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Old 08-12-2017, 01:06   #16
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Re: RS-422 or RS-232 into NMEA

Another possibility: if the MFD can output electronic compass and/or knotmeter information that can be used by the AIS to provide slightly more info in its broadcast. For instance, if you have an autopilot that connects to the MFD through SeaTalk you will have at least heading information, and of course a knotmeter will have speed-through-water. In that case you could connect both TX and RX pairs on the AIS to port 1 In and Out on the MFD. Its pretty marginal, and Class B doesn't transmit as much info as Class A so it may not help much if at all. Read the AIS manual and see if it accepts any input sentences, and get back to us if you want to go that way.

Greg
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Old 08-12-2017, 18:31   #17
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Re: RS-422 or RS-232 into NMEA

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RS232 and RS422 are defined by voltage levels, not technology. The circuitry may use bipolar or (more likely) CMOS technology.

RS232 is specified to have a voltage swing between +/- 5V and +/- 15V. There is a signal wire and a ground wire -- this is called "single-ended". For many years +/- 12V was the typical output, with +12 being a logic zero (or "space") The negative voltage was a logic one (or a "mark"). These days, many "RS232" devices use +/- 5V or +/- 10V, and some even use +5V/0V. Most RS232 receivers consider 0V to be a "mark", so the +5V/0V signal, while not standard compliant, will work OK. Usually.

RS422 uses two signal wires -- this is called "balanced" or "differential". The voltage range on either wire is -3V to +3V, and when one wire goes negative the other goes positive. RS422 is relative immune to coupled noise, at least when compared to RS232.

continuouswave describes how to hook up these different standards.

The challenge with connecting RS422 and RS232 together, at least as I read the specs, is that the RS422 output is the DIFFERENCE between the A and B outputs. That varies +-5v (give or take), but the A and B output voltages relative to ground always run positive at 0V to +5V. So there is never a negative voltage relative to ground, and that's what a compliant RS232 interface wants to see for a logic 1.

So to work, you are counting on the RS232 receiver being non-compliant and accepting 0V as a logic 1. In practice, many do, but in the spec it's an undefined result. I wouldn't want my nav system depending on that.
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Old 08-12-2017, 22:16   #18
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Re: RS-422 or RS-232 into NMEA

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The challenge with connecting RS422 and RS232 together,
leave the TXB disconnected and ground the RX-
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Old 09-12-2017, 02:55   #19
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Re: RS-422 or RS-232 into NMEA

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leave the TXB disconnected and ground the RX-
Yes, that's the practice, and it often works, but only because the RS232 receiver happens to be treating the undefined receive voltage levels as 1 rather than 0. That's because TxA (the signal you are using) toggles between 0v and +5V. +5V is a valid "0", but 0V is not a valid "1". It's undefined behavior at that voltage level.

If your particular RS232 receiver chip happens to treat 0V as a "1", then it will work, or at least appear to work at the moment. But it might not keep working, or work reliably across a variety of operating conditions.

I experienced exactly this with an RS232 PC adapter that I tried for a quickie experiment. By all appearances it worked, but if you looked at the actual data stream, maybe 10% of the sentences were corrupted gibberish. It "appeared" to work because of the checksum on the sentences, and a receiving device that was dropping the bad sentences.

So the question is whether you are OK with only some of the sentences coming through correctly?
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Old 09-12-2017, 11:32   #20
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Re: RS-422 or RS-232 into NMEA

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My eyes are more glassed over than before after trying to understand...

Continuous wave suggests 10 different possible combinations?
Yes, that is correct. Depending on the devices to be interconnected, there are ten possible arrangements. But with any two specific devices, there is only ONE arrangement.

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Can someone please give me a simple connect this wire to that wire explanation?
No, I don't give advice about "connect red to blue" or something like that. No one has every learned anything by following advice like that. I am trying to help you figure out how to do it.

Re-read my initial post. I gave you explicit instructions on what signal to connect to what signal. I have no idea what "wire" those signals are on with your gear because I don't own that gear and don't have the instruction manuals.
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Old 09-12-2017, 11:52   #21
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Re: RS-422 or RS-232 into NMEA

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Yes, that's the practice, and it often works, but only because the RS232 receiver happens to be treating the undefined receive voltage levels as 1 rather than 0. That's because TxA (the signal you are using) toggles between 0v and +5V. +5V is a valid "0", but 0V is not a valid "1". It's undefined behavior at that voltage level.

If your particular RS232 receiver chip happens to treat 0V as a "1", then it will work, or at least appear to work at the moment. But it might not keep working, or work reliably across a variety of operating conditions.
These days there are plenty of devices sold as a "RS-232" interface that are not exactly following the specifications.

I have not experienced any problems reading NMEA-0183 data that is interfaced to an "RS-232" interface. I use a Keyspan USA-19 HS.

These days you don't find any real RS-232 physical ports on computers. They're mostly implemented by adaptors to USB ports. Errors in those data links are likely to be attributable to software problems in the device driver, more than electrical errors at the hardware level.

If a device required minus-3-Volts to be present to recognize a data state, it seems to me it would NOT randomly work when 0-Volts were present.

There is some good advice about hardware and voltage in an earlier discussion. See

NMEA-0183 and RS-422 vs RS-232
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Old 10-12-2017, 17:57   #22
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Re: RS-422 or RS-232 into NMEA

I have it sorted now and thanks to all
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Old 10-12-2017, 17:59   #23
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Re: RS-422 or RS-232 into NMEA

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Another possibility: if the MFD can output electronic compass and/or knotmeter information that can be used by the AIS to provide slightly more info in its broadcast. For instance, if you have an autopilot that connects to the MFD through SeaTalk you will have at least heading information, and of course a knotmeter will have speed-through-water. In that case you could connect both TX and RX pairs on the AIS to port 1 In and Out on the MFD. Its pretty marginal, and Class B doesn't transmit as much info as Class A so it may not help much if at all. Read the AIS manual and see if it accepts any input sentences, and get back to us if you want to go that way.

Greg
Going this route - thanks
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Old 11-12-2017, 13:09   #24
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Re: RS-422 or RS-232 into NMEA

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Going this route - thanks

you can't with the ins / outs you have. a class B AIS has no use for that info anyways.

put the pilot in / out on port 1. AIS in on port 2
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Old 11-12-2017, 18:09   #25
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Re: RS-422 or RS-232 into NMEA

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you can't with the ins / outs you have. a class B AIS has no use for that info anyways.

put the pilot in / out on port 1. AIS in on port 2
Done
Thanks so much
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Old 13-12-2017, 14:59   #26
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Re: RS-422 or RS-232 into NMEA

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That's an excellent compilation of useful information, thanks James!
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