[aprssig] Argent open tracker

Thu Jul 14 13:47:28 EDT 2016

To be very explicit...

The common RS-232 receivers will accept the TTL *levels* because most of them use TTL-like circuits (or CMOS circuits designed to do TTL levels) and have a threshhold around 1.5 volts. This is well within the RS-232 spec.   RS-232 thresholds are >+3 volts for high "space" and <-3 volts for low  "mark"...

As others say, this serial data polarity must be correct and the common GPS receiver (& transmitter) internal circuits have an inverted sense because the common RS-232 level-converting IC's were inverting. This is because the RS-232 levels are "Upside-Down" with respect to mark and space when compared to logic levels (and an inverting level converter is simpler circuit-wise).   The circuit level Mark is a logic "1", high and logic space is logic "0" low.

Background:
 Serial teletype used a "Mark" level that has the line current ON to stop the receiving mechanism. The current controlled a clutch on a continuously rotating motor.  It started rotating when the first space "start bit" came along and the subsequent marks and spaces alligned some bars to make the given character print.  Then the trailing mark "stop bit" disengaged the clutch, stopping the mechanism until the next character.

Everything back in "those days" was built with TTL circuitry.  This is why the RS-232 level converters have 1.5 V levels on both the TTL side and 232 side ... and everything was made to interface with TTL when necessary. 

I still to this day have to look at the spec EVERY time I want to look at RS-232 levels just because it is upside down...  That and to get the pins correct because of the DCE and DTE differences...null modems come in real handy..........

Yes ... From Sparkfun (thanks to Google):
"By the RS-232 standard a logic high ('1') [the Mark - Ed] is represented by a negative voltage – anywhere from -3 to -25V – while a logic low ('0') [the Space - Ed] transmits a positive voltage that can be anywhere from +3 to +25V. 

-- 
 Regards, Steve Noskowicz
 Science & Technical Advisor
 http://www.challengerillinois.org/

--------------------------------------------
On Thu, 7/14/16, Randy Love via aprssig <aprssig at tapr.org> wrote:

 Subject: Re: [aprssig] Argent open tracker
 To: "Tom Hayward" <esarfl at gmail.com>, "TAPR APRS Mailing List" <aprssig at tapr.org>
 Date: Thursday, July 14, 2016, 12:14 PM

 I have
 used Garmin GPS18 pucks with the DE9 Femail interface
 intended for connection to PC's directly into OT+
 devices without concerns for levels.
 They have worked without
 issue. 
 If you hook a
 GPS18 up to a PC and verify that it is sending NMEA
 sentences at 4800 baud 8N1, then it will work correctly with
 an OT+, OTUSB, T2, or T3.
 That has been my experience with the
 GPS18's that have the DE9 Female interface with
 OpenTracker devices. I have 4 GPS18's and have used them
 with many different OpenTracker devices as well as Byonics
 TT3's and Knwd APRS capable radios ( TM-D700, D7, D710,
 and D72 ). No additional level sense or polarity
 modifications were needed.
 RandyWF5X

 On Thu, Jul 14, 2016 at
 12:19 PM, Tom Hayward via aprssig <aprssig at tapr.org>
 wrote:
 On Thu, Jul 14, 2016 at 8:59 AM,
 Andrew rich via aprssig

 <aprssig at tapr.org>
 wrote:

 > Thats my point - they want to act at RS232 but they can
 only go TTL levels.

 >

 > RS232 height would be > 7 volts

 > RS323 height would be < 7 volts

 >

 > Garmin

 >

 > RS232F is VCC

 > RS232F is 0 Volts

 Andrew,

 The piece I'm missing here is your question/problem.
 What hardware do

 you have, how do you want to use it, and what are you
 observing?

 The Argent Data OpenTracker OT1+ uses a 2N7002 on the GPS
 RXD line,

 which should trigger down to 1V. TTL levels from the GPS are
 probably

 not your problem here. When I have used TTL GPS modules
 with

 OpenTracker, I've had to invert the RXD line. If you
 have already

 confirmed your data is the right polarity, maybe something
 else is

 going on, but I don't have enough information about your
 setup to help

 you find it.

 https://www.argentdata.com/support/ot1plus-smt-1_5-schematic.png

 Tom KD7LXL

 P.S. Some posts from other times this has been asked that
 may be helpful:

 https://groups.yahoo.com/neo/groups/opentracker/conversations/topics/11672

 https://groups.yahoo.com/neo/groups/opentracker/conversations/topics/11652

 https://groups.yahoo.com/neo/groups/opentracker/conversations/topics/11419

 (you, Jason, and I all posted in this thread!)

 https://groups.yahoo.com/neo/groups/opentracker/conversations/topics/10986

 https://groups.yahoo.com/neo/groups/opentracker/conversations/topics/9833

 https://groups.yahoo.com/neo/groups/opentracker/conversations/topics/8763

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