EXT power + FTDI comm?

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rbgorbet
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Joined: Wed Jan 27, 2010 1:21 am

EXT power + FTDI comm?

Post by rbgorbet » Wed Jan 27, 2010 10:33 am

HI Paul (and others).

I'm using BBB.C for an installation where the board is powered using the external barrel connector, at 12V (nominal), from a power supply which might be several dozen feet from the board.

The BBB has a custom shield with an RS232/485 transceiver on it and communicates out to a bus with several dozen other BBBs. This particular BBB is the Master on the bus and the others are Slaves.

I would like to connect an FTDI cable to the Master in order to monitor bus traffic via the SERIAL object in MAX.

I have three questions:

1. With the power jumper in the EXT position, do I need to worry about damaging the USB port on the MAX laptop due to the shared GND between the 12V remote power supply and the laptop? I know that in theory this shouldn't be a problem; I wonder if anyone has any experience with this situation.

2. ATMega168 TX is connected directly to RX on the FTDI header, so in theory the MAX serial object should be able to listen no problem. ATMega168 RX is connected to FTDI TX through a 10k resistor, however. Does anyone know if this will cause an issue with MAX trying to talk onto the bus? In my current application, I don't need this (in fact, it would be a problem and I might physically remove the TX wire from the FTDI connector), but I'm curious.

3. How might this have changed with future revisions of the BBB, because we have some of those boards as well.

Thanks for any help you can provide!
Rob.

paul
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Posts: 735
Joined: Mon May 12, 2008 4:19 pm

Re: EXT power + FTDI comm?

Post by paul » Sat Jan 30, 2010 1:13 pm

Hi Rob,
I have three questions:

1. With the power jumper in the EXT position, do I need to worry about damaging the USB port on the MAX laptop due to the shared GND between the 12V remote power supply and the laptop? I know that in theory this shouldn't be a problem; I wonder if anyone has any experience with this situation.
I wouldn't worry about power issues if the clip is in the external position, since only the grounds are connected. In rev C - the USB runs through the regulator (but I also think you replaced the regulators with 1 amp variety) If someone forgets and swaps the clip to USB, you'll probably get the alarming "USB over current" alert box. Maybe you're beyond swapping clips now.
2. ATMega168 TX is connected directly to RX on the FTDI header, so in theory the MAX serial object should be able to listen no problem. ATMega168 RX is connected to FTDI TX through a 10k resistor, however. Does anyone know if this will cause an issue with MAX trying to talk onto the bus? In my current application, I don't need this (in fact, it would be a problem and I might physically remove the TX wire from the FTDI connector), but I'm curious.
Not quite sure of the question. Is there handshaking going on with serial protocol ? I don't think so. With upload? - yes. The 10k line in theory shouldn't effect the serial bus is it's driven from a low impedance source? But the TX/10k could put noise on the line when no one else is talking , eh? (Practicing my Canadian ;)
3. How might this have changed with future revisions of the BBB, because we have some of those boards as well.
The only real change with rev E is that the USB power line no longer goes through the regulator. This doesn't seem like it should effect what you are doing now as you're not using USB power for anything. Testing?
I don't contemplate putting out another rev on BBB anytime soon.



Maybe post a link / dates for latest iteration of the project in my exhibition section. For others wanting to know where all these BBB's are going, here's a link to youtube videos of Rob/ Philip Beesley's project. . http://www.youtube.com/results?search_q ... type=&aq=f

Paul

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