Maker Pro
Maker Pro

How to use TX/RX on a PIC

H

hybrid_snyper

Jan 1, 1970
0
Hi,

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.

yours

Wayne
 
P

petrus bitbyter

Jan 1, 1970
0
hybrid_snyper said:
Hi,

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.

yours

Wayne

Wayne,

The MAX232 is necessary. It inverts the signals from the PIC and converts
them to the voltages an RS232 needs. You need to do the same when you want
to talk with a PIC to a COM port of a PC.

petrus bitbyter
 
C

Costas Vlachos

Jan 1, 1970
0
hybrid_snyper said:
Hi,

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.

yours

Wayne

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.

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email: [email protected]
SPAM-TRAPPED: Please remove "-X-" before replying
 
T

Tim Auton

Jan 1, 1970
0
Costas Vlachos said:
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.


Tim
 
H

hybrid_snyper

Jan 1, 1970
0
Thank you very much for that, I will plod on with that and see what i
can come up with. If i have any problems i will let you know.
 
C

Costas Vlachos

Jan 1, 1970
0
Tim said:
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.


Tim

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.

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email: [email protected]
SPAM-TRAPPED: Please remove "-X-" before replying
 
P

petrus bitbyter

Jan 1, 1970
0
Tim Auton said:
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.


Tim


Hmmm. I carefully reread the original question. The OP tells he knows how to
connect the COM port of its PC to the phone. If he did so and the phone has
3.3V or 5V logic inputs, it's blown by now. So I concluded the phone to have
an RS232 interface. I agree, it's not very likely the phone to have this
build in, but you can buy interface cables. The ones I'm aware of have a
MAX3232 or similar in their connector. So you are right, it will end up
converting two times. 5V <--> RS232 <--> 3V3. Which will be the most safe
way for someone asking questions like this.

petrus bitbyter
 
T

Tim Auton

Jan 1, 1970
0
Costas Vlachos said:
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
 
C

Costas Vlachos

Jan 1, 1970
0
Tim said:
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).

I see what you mean. If the inversion/level-shifting is done in the
phone cable, then he should be able to do it like you said in your first
post (direct connection, resistors for protection). It makes sense for
the phone manufacturer to move the MAX232 or similar IC outside of the
phone for size/cost issues. The OP can connect directly like you said
and see what happens. If it doesn't work, invert the signals and try
again. I don't think level shifting will be necessary for such a small
device anyway (it definitely isn't for modern PCs). I was just assuming
that the Tx/Rx signals from the phone would match the standard polarity
of the RS-232 in which case inversion would be necessary. I suppose he
could measure the phone's Tx line when idle to determine whether
inversion is needed.

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email: [email protected]
SPAM-TRAPPED: Please remove "-X-" before replying
 
H

hybrid_snyper

Jan 1, 1970
0
Costas said:
I see what you mean. If the inversion/level-shifting is done in the
phone cable, then he should be able to do it like you said in your first
post (direct connection, resistors for protection). It makes sense for
the phone manufacturer to move the MAX232 or similar IC outside of the
phone for size/cost issues. The OP can connect directly like you said
and see what happens. If it doesn't work, invert the signals and try
again. I don't think level shifting will be necessary for such a small
device anyway (it definitely isn't for modern PCs). I was just assuming
that the Tx/Rx signals from the phone would match the standard polarity
of the RS-232 in which case inversion would be necessary. I suppose he
could measure the phone's Tx line when idle to determine whether
inversion is needed.

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email: [email protected]
SPAM-TRAPPED: Please remove "-X-" before replying


So direct connection is the way to go. I found these two links a while
back. The first is what I believe to be the circuitry within the
datacable and to which would allow me to eliminate the cable.

http://pinouts.ru/CellularPhonesCables/ericsson_cable_pinout.shtml

this next link shows a guy which has a Tiny12 connected directly via a
datacable to his MCU. I take it would be a similar setup to this. He
has the data entering the MCU fed direct with a 4700 resistor in
between. However he has added circuitry to create a true bi-polar
signal. Im not sure what this means.

http://www.riccibitti.com/tinyplanet/tiny_article.htm
 
C

Costas Vlachos

Jan 1, 1970
0
hybrid_snyper said:
So direct connection is the way to go. I found these two links a while
back. The first is what I believe to be the circuitry within the
datacable and to which would allow me to eliminate the cable.

http://pinouts.ru/CellularPhonesCables/ericsson_cable_pinout.shtml

this next link shows a guy which has a Tiny12 connected directly via a
datacable to his MCU. I take it would be a similar setup to this. He
has the data entering the MCU fed direct with a 4700 resistor in
between. However he has added circuitry to create a true bi-polar
signal. Im not sure what this means.

