A board designed to log weather NMEA strings at specific intervals whilst keeping power consumption to a minimal. The NMEA strings are parsed, checked for validity and stored in the onboard 512K SRAM for later retrieval. The board can sample the strings at 1,5,10 or 20 minute intervals and can do so for 3 days to over a week depending on the interval setting. Data can be extracted at any time by connecting a PC to the serial out and sending the relevant retrieval commands. Other commands include reset, interval setting, data stats and info on the board revision.
The board is based around the PIC16F876A chip, which controls the two serial interfaces via a MAX232 chip and also the SRAM communication. Eight I/O lines are multiplexed to provide the RAM interface. The address is latched 8 bits at a time into the three 74HCT573 latches. The RAM data is then either sent or received on the same eight lines. This reduces the amount of wiring on the PCB and also prevents the need for a larger more expensive PIC chip. The serial interface to the PC uses the onboard PIC USART at 19200bps and the interface to the weather head uses a bit banging technique at 4800bps. The whole board runs at 4MHz.