System block diagram

This page describes the interface circuit which is used at seismological stations running SEISLOG at Institute of Solid Earth Physics, University of Bergen.

The interface is used between a GPS and a PC. The PC is running SEISLOG-software and the GPS (Garmin GPS 35-HVS) is providing accurate synchronization of the PC's real-time clock.

The GPS outputs NMEA0183 messages (GPGGA & GPRMC) at 9600 baud. It also has an output called 1PPS (meaning One Pulse Per Second). NMEA messages are output at RS-232 levels and is directly connected to one of the PC's COM-ports.

The 1PPS signal is (by default) a positive pulse of 100ms. Voltage levels are 4.4V and 0V for logical high and low, respectively. Output impedance is 250 ohm. If this signal is directly connected to a PC parallel port input with internal pull-ups (most PCs have internal pull-ups at approx. 1 kohm) the voltage levels will be pulled to 4.5V and 1V for high and low. The high level is OK, but the low level is out of specification for TTL-signals. Low level should be maximum 0.8V at an input, but in order to obtain required noise margins a TTL-output shall not deliver more than 0.4V.

The OCCC solves this problem by adding an emitter-follower buffer to the 1PPS signal. The transistor used is a PNP germanium transistor (AC128). The voltage levels at the PC's parallel port (or at the Emitter of transistor) is 4.6V and 0.2V for high and low, respectively.

If the OCCC is used in a very noisy environment one should keep in mind that germanium transistors are sensitive to high voltages. Some kind of EMP-protection (transient diodes) may be added at Basis of the transistor.

The original schematic diagram has been scanned and is available as an Adobe Acrobat PDF-file.

Click to download:

occc.pdf

Here is also the GPS 35 "Technical Specification".
There may be a newer revision available at Garmin's WWW-site.

Click the icons below to see full-size images of the different parts of the interface.