http://www.riccibitti.com/tinyplanet/tiny_article.htm

OK, these are nice links relating to what you want to do. From the first
link it's clear that there's no need to invert the signals when you
connect the phone *directly* to the PIC (i.e., *without* using the
special Ericsson cable which has circuitry inside). If you do it like
this you should be fine using only resistors for protection (as Tim
Auton suggested earlier):

Phone Tx -----\/\/\----- PIC Rx
Phone Rx -----\/\/\----- PIC Tx
Phone GND --------------- PIC GND

I don't know about the voltage levels of the T28. Are they 5V or 3.3V?
Probably the latter. As Tim said, run the PIC at the same voltage. I
don't think you need to worry about true bipolar signals, the other guy
in the second link probably uses the proper Ericsson cable (with the
circuit inside) and so has to deal with negative voltages (hence the
Zener), and uses the transistor to "steal" negative voltage from the
incoming signal to generate his own bipolar Tx signal. The transistor
also inverts the MCU's Tx signal. He probably does the inversion of the
Rx signal in s/w (inside the MCU - no h/w UART used).

So, to recap, if you're somehow connecting the phone directly to the PIC
(no cable) or you're using just a passive connector or a dummy cable (no
circuitry involved) then use the above wiring (PIC's voltage same as
phone's logic levels, 5V or 3.3V). If you *are* using the proper
Ericsson cable (with the level shifter/inverter inside), then you need
to invert the signals again so that the PIC & phone can talk to each
other. Hope this is now clearer.

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email: [email protected]
SPAM-TRAPPED: Please remove "-X-" before replying
 
H

hybrid_snyper

Jan 1, 1970
0
If you *are* using the proper
Ericsson cable (with the level shifter/inverter inside), then you need
to invert the signals again so that the PIC & phone can talk to each
other. Hope this is now clearer.

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email: [email protected]
SPAM-TRAPPED: Please remove "-X-" before replying

So to use the proper connector is going to require the max232 anyway,
so im going to be better off using some copper wire some resistors and
try and wire the phone pins to the PIC pins directly. I bought the data
cable on the assumption it would be the simpler option, connect the
data cable to a RS232 port wired to the PIC. But in this case it is
not, its def not the simpler way,

Thanks for that guys I appreciate it, i will be in touch with the
outcome
 
C

Costas Vlachos

Jan 1, 1970
0
hybrid_snyper said:
So to use the proper connector is going to require the max232 anyway,
so im going to be better off using some copper wire some resistors and
try and wire the phone pins to the PIC pins directly. I bought the data
cable on the assumption it would be the simpler option, connect the
data cable to a RS232 port wired to the PIC. But in this case it is
not, its def not the simpler way,

The simplest method is the direct connection (no cable). But since you
already have the proper cable, you could implement the circuit of the
Tiny MCU project (second link you posted earlier). You just need a
second transistor (NPN this time) between the Zener & the PIC, like this:

.. +5V
.. |
.. |
.. 10k
.. |
.. +----------> To PIC Rx
.. |
.. C
.. From Zener/4.7k ----------> B BC547
.. E
.. |
.. |
.. GND

No real need for the MAX232, and in this way your PIC circuit alone (and
phone alone) will be able to talk to other devices too, like a PC. Plus
your PIC can operate at standard +5V (no need to match the phone's logic
levels). If I was you I'd do it like this. I've used the transistor
approach to connect a PIC to a PC with no problems. Your phone+cable is
like a PC as far as RS-232 signals are concerned, so I don't see why it
wouldn't work.

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email: [email protected]
SPAM-TRAPPED: Please remove "-X-" before replying
 
H

hybrid_snyper

Jan 1, 1970
0
Costas said:
The simplest method is the direct connection (no cable). But since you
already have the proper cable, you could implement the circuit of the
Tiny MCU project (second link you posted earlier). You just need a
second transistor (NPN this time) between the Zener & the PIC, like this:

. +5V
. |
. |
. 10k
. |
. +----------> To PIC Rx
. |
. C
. From Zener/4.7k ----------> B BC547
. E
. |
. |
. GND

No real need for the MAX232, and in this way your PIC circuit alone (and
phone alone) will be able to talk to other devices too, like a PC. Plus
your PIC can operate at standard +5V (no need to match the phone's logic
levels). If I was you I'd do it like this. I've used the transistor
approach to connect a PIC to a PC with no problems. Your phone+cable is
like a PC as far as RS-232 signals are concerned, so I don't see why it
wouldn't work.

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email: [email protected]
SPAM-TRAPPED: Please remove "-X-" before replying


i think i will go with direct connection, just because i want to move
from a mobile phone to a standalone GSM modem make it my device into a
tidy unit, with modem mounted to the circuit board.
 
Top