2.8v RS232 driver

[newbie]

Hi, things are going well with my project, but I want to talk to the GSM to do tests (emulate the PIC) as it's talking at a max voltage of 2.8v as is the PIC, I can use a normal max232 etc. to talk to it via PC, is there a RS232 line driver that does 2.8v levels?

Thanks
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This message was ported from CCS's old forum
Original Post ID: 12245

[PCM programmer]

:=Hi, things are going well with my project, but I want to talk to the GSM to do tests (emulate the PIC) as it's talking at a max voltage of 2.8v as is the PIC, I can use a normal max232 etc. to talk to it via PC, is there a RS232 line driver that does 2.8v levels?
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There are chips available, but the packages are not that
convenient for breadboarding. You can run them at +2.8v.
<a href="http://pdfserv.maxim-ic.com/arpdf/MAX3380E-MAX3381E.pdf" TARGET="_blank">http://pdfserv.maxim-ic.com/arpdf/MAX3380E-MAX3381E.pdf</a>

It might be easier just to use a MAX232A, and then use
level converters on the GSM side. In the MAX232A data
sheet, it gives the "Input Logic Threshold" levels for
the RS-Transmitter. While it's typically 1.4v for both,
the real range is Vih = 2.0v and Vil = 0.8v. So your
GSM can likely drive the transmit buffer of the MAX232A
directly. No logic level conversion is required.
And, the MAX232A is running with a Vcc of +5v.

On the receiver side of the MAX232A, it's going to be
putting out, typically, 4.8v for Voh, and 0.4v for Vol.
Unless your GSM has 5v tolerant inputs, the Voh is too high.
So you need a level conversion chip. You could use LVC or LVT
logic families. This chip would work, I think:
<a href="http://www-s.ti.com/sc/ds/sn74lvc1g125.pdf" TARGET="_blank">http://www-s.ti.com/sc/ds/sn74lvc1g125.pdf</a>
You could run it at 2.8v, and it can accept the 4.8v coming
from the MAX232A receiver's output, and convert to a output
high level of probably 2.7v. The low level output would be
probably 0.1v. Digikey has those chips available in the
SOT23-5 package for about $.50 (US).(T.I. p/n SN74LVC1G125DBVR)
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This message was ported from CCS's old forum
Original Post ID: 12246

[Hans Wedemeyer]

:=On the receiver side of the MAX232A, it's going to be
:=putting out, typically, 4.8v for Voh, and 0.4v for Vol.
:=Unless your GSM has 5v tolerant inputs, the Voh is too high.
:=So you need a level conversion chip.

The Rx output side is easiest to convert by using an old fashioned potential divider i.e. two resistors.

With a litle Ohm's law it can be easier than adding chips !
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This message was ported from CCS's old forum
Original Post ID: 12266

[Willie Erasmus]

:=:=On the receiver side of the MAX232A, it's going to be
:=:=putting out, typically, 4.8v for Voh, and 0.4v for Vol.
:=:=Unless your GSM has 5v tolerant inputs, the Voh is too high.
:=:=So you need a level conversion chip.
:=
:=The Rx output side is easiest to convert by using an old fashioned potential divider i.e. two resistors.
:=
:=With a litle Ohm's law it can be easier than adding chips !


I've found that you can drive the MAX232 chip at 3.3V and then you can connect it directly to the GSM modem because the voltage levels on the GSM side will be 3.3V max.
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This message was ported from CCS's old forum
Original Post ID: 12297