Universal NMEA-0183 Interconnection Method

Electrical and electronic topics for small boats
jimh
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Universal NMEA-0183 Interconnection Method

Postby jimh » Mon Feb 08, 2016 6:45 am

Several years ago I developed a very useful method of interconnecting NMEA-0183 serial data interfaces using a five-pole connector wiring arrangement and a simple-to-construct interface backplane. The connectors and backplane were very inexpensive and very easily obtained.

For the past five years I have employed this method for wiring all the NMEA-0183 ports on all the devices I own that have such a port. This method has enable me to easily interconnect any NMEA-0183 device with another without having to re-engineer the wiring of the interface for any individual connection. The method fully accommodates the ten possible interface situations involving ports of differential signals and single-ended signal--it is a truly universal approach.

I also wrote an article five years ago to describe in great detail the development and implementation of the method. See

Universal NMEA-0183 Interface
http://continuouswave.com/whaler/refere ... A0183.html

The article points to this thread and invites readers to use it for posting comments or questions. I will be glad to answer any questions regarding the universal NMEA-0183 wiring method I have developed.

Jefecinco
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Re: Universal NMEA-0183 Interconnection Method

Postby Jefecinco » Mon Feb 08, 2016 9:39 am

That's good timing for representing your article. The communication between my Standard Horizon VHF and my Garmin MFD has failed. Rather than trying to trouble shoot the old connections made by me in 2009 I believe I'll simply redo the connections per your article.

That will have to wait for a warmer day. It's only in the mid-fifties this morning.
Butch

jimh
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Re: Universal NMEA-0183 Interconnection Method

Postby jimh » Mon Feb 08, 2016 11:38 am

I was a bit worried that Radio Shack had gone into bankruptcy and the experimenter's circuit board I was using was no longer available. I visited my local Radio Shack last week, and found they were still in business and had plenty of those experimenter's circuit boards for sale. They have changed the part number for them, but I updated my article to reflect that. I also checked the other links to sources, and updated the pricing, too.

The Radio Shack circuit board that I obtained a few years ago did not have the copper surface of the board tinned with solder. That omission makes it a bit more difficult to work with. In the latest on-line catalogue image of the circuit board at

https://www.radioshack.com/collections/ ... 5717554885

the copper circuits of the board appear to now be pre-tinned. That would be a nice improvement. However, the store I visited had these boards in-stock and they did not have pre-tinned copper lands. I don't know if that was because they were old stock, or if the picture on the website is misleading in its appearance. If anyone happens to buy a Radio Shack experimenter's board with pre-tinned copper lands, please comment about it.

BugMaster
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Re: Universal NMEA-0183 Interconnection Method

Postby BugMaster » Tue Apr 05, 2016 9:34 am

Hello, I think your interconnection method is very useful.
But as far as I know, RS-232 and RS-422 signals have different voltage (see https://en.wikipedia.org/wiki/RS-232#Voltage_levels and https://en.wikipedia.org/wiki/RS-422#Characteristics):
- RS-232 has from +3V to +15V for line asserted and -15V to -3V for line deasserted
- RS-422 has from -6V to +6V

So I am wondering how you can connect a "single ended" line (RS-232) to a "double ended" line (RS-422) when they have different signal voltage.
From what I wrote above, I thnk RS-232 can easily accept RS-422 signals, since it has a great allowance (from -15 to +15).
Instead I am afraid that a RS-422 device would be broken when receiving the signals from a RS-232 since it is not designed to accept more than 12V of differenzial voltage.
This is why converters exists, as http://www.serialcomm.com/serial_rs232_ ... rters.aspx

So, simply put: are you sure I will not destroy my devices?
Thank you.

jimh
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Re: Universal NMEA-0183 Interconnection Method

Postby jimh » Tue Apr 05, 2016 10:53 am

My remarks are in regard to interconnection of devices using NMEA-0183 standards.

As for "RS-232", in the real world there are many interface circuits labeled as RS-232 which do not really adhere to the specifications of the formal standards. (There is a good article at Wikipedia that points this out.) For example, on my Lowrance HDS chart plotter it is possible to select in software the configuration of the NMEA-0183 serial ports to use a "protocol" of RS-232 or RS-422. See the illustration at

http://continuouswave.com/whaler/refere ... l#NMEA0183

I really doubt that the voltage levels at those interfaces change when the "protocol" is changed. What actually happens is that in the "RS-232" mode you get two serial interfaces on the four wires of the cable connected to the port (because they are single-ended interfaces), and on the "RS-422' mode you get one serial interface on those four wires (because they are differential interfaces). But the electrical interface is still according to NMEA-0183 standards. (See more below on the electrical standards of NMEA-0183).

