How to send a "BREAK" to a modem?

runtime · Joined: 15 Sep 2003 Posts: 36

Hello,

I'm using a PIC18F252 interfaced to a Wavecom modem (GSM/GPRS), I'm only using the TX, RX and GND (RS232) pins of the modem to comunicate with the PIC. Sometimes the PIC looses comunication with the modem, maybe because I'm using software uart or maybe because the other pins are not used (CTS, RTS, DTR, DSR).

Anyway, I put a check subroutine that will check if communication is ok every minute sending an AT command to the modem, if it responds OK then everything is working well, otherwise I want to send a BREAK to the modem to reset the TX and RX lines...

Hope is clear enough.
Thank you in advance
Peter

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

As <break> is not an ASCII character you will have to bit-bang it. Between chars the line should be at Stop bit level. Set it to Start bit level for 10ms (IIRC) and then back to Stop bit level. That is all there is to it.
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runtime · Joined: 15 Sep 2003 Posts: 36

hello SherpaDoug,

what do you mean for "Stop bit level" and "Start bit level", and what line should I use, TX or RX?

Thanx

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

You are sending the <break> to the modem so you would use the TX line. Normally Start is a logic 0 and Stop is a logic 1, unless you are using the "invert" software UART option option.
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runtime · Joined: 15 Sep 2003 Posts: 36

Hi SherpaDoug,

you said "Between chars the line should be at Stop bit level. Set it to Start bit level for 10ms (IIRC) and then back to Start bit level. That is all there is to it."

should it be: Set it to Start bit level for 10ms (IIRC) and then back to Stop bit level ?

thank you

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Yes, you are right. I corrected my post.
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Guest · Posted: Wed Sep 15, 2004 11:05 am

Don't worry about modem handshake signals.
Just connect pins 7-8 (one short circuit) and 11-12 (another short) (assumed that you use Fastrack series and not the Integra line Smile

).

Jeff King · Posted: Wed Sep 15, 2004 4:22 pm

You also need to turn the uart off to gain control of the pin, if using a hardware uart. There is a command in CCS to do it or you can do it direct.

Kieran · Joined: 28 Nov 2003 Posts: 39 Location: Essex UK

How about putc(0xFF); ?
Should send a start followed by stop level for one char duration.
saves the Tx pin bit banging.

Ttelmah · Guest

Guest · Posted: Thu Sep 16, 2004 6:10 am

Maybe 0x00 would be better. Sending 0xFF contains only one-bit time of break level (the start bit itself).

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

Ttelmah · Guest