Note: Although Lorne designed this circuit for use with a Garmin GPS-50, it should work with most other receivers. /pb Date: Fri, 26 May 1995 09:05:21 EDT From: Newsgroups: sci.geo.satellite-nav Subject: Garmin GPS50 / computer interface circuit Since asking for assistance a couple of weeks ago, I received many notes giving hints and suggestions. Among those providing assistance and support were Rane Curl, Reg Neale, Peter Bennett, Phil Sohn, Larry James, Bob Winingham, Tom Dawes-Gamble, Mike Walters, Randy Bowman. I borrowed an oscilloscope and other electronic gadgetry and with the help of local computer techs set down to explore the problem. The summary is that the GPS50 provides NMEA output, not RS232 or RS422. It's a current loop output device, and depends on a resistive load to provide RS232 and RS422 voltage swings. According to the computer techs here, there is wide variety in the design of COM ports and the impedance presented to current loop devices. In a following post, I am publishing a computer interface which eliminated connection problems for me and my GPS50. It cost me about $5 in Radio-Shack parts to build. It has worked successfully on two different PCs, one RS232, the other RS422, and a PowerMac with RS422 geoport. Thanks to all who provided advice and assistance. Regards Lorne Dudley Kingston, Ontario. Circuit for converting Garmin GPS50 NMEA output to RS232, RS422 =============================================================== GPS50 output is current loop. Non-loaded output is +4.8v steady state to +4.95v . Through a 2.2K load, +2.1v steady state to +4.95v . Through a 1 K load, +1.4v steady state to +4.8v . I found that various PC and Mac COM ports offered different impedances when the GPS50 was connected directly. Also, some RS232 TxD connections would not properly drive the GPS50 directly. This circuit has operated successfully on every PC / Mac I have tried to date. It can be powered from any 9 - 12 v DC source. Current draw is approximately 10 ma. It produces 0.0v steady state to +9.0v peaks to the RxD connection and +0.5v steady state to +5.0v peaks to the GPS input connection. 1995-May-26, Lorne A. Dudley ------------------------------------------------------------------------- + - V+ 10v --------->|-----------+ diode \ | | \| 4 from GPS |3 \ >---------+--------------|+ \ | | 1\------------------> RxD RS232 Z | / 1K Z +------|- / Z | |2 / | | | /|11 ---- | / ---- / / / | / / / ground +-----------------+ Pin 11 | | | V+ (Pin 4, +10 v) | | | | | Z \ | 16.4K Z | \ | (2 x 8.2K) Z 3.3 v |5 \ | |--------------|+ \ | Z | 7\--+ 3.3 v 8.2K Z | / | Z +------|- / | | | |6 / | | | | / | ---- | / | / / / | | +------ZZZZZZ-----+ 8.2K | | +-------------------------+ | | \ | | \ \ +-----------+--|+ \ | \ + | |10 \ |12 \ diode V | 8\--ZZZZZZ-------| \ to GPS - | | / 1 K | | 14\--+---------> TxD >------ZZZZZZ----+--|- / | | / | RS232 1K |9 / | | / | | / | ---|13 / | / | | | / | | | / | | +-------------+ | | - | | --- Z + ^ 5.1v Z 8.2K + zener Z | diode | ----- ---- / / / / / / Parts List ========== Qty 1 quad op amplifier, LM324N, Radio Shack 276-1711, $2.99 CDN. 2 diode, 1N4007, or equivalent (not critical) 1 zener diode, 5.1v, 1N4733, Radio Shack 276-565, $1.69 CDN. 5 resistor, 8.2K, 1/4 watt 3 resistor, 1K, 1/4 watt