ...has anyone expereinced this sort of thing before? Could it possibly be a problem of the phone itself? (some internal hardware/software defect)
Should I spend more cash to get another phone and test it out?
if it helps the cable looks like this (see link)
**broken link removed**


The end=)

 

[Diver300]

Have you tried a different cable?

There are differences in the RS232 levels. Desktop PCs will often swing more. I know you measured.

Are you running the same baud rate as the PC? It could be that there is a bad connection which slows the rise and fall times, so only slower baud rates are supported.

AT+CMGF=1<CR> should switch a phone to text mode from PDU mode.

 

[qtommer]

hi thanks alott for your reply=)

Have you tried a different cable?

no i haven't because the cables are hard to obtain in my area of the world. I even had a hard time finding the phones cuz they're obsolete. But the cable should be okay i guess cuz it the phone communicates well with byper terminal

There are differences in the RS232 levels. Desktop PCs will often swing more. I know you measured.

hmmm..are there possibilities that certain max232s due to lot variations will swing less? I measured the voltages on pins 2 and 6 and I got about 9V and -8.6V++ respectively. It oughta be 10V and -10V each right? Would that pose a problem? (i use 1uF 50V rated caps)

Are you running the same baud rate as the PC? It could be that there is a bad connection which slows the rise and fall times, so only slower baud rates are supported.

Yes the baud rates are the same. pretty slow decent rate of 9600.

AT+CMGF=1<CR> should switch a phone to text mode from PDU mode.

Yes I do know that. Unfortunately that phone model does not support text mode. By sending AT+CMGF=?<CR> to that particular phone, only '0' was returned.

 

[qtommer]

i was wondering if there is such thing whereby the phone by default requires CTS /RTS handshaking and there is a way to unlock it in the phone so that only Tx /Rx and GND is required for communication. Is there such a unlock feature as this for phones?

i wondered so cuz certain phones works fine with a PC but not with the board..but other phones work fine with both.

 

[qtommer]

Ericsson T-28, T28z, T28-world, T39, T39m, T60, T60d, T60lx, T61z, T68, A2218z, R300d, R300lx, R300z, R380-world and Sony Ericsson T68i, T100, T200, T300, T310, T600,T610, P800 cellular phones data and service cables scheme pinout and wiring @ pinout

I found out the schematic of the internal cable and it uses a max3232 to interface the logic levels of the handphone (weak TTL 0-3V) to rs232..The 3Volts was measured directly on the Tx pin from the output of the handphone (Tx idle state is always high)
Any idea on how this cable might work well with PCs but not with PIC via MAX232s?

There are differences in the RS232 levels. Desktop PCs will often swing more. I know you measured.

You were right, It was an error on my part.. The PC serial port Tx has a +/- 11V swing and my max232 has a +/- 9V swing..My bad..*blushing*

 

[qtommer]

i've managed to find out the cause of the problem..the given schematic from the source is incorrect. There is flow control available through the DRS-11 cable unlike shown in the schematic link where the RTS is looped back to the RTS..

I shorted CTS to RTS on the DB9 end of the cable and everything is working fine. Writing this just to inform any one out there who might possibly employ a cable of this sort