Plevier
Well-Known Member
I don't think that's going to work.
ST60 NMEA OUT ON = A 11V, B 0V = current in inverting opto = output LOW
ST60 NMEA OUT OFF = A 11V, B 11V = no current in inverting opto = output HIGH
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PCB to convert Differential NMEA to Single Ended
- Thread starter nigelmercier
- Start date
I don't think that's going to work.
ST60 NMEA OUT ON = A 11V, B 0V = current in inverting opto = output LOW
ST60 NMEA OUT OFF = A 11V, B 11V = no current in inverting opto = output HIGH
ianj99
Well-Known Member
Please see post 19 - that's why I obliged with an inverting opto coupler suggestion, so Billjrat was wrong.
The ST60's output don't make sense. I can't believe Raymarine would release a product that does not abide by the NMEA standard. After all no one can use the term NMEA on their product, literature or marketing unless they've paid nmea the fee and in doing so, agree to adhere to the standard.
More like the A out is knackered and not switching to 0v when B goes high.
Why are you bothering with output A since it doesn't change? Just connect B to the led +ve and the led -ve to 0v. Then St60 out on = no led current = output high and St60 output off, = led current so output = low for an inverting opto coupler.
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Plevier
Well-Known Member
You are just refusing to see it!
One last try.
NMEA out terminal A is constant 11V whether ON or OFF
NMEA out terminal B for OFF is floating, not connected to anything (until you put a load between A & B).
NMEA out terminal B is 0V for ON.
It is not faulty. This is how Raymarine designed it. Look at post #8 you have the circuit.
So connecting B to opto +ve achieves nothing.
You need to connect opto +ve to A (with a resistor) and opto -ve to B.
Which is what I did, 2 years ago, and whatever you say, it works!
Billjrat is right in that the output needs voltage inverting to be anything like NMEA standard. However the opto is working from current - and that does not need inverting (in the logic sense) - current means ON, no current means OFF.
Finished. If you still don't understand, please go back and read the thread properly.
Billjratt
Well-Known Member
Ian99 - stop hypothesising and get on the bench with some tools and components.
Then come back with the results. If you find a new way to do it then super.
WE KNOW what works 'cos it's already happening on our boats.
Then come back with the results. If you find a new way to do it then super.
WE KNOW what works 'cos it's already happening on our boats.
ianj99
Well-Known Member
Yes sir, how high sir!
Nmea compliant differential does not have floating outputs which seems to be the reason for the issue covered in this thread. (RS485 does require the outputs to be tristated (Hi impedance) when inactive but I can't find any reference to RS485 type outputs for nmea 0183)
I have several devices with differential (+ and -) nmea outputs and many with differential inputs and I have had no problem interconnecting them either single ended or differentially, which is what NMEA intends.
So its a non issue unless the equipment is odd ball, which seems to be the case according to the comments made about Raymarine and Garmin not conforming to the standard. (I find it ,hard to believe that these manufacturers would breach the terms of their NMEA licence by failing to agree to adhere to the standard, but in the good old days, there were probably ambiguities)
From Peter Bennets nmea faqs. http://www.kh-gps.de/nmea.faq
Electrical Interface
These standards allow a single "talker", and several "listeners"
on one circuit. The recommended interconnect wiring is a
shielded twisted pair, with the shield grounded only at the
talker. The standards do not specify the use of any particular
connector.
The NMEA-0180 and 0182 standards say that the talker output may
be RS-232, or from a TTL buffer, capable of delivering 10 mA at
4 V. A sample circuit shows an open collector TTL buffer with a
680 ohm resistor to +12 V, and a diode to prevent the output
voltage from rising above +5.7 V.
NMEA-0183 accepts this, but recommends that the talker output
comply with EIA-422. This is a differential system, having two
signal lines, A and B. The voltages on the "A" line correspond
to those on the older TTL single wire, while the "B" voltages
are reversed (while "A" is at +5, "B" is at ground, and vice
versa)
In either case, the recommended receive circuit uses an
opto-isolator with suitable protection circuitry. The input
should be isolated from the receiver's ground.
In practice, the single wire, or the EIA-422 "A" wire may be
directly connected to a computer's RS-232 input.
Ian
Gypsy
Well-Known Member
In my experience NMEA is usually quite interconnectable across various models and manufacturers but there are certainly many flavours of NMEA output and input. I recently came across a problem to connect my GME GX600D VHF radio NMEA out to my Raymarine E120W plotter. When connected the output of the GX600D data level dropped to 2.5V p/p going from 0V to +2.5V (measured on a 'scope). Open cct the output was 0-5V p/p. I am assuming the GX600D is TTL, but the E120W is 600ohm terminated opto-isolator input which is clearly too low an impedance for the GX600D.
After experimenting with other Raymarine plotters I have, I can see they regularly use 600ohm input impedance but their output drivers are also able to drive relatively low impedance ccts as I have used more than one 'listener' on them from time to time, but probably no more than 2 at a time.
