How to use TX/RX on a PIC

"hybrid_snyper" schreef in bericht news: snipped-for-privacy@i42g2000cwa.googlegroups.com...

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

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
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Costas Vlachos  Email: c-X-vlachos@hot-X-mail.com
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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

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.

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
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Costas Vlachos  Email: c-X-vlachos@hot-X-mail.com
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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

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

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
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Costas Vlachos  Email: c-X-vlachos@hot-X-mail.com
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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.

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.

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
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Costas Vlachos  Email: c-X-vlachos@hot-X-mail.com
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If you *are* using the proper

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

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.

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Regards,
Costas
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Costas Vlachos  Email: c-X-vlachos@hot-X-mail.com
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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.