The "One Component Carlos'
System block diagram
This page describes the interface
circuit which is used at seismological stations running
SEISLOG at Institute of Solid Earth Physics, University
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
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:
Here is also the GPS 35 "Technical
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.
Last update: January 24, 2000 by E.G.
This is a
construction zone ....