Re: How to use TX/RX on a PIC

Costas Vlachos <c-X-vlachos@xxxxxxxxxxxxxx> wrote:
Tim Auton wrote:
Costas Vlachos <c-X-vlachos@xxxxxxxxxxxxxx> wrote:
hybrid_snyper wrote:

I'm looking for some advice, i am trying to control an Ericsson T28
mobile phone by using a PIC16F87X. I understand the how to talk to a
mobile phone by using my COM port on my PC. Is it as simple as
connecting the relevant TX/RX pins to the correct pins on the mobile
phone? Do i need any circuitry between. Ive seen something about MAX232
when using serial communications any tips would be appreciated.
The proper way is to use the MAX232. Another nice IC for this is the
SN75155 (8-pin DIL). Depending on the voltage levels recognised by the
T28, you may be able to do it using simple transistors for the signal
inversion.

Have I read a different post to everyone else?

The guy asks how to connect a PIC to a mobile phone. Neither uses RS232
levels.

PIC <> PC RS232 = MAX232
T28 <> PC RS232 = MAX232
T28 <> PIC = NO MAX232

Find out what voltage levels the pins on the T28 are at. I suspect it'll
be 3.3V logic, but it might be 5V. The easiest thing to do is run the PIC
at the same voltage as the port on the T28, then you don't need a level
shifter and can connect them directly. Series resistors of a few hundred
ohms on the TX and RX lines won't affect the functioning but might save
your PIC or phone if you hook it up wrong.

I think the signals may need to be inverted, in which case the above
won't work. For a PC <--> PIC connection you don't really need level
shifting but you must invert the signals. I don't know how the T28
serial works. If it's non-standard, the above might work, but I suspect
inversion will be necessary. Two transistors should do the trick.

Thinking about this more I think we might be talking about different
things. I would be very surprised if the T28 had on-board circuitry to
generate RS232 levels - ie something like a MAX232 on board. Mobile
phones are too cost, size and power sensetive for that. So I expect on
the connector on the bottom of the phone there is a standard
(non-inverted) logic-level UART connection, which you could connect to a
PIC directly. However, if there is a cable with a DB9 plug on the end to
connect to a PC serial port (ie RS232) I would expect that to provide
RS232 levels, with the conversion provided by something like a MAX232 or
DS276 (like a MAX232 but steals power from the signal lines) inside the
cable. I was talking about the phone end of that cable, you seem to be
talking about the PC end.

So, if the OP has a cable with a DB9 plug designed to plug into a PC and
wants to connect it to their PIC they would need a MAX232. I was
assuming they just had a connector, so wouldn't need one (or an inverter).


Tim
.

Relevant Pages

  • Re: How to use TX/RX on a PIC
    ... i am trying to control an Ericsson T28 ... mobile phone by using my COM port on my PC. ... connecting the relevant TX/RX pins to the correct pins on the mobile ... Find out what voltage levels the pins on the T28 are at. ...
    (sci.electronics.basics)
  • Re: How to use TX/RX on a PIC
    ... Tim Auton wrote: ... mobile phone by using my COM port on my PC. ... Depending on the voltage levels recognised by the T28, you may be able to do it using simple transistors for the signal inversion. ... For a PC PIC connection you don't really need level shifting but you must invert the signals. ...
    (sci.electronics.basics)
  • Re: How to use TX/RX on a PIC
    ... mobile phone by using my COM port on my PC. ... Depending on the voltage levels recognised by the T28, you may be able to do it using simple transistors for the signal inversion. ... For a PC PIC connection you don't really need level shifting but you must invert the signals. ...
    (sci.electronics.basics)
  • Re: How to use TX/RX on a PIC
    ... i am trying to control an Ericsson T28 ... mobile phone by using my COM port on my PC. ... connecting the relevant TX/RX pins to the correct pins on the mobile ... Find out what voltage levels the pins on the T28 are at. ...
    (sci.electronics.basics)
  • Re: PIC USART troubles
    ... If your T28 outputs a signal that an ordinary PC serial port ... then you will need to invert the output of the PIC so that ... I assume you are powering the PIC from 5V? ... This is what a MAX232 would do to the signals. ...
    (sci.electronics.basics)
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