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Forum Index : Microcontroller and PC projects : Serial Comms woes...
| Author | Message | ||||
| vk5la Newbie  Joined: 05/03/2017 Location: AustraliaPosts: 13 |
Hi all, Complete beginner alert! I’m trying to read the output from a Garmin GPS18x LVS into a micromite plus. This device is NMEA sentences at 4800 baud 5v TTL level out of the box, I’ve not changed it. I gave up with the micromite so I thought I’d see if the gPS was actually working. I connected it to terra term via a couple of different USB to serial converters... I can see the data coming out, but it’s garbage. Unreadable. At any baud. Tried Different data bits, parity, ya da ya da tried it all nothing appears to work. Should se a readable sentence but I just get garbage. Started at the beginning with a loopback test on the USB to serial converter, that works. I know I’m connected to the right pin because I can see the garbled data spewing out. Any suggestions greatly appreciated. I suppose my next step is to try another GPS unit...? Cheers Andy VK5LA Andy (VK5LA) Winkie, S.A. |
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| matherp Guru  Joined: 11/12/2012 Location: United KingdomPosts: 10572 |
Sounds like the output may be inverted. Get a copy of realterm and use this to invert the data to see if it then makes sense. |
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Geoffg Guru Joined: 06/06/2011 Location: AustraliaPosts: 3308 |
The Garmin GPS18x LVS uses RS232 signalling which is the inverse of the normal TTL signal. See page 88 of the latest Micromite User Manual for an explanation. You need to open COM1: with the invert option. For example: OPEN "COM1: 4800, INV" AS #1 Geoff Geoff Graham - http://geoffg.net |
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| vk5la Newbie Joined: 05/03/2017 Location: AustraliaPosts: 13 |
Ok, but does not this code check for inverted data with ",INV"? For i = 1 To 5 WatchDog 4000 Open "COM1:" + baud(i) As #1 Pause 1000 If Instr(Input$(255, #1), "$GP") > 0 Then Exit Do Close #1 WatchDog 4000 Open "COM1:" + baud(i) + ",INV" As #1 Pause 1000 If Instr(Input$(255, #1), "$GP") > 0 Then Exit Do Close #1 Next i this is from the GPS boat computer V6 of whic I'm trying to connect the Garmin GPS cheers Andy (VK5LA) Winkie, S.A. |
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| vk5la Newbie Joined: 05/03/2017 Location: AustraliaPosts: 13 |
I have also tried Realterm and inverting the data, still garbage at any baud, any setting. Thanks for the suggestion, Andy VK5LA Andy (VK5LA) Winkie, S.A. |
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goc30 Guru Joined: 12/04/2017 Location: FrancePosts: 435 |
Hi vk5la with gps, I think that you can't use "instr" instruction. Don't forget that Micromite is an interpreted language, not compiled, and for that each instruction spend more time The best is to buffered your sentence before you receive CRLF, and after only you can process your buffer |
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| Geoffg Guru Joined: 06/06/2011 Location: AustraliaPosts: 3308 |
To check the output from the GPS it would be best to simplify the code. The following will echo anything from the GPS to the console. With this you can easily test variations of INV or not, baudrates, stop bits, etc. OPEN "COM1: 4800, INV" AS #1 DO PRINT INPUT$(1, #1); LOOP Geoff Graham - http://geoffg.net |
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| Pluto Guru Joined: 09/06/2017 Location: FinlandPosts: 398 |
I had problems with some cheap chinese GPS modules (based on UBLOX) some days ago. The problem was that the TX-signal from the module never came down low enough for Micromite Plus to recognize that it was at a low level. Did some experimenting and found that by pulling the TX (on the GPS module) down to ground with a 10k resistor helped. ( 1k resistor was too small. 10k is probably not optimal, but works OK). It has been running now for 3-4 days w/o problems. If you have an oscilloscope, check the levels of the signals when the Garmin is connected to Micromite Plus. I had never any problems like this earlier, when I used standard MicroMites. (Maybe there are some pull-ups active in the Micromite Plus???) Anyhow, the solution was so simple that I did not bother to investigate further. |
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| Geoffg Guru Joined: 06/06/2011 Location: AustraliaPosts: 3308 |
Yes, a pullup is enabled (to stop the input from floating). You can use the CSub CHANGEPIN (included in the MMBasic ZIP) to turn it off. Geoff Graham - http://geoffg.net |
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| vk5la Newbie Joined: 05/03/2017 Location: AustraliaPosts: 13 |
Hi all, *Update* So I tried (thanks Geoff!)this with the GPS connected to the Micromite OPEN "COM1: 4800, INV" AS #1 DO PRINT INPUT$(1, #1); LOOP and got readable NMEA sentences out - great! My Garmin GPS is obviously working. Downloaded the SuperClock program, got that to go but it told me that no RTC or GPS was found...Hmmm But, now it's working? The only thing I can think of is, is the COM1 port used by the USB connection to the console? Maybe I had the Micromite connected to the console at the time when it was looking for a GPS...then a reset and not connecting let it get the GPS? Anyway, thanks for steering me in the right direction, I'm slowly beginning to get a bit of a grasp on these Micromites. Andy (VK5LA) Winkie, S.A. |
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| Geoffg Guru Joined: 06/06/2011 Location: AustraliaPosts: 3308 |
No, they are completely separate and independent. Geoff Graham - http://geoffg.net |
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