There is no way for me to know if the particular devices you have actually follow the electrical signal specifications of the formal standards, or, if like many devices, they just use some voltages that are more like 0-Volts, 5-Volts, and (-)5-Volts. I recommend that you use an oscilloscope probe on your devices and see what voltages they are using, and check the specifications of your devices to see if they are compatible.

I'd like to be able to point you to an on-line document that gives more details of the latest NMEA-0183 electrical standard, but the NMEA organization does not provide free on-line access to any of their standards. You have to buy them if you want to see them. Send NMEA about $300 and you can get all the information you may need to make this determination.

By the way, ALL NMEA-0183 interfaces are NOW supposed to be differential and follow RS-422 signal standards. I believe--again I cannot know because I have not sent a big check to NMEA to see their official standards--that very early versions of NMEA-0183 may have allowed single-ended interfaces. I do not know what electrical standards were used in those early interfaces.

jimh
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Re: Universal NMEA-0183 Interconnection Method

Postby jimh » Tue Apr 05, 2016 11:15 am

You can find some older versions of the NMEA-0183 standard on-line if you search. Here is an excerpt from NMEA-0183 version 3.0 dated January 2002 I found available on-line:

3.5 Electrical Signal Characteristics

This section describes the electrical characteristics of transmitters and receivers.

3.5.1 Signal State Definitions

The idle, marking, logical "1", OFF or stop bit state is defined by a negative voltage on line "A" with respect to line "B".

The active, spacing, logical "0", ON or start bit state is defined by a positive voltage on line "A" with respect to line "B".

Note that the above "A" with respect to "B" levels are inverted from the voltage input/output requirements of standard UARTs and that many line drivers and receivers provide a logic inversion.

3.5.2 TALKER Drive Circuits

No provision is made for more than a single TALKER to be connected to the bus. The drive circuit used to provide the signal "A" and the return "B" shall meet, at a minimum, the requirements of EIA-422-A (December 1978).

3.5.3 LISTENER Receive Circuits

Multiple LISTENERs may be connected to a single TALKER. The LISTENER receive circuit shall consist of an optoisolator and should have protective circuits to limit current, reverse bias and power dissipation at the optodiode as shown in Figure 1. Reference is made to example circuits in Section 7.2 of this Standard.

The receive circuit shall be designed for operation with a minimum differential input voltage of 2.0 Volts and shall not take more than 2.0 mA from the line at that voltage.

For reasons of compatibility with equipment designed to earlier versions of this standard, it is noted that the "idle, marking, logical "1", OFF or stop bit state" had previously been defined to be in the range -15 to + 0.5 Volts. The "active, spacing, logical "0", ON or start bit state" was defined to be in the range +4.0 to +15 Volts while sourcing not less than 15 mA.

jimh
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Re: Universal NMEA-0183 Interconnection Method

Postby jimh » Tue Apr 05, 2016 11:20 am

According to some information at

http://freenmea.net/docs

the NMEA-0183 specification migrated to RS-422 from RS-232 in January 1992. I would hope that any device you buy today, in 2016, is using the specification that has been in effect for 24-years and four months. I cannot imagine any electronics industry would still be manufacturing products and calling them "NMEA-0183" but following an electrical standard that has been obsolete for 24 years.

jimh
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Re: Universal NMEA-0183 Interconnection Method

Postby jimh » Tue Apr 05, 2016 11:28 am

Regarding the method I describe for interconnecting single-ended and differential NMEA-0183 signals, I am confident in the method. I have seen the exact same method specified by many sources, including some advice from the USCG. Compare my methods with the USCG method shown at

http://www.navcen.uscg.gov/pdf/gmdss/ta ... nmea_7.pdf

jimh
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Re: Universal NMEA-0183 Interconnection Method

Postby jimh » Tue Apr 05, 2016 1:20 pm

BugMaster wrote:I think your interconnection method is very useful.


Hi--yes, I have found my five-pole standardized connector wiring and simple backplane motherboard approach to be extremely useful for my own purposes. Let me know if you construct a similar backplane with the five-pin headers, or if you find some other useful arrangement. I have lost track how many times I have used this approach and been able to simply just plug together two devices I have pre-wired with my connector arrangement, and have the pleasure of seeing them communicate flawlessly via NMEA-0183.