Serial port for the new GPS

Version 2 of the GPS has a more complicated serial port interface, as it has two functions. The original only had to read from the GPS, and store the results in memory, the new version also has to be able to send the result to a PC. Unlike version 1, which used an external GPS and therefore used a regular serial input, version 2 uses an actual GPS module. The second version also has to be able to output to a PC, something that was not covered at all in the first version.

The connection to the GPS module has some intricacies, because the GPS module, despite having a 4-6V input, had only a 3.3V output. Glancing through the data sheets of the PIC16F628A this might seem to be no problem, until one notices that the serial input port of this microcontroller uses Schmitt Trigger inputs, which means the minimum voltage for a '1' is 4V. To do the conversion I used a 74HCT125, which accepts a large range of input voltages, and converts them to whatever voltage is set on Vcc. Of course it is a bit of a waste of the other 3 buffers inside this chip, but it works and makes it very unlikely to damage the GPS module. An alternative would be to run the PIC16F628A on 3.3V, but this would cause other difficulties, for example with the interface to the LCD display, which needs 5V. Apart from the buffer, the connection to the GPS is very simple, as there is no other conversion necessary.

To connect to the PC we would officially have to output +12V and -12V signals. Clearly this is not easy with 5V power, and even if we used the 9V battery power we would still run into trouble with negative voltages. Fortunately there is the DS275. This small component leeches voltage from the RS232 interface of the PC and uses that voltage to send the messages to the PC. Of course, a PC might not have the right voltage (in fact, often they do not) but you would expect a PC to be able to read its own voltage levels back. The DS275 covers the -12V, for the +12V it uses Vdrv, which in this case is still connected to +5V. As the RS232 specification states that anything above +3V should be considered legal, +5V is fine, though barely. Hooking up the DS275 is very staightforward. As you can see it handles reads as well, but in this case it is not used. However, on the PC side the read should still be connected to allow access to the voltage levels. Note that if you decide to use the DS275 for sending AND receiving you should realise the component is NOT full duplex. You can either send or receive, but not both at the same time. If you need to do this, you might want to consider the Maxim 232 range of devices, which will be covered at another time.

Of course, you can see the decoupling capacitors of 0.1uF everywhere in the schematic. The GPS, the DS275 and the 74HCT125 all need them. Also, the plug used for the serial port is female, in this case, and this is why pins 2 and 3 were swapped compared to the previous schematic.