My warning is that whilst some devices will be able to drive more than one 'listener' it is far from guaranteed as it will depend upon the load impedance of the listeners and the ability of the 'talker' to drive them sufficiently to achieve the required voltage levels.
My solution was to build a small transistor buffer giving 12V p/p output installed between the GX600D and the E120W. Much the same as Plevier had to do to drive the Garmin although his issue and solution were different to mine.
After experimenting with other Raymarine plotters I have, I can see they regularly use 600ohm input impedance but their output drivers are also able to drive relatively low impedance ccts as I have used more than one 'listener' on them from time to time, but probably no more than 2 at a time.
My warning is that whilst some devices will be able to drive more than one 'listener' it is far from guaranteed as it will depend upon the load impedance of the listeners and the ability of the 'talker' to drive them sufficiently to achieve the required voltage levels.
My solution was to build a small transistor buffer giving 12V p/p output installed between the GX600D and the E120W. Much the same as Plevier had to do to drive the Garmin although his issue and solution were different to mine.
ianj99
Well-Known Member
The other way to look at it is the older 232 based devices are voltage driven and were not intended to drive more than one input and because 232 inputs are fairly high impedance there can be a problem connecting old to new because the outputs can't supply the required current.
Whereas opto isolators are current driven and don't care what the voltage is, provided its enough to drive a couple of ma through the led. In single ended use, the voltage is relative to ground so needs to be at least +3v but for differential connections, the actual voltages relative to ground do not matter too much, there just needs to be at least 3v between the + and - inputs to turn on the led.
So provided you have a signal that changes by more than 3v AND can supply 2ma at then there shouldn't be a problem otherwise the buffer such as you use is needed.
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Billjratt
Well-Known Member
The proof of the pudding ... as they say.
With and without the wee transistor thingy.
Now show me yours.
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ianj99
Well-Known Member
I received a 3rd reply from RM today which included a pdf of the NMEA differential spec.
They just hoisted themselves by their own petard!
As we know, the spec states that:
1) the Off, logical 1 or stop bit is defined by a negative voltage on the A line with respect to the B line and
2) the logical 0, ON state is defined as a positive voltage on the A line with respect to the B line.
3) the drive circuits for outputs A and B should meet at a minimum EIA-422 Dec 1978
And the email states that "....NMEA 0183 version 2.01 clearly identifying TWO signal lines – data +ve and Data-ve."
So if output A is not driven, has no data on it, and sits at 11.2v, then RM have not complied with the NMEA spec their own support person has sent me.
Its academic but shows RM are far from infallible but I wanted to hear it from the 'horses mouth' so to speak but without feeling smug.
They just hoisted themselves by their own petard!
As we know, the spec states that:
1) the Off, logical 1 or stop bit is defined by a negative voltage on the A line with respect to the B line and
2) the logical 0, ON state is defined as a positive voltage on the A line with respect to the B line.
3) the drive circuits for outputs A and B should meet at a minimum EIA-422 Dec 1978
And the email states that "....NMEA 0183 version 2.01 clearly identifying TWO signal lines – data +ve and Data-ve."
So if output A is not driven, has no data on it, and sits at 11.2v, then RM have not complied with the NMEA spec their own support person has sent me.
Its academic but shows RM are far from infallible but I wanted to hear it from the 'horses mouth' so to speak but without feeling smug.
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Billjratt
Well-Known Member
And there's more... I've got the ST60 talking to the Garmin 4008 but the Cobra is ignoring it so it seems there's a whole stack of "interpretations" out there.
Gypsy
Well-Known Member
Since I posted this GME, the VHF radio manufacturer, replied to my support email relating the problem of the GX600D not driving the 600ohm differential input of the Raymarine plotter. They acknowledged the problem and offered to change the driver impedance. Apparently most of their testing was with Garmin plotters which, as is well explained in this thread, have single sided hi/med Z NMEA input ccts. The radio has been modified and now is quite happy driving the RM plotter. I have removed my little buffer cct and put it in my 'useful bits' drawer for another challenge which will no doubt come along.
Well done GME.
pblancha
New Member
My Raymarine ST60+ Graphic's NMEA OUT is now successfully sending Depth to my Garmin GPS518i. This is thanks to Mike Wylde's solution, see circuit diagram below. I soldered the components to a small circuit board with a connector block.
https://drive.google.com/open?id=1X5pY9-WG2hLpqaLd7mfncAtV4Xn3M_G2CQ
https://drive.google.com/open?id=1z0_ZdaJJarAV6SdBbCbyut7yKIR5QWoYow
https://drive.google.com/open?id=1X5pY9-WG2hLpqaLd7mfncAtV4Xn3M_G2CQ
https://drive.google.com/open?id=1z0_ZdaJJarAV6SdBbCbyut7yKIR5QWoYow