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I'm working with a driver board which communicates with a PC program via USB -> FTDI -> TX/RX into the microcontroller. The serial pins are not broken out on the board and I need to talk to the board from another microcontroller (like Arduino). My plan is to just solder directly to the RX/TX pins on the microcontroller despite the fact that the FTDI is powered on and RX/TX of the FTDI will be electrically connected to those pins. I will not be transmitting anything via USB. My assumption is that the FTDI chip will be passive and I can just drive those pins from my Arduino.

Will this cause me any issues? I'm just concerned if the FTDI passively drives those pins low or high, I'll get "smoke" if I follow through with my plan. Perhaps I should put some resistors on my Arduino's RX/TX just as a safeguard.

It's very possible some of my logic is flawed here. Thanks in advance for your advice!!

Cheers

nargetdev
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    Your verbal description makes no sense. Please provide a block diagram or schematic that clearly explains what you have and what you propose. – sawdust Apr 04 '15 at 04:32
  • you need a common ground and isolated reference for it to work right. will it do harm? if there is no reference at all (the arduino is battery powered and no electrical connection to wall power/earth ground, etc) not sure how this would let smoke out. but not sure how it would work either except for some dumb luck. if both sides are for example plugged into the same computer via usb, the ardunino using it as power, that is a common ground so long as neither side has their ground different from that. this is an electrical stack exchange question btw. not stackexchange. – old_timer Dec 26 '18 at 16:43
  • @sawdust just rereading this old post, haha yes I agree it's pretty poorly described. A few years of embedded experience later and perhaps I could have directly asked whether these could operate as open drain. – nargetdev Jan 07 '19 at 02:04

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You need to check with the particular FTDI chip you're using on that board.

Some FTDI chips support RS485 and because of the required multidrop capability they operate as open-drain allowing to connect multiple devices.

Even if this solves your hardware problem, you still need to consider how the (live) FTDI chip may respond to your 'unexpected' incoming (RX) data stream. For example, possible XON/XOFF flow control appearing on TX especially since there won't be a consumer from the PC side and the internal FTDI buffers will eventually fill up.

tonypdmtr
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  • Your answer makes too many assumptions, e.g. the PC (?) is going to use the USB connection and no level shifter is needed. You don't know what *"bypassing the powered-on FTDI"* or *"will not be transmitting anything via USB"* really means. Besides the FTDI chip there's mention of a *"driver board"*, a PC, a microcontroller, and *"**another** microcontroller"*. Instead of guessing how all this is supposed to be connected together, why not let the OP explain first? – sawdust Apr 04 '15 at 23:03
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    I do not make any assumptions. I simply understand the problem better than you did, apparently. And that's because I've done similar things myself. So, just because a question is not clear to you does not mean it is not clear to everyone else. (Maybe you overestimate your skills while underestimating others' ?) PC is just an example as it can be anything else with similar role (i.e., attached on the USB side of the FTDI). He wants to connect another MCU from the in-side of the FTDI. Simple! Get it? – tonypdmtr Apr 05 '15 at 12:31
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    BTW, voting down an answer because YOU do not understand the issue is pretty low! – tonypdmtr Apr 05 '15 at 12:43
  • It's alright guys. I think it was helpful information. The open drain bit was useful. – nargetdev Apr 05 '15 at 16:41
  • I think I will just play it safe and cut the traces for RX and TX to the FTDI. I can just solder-bridge them back together later if I need to talk to the PC again. – nargetdev Apr 05 '15 at 16:44