Subject:
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Re: Connecting two HandyBoards through their serial ports
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Thu, 12 Nov 1998 14:44:19 GMT
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Original-From:
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Bill Bynum <bynum@csNOMORESPAM.wm.edu>
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Viewed:
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2103 times
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Hello, everyone:
Richard Drushel helped me solve the problem that I was having
in getting two HandyBoards to communicate through their serial
ports by suggesting that I check to be sure that the wiring on the
RJ11 cable I was using corresponded to a "null modem" connection.
Indeed, the connecting RJ11 cable I was using turned out NOT to be
a null modem cable.
On the HandyBoard FAQ, the following question tells how to
connect two HandyBoards:
> I want to plug two Handy Boards together so that they can
> communicate. What kind of cable should I use?
>
> For this application, you must use the network style RJ11 cable
> (as described above). Do not use the normal telephone style
> RJ11 cable used to connect the HB to the serial interface, or
> you will suffer the smoke and burn consequences noted above.
The question before this one in the FAQ -- "The R11 on my
Interface/Charger is smoking hot and is turning black! ..." --
describes a network-style RJ11 cable by saying that if you hold
the the ends of the cable plug-to-plug, both contact side up, the
wires line up as follows:
-------+ +--------
--------- black| |yellow -----------
--------- red| |green -----------
--------- green| |redn -----------
--------- yellow| |black -----------
-------+ +--------
Wiring an RJ11 cable this way avoids smoking the power supplies
on the two HandyBoards, because the unswitched power leads of the
two HandyBoards are connected, the ground leads are connected, the
RxD leads are connected and the TxD leads are connected.
Either the above diagram is wrong (I think that it's right,
actually) or the advice in the FAQ to use this kind of cable is
wrong, because it is necessary for the RxD and TxD leads on the
two HandyBoards to be swapped and the ground leads to be matched.
The unswitched power leads aren't needed for serial communication,
but if they are connected, then they have to be matched. Namely, if
you wire your RJ11 cable so that when you compare the cable plugs
contact side up you get the pairings:
-------+ +--------
-------- black| |black ------------
-------- red| |green ------------
-------- green| |red ------------
-------- yellow| |yellow ------------
-------+ +--------
This wiring switches TxD and RxD, but matches up the unswitched power
leads and the ground leads on the two robots.
When you use an RJ11 cable wired this way to connect two HandyBoards,
then the serialio.c functions implement serial communication between
the two HandyBoards quite nicely.
Bill Bynum
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