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MamBo
3 Posts |
 Posted - 10 août 2008 : 11:24:39
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Recently I got myself Holux M1000 with B-core firmware 1.0, and decided to upgrade it to 1.2 (it should solve LSB problem).
I decided to build myself Holux -> RS232 cable. (from the datasheet provided in holux manulal it has regular com interface not USB).
So my question is do I need to supply +5 to to holux unit or it's enouht to connect grouncd RX,TX ? |
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saimhe
Lithuania
134 Posts |
Posted - 12 août 2008 : 04:50:38
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| Since the receiver has its own power source, I believe the +5V lead is for charging only. A similar cable for Holux GR-236 works exactly this way. |
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MamBo
3 Posts |
Posted - 12 août 2008 : 07:12:52
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| Thank you for your answer. If I understood right you are the author of this datacable. Maybe you could explain is this ST3232 chip is really nesesary ? (why we can't connect dirrectly rx,tx, ground to com port ? |
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saimhe
Lithuania
134 Posts |
Posted - 12 août 2008 : 15:36:54
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The manufacturer's specification is rather vague:
"In/Out Port. GPS signal (Out)/Command(In) with CMOS Level / Mini USB Type B Connector and Cable option: GR230-A2 (USB data cable)"
They should simply say "UART at CMOS levels" or something similar. Therefore some educated guess (and testing) is needed. Anyway, it has been already done.
UARTs are available in almost every microcontroller -- attractive as there's no need for additional chip. The GSC3 chip in SiRF III has, like majority in ARM architecture family, at least two. I know very little about MTK chips however they should be similar.
And CMOS levels are... well, just rail-to-rail (0...5V or, most likely in this case, 0...3.3V). Furthermore UART signals are active low -- start bit is zero, stop bit is power.
However CMOS and similar "core" interfaces are only good for tens of cm (remember Parallel ATA). To interconnect at greater distances, RS232 was invented. There the "logical zero" is +3...+15V and "one" is -3...-15V; the undefined area near 0V was perhaps meant to detect a broken line. That is, voltages are not only amplified but also shifted and inverted. Sometimes, if you are lucky, a digital inverter like 74HC04 could also work as a "level converter" because threshold levels at the com port side are a bit asymmetric (for example, ST3232 datasheet specifies about +1.5V with tiny hysteresis).
Holux are merely saving money. If a ST3232 were put into the receiver, a simple "wiring adapter" (instead of that converter cable) would be enough for old, outfashioned computers which are not worth such exclusive support; and a USB adapter for modern folks would require not only the FT232R (or similar) chip but also an additional ST3232. On the other hand, a brave move would be to use a newer ARM chip with built-in USB Device port -- but that's up to SiRF and MTK, and incompatible with many devices: for example, there are almost no PDAs with USB Host capability. |
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MamBo
3 Posts |
Posted - 13 août 2008 : 15:23:40
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thank you for your extensive explanations. it seem's that it's cheaper to get myself original holux cable than to play aroound with all these cips, capacitatoes etc.
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bkbbk
1 Posts |
Posted - 18 août 2008 : 04:46:57
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| I have upgraded my m1000b to B-core_1.20 using the cable which built by myself. I used an MAX232 to convert between EIA levels and CMOS levels. I am testing the new firmware, but seems still having LSB... |
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masster
13 Posts |
Posted - 07 juin 2012 : 09:35:00
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| what is the schematics of data cable for Holux M-1000C? |
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Holux M1000/M1200 cable
