Bi-directional level shifter

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Warning: not yet tested !

The circuit below shifts the logical levels between the TomTom's serial IO (at 3.3V) to RS232:

Image:dshift_bi.png

The 100k resistor keeps the input from floating when the TomTom is not connected. This variant of the circuit shown on the Serial Console page has only been tested very superficially. If you build the uni-directional version and leave some space on the PCB, you can easily upgrade to the bi-directional version later.


parts list to build the serial converter

1x 27k

2x 1N4148

1x 9 socket (female) D-SUB connector

2x 33k

1x 78L05

1x printed circuit board

1x 68k

1x 74HCT04

1x 100k

1x 100nF


Since the circuit handles neither modem nor flow control, the communication program has to be configured accordingly. If using kermit, the following commands will accomplish this: 

set port /dev/ttyS0
set speed 115200
set flow-control none
set carrier-watch off
Retrieved from "http://opentom.org/Bi-directional_level_shifter"
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