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Topic: Arduino Nano resistors on Rx/Tx (Read 1 time) previous topic - next topic

noob314

Dec 05, 2019, 07:04 am Last Edit: Dec 05, 2019, 08:03 am by noob314
According to schematic, Arduino Nano have series resistors of 1K on Rx and Tx. What for?

They theoretically may involve some issues on high speed baud rate. Some older posts found here indicate it gives some protection, well not certain what exactly.

Or perhaps I read this schematic wrongly?

larryd

If you were to use D0 as an output in a project (i.e. not as a RX pin), the 1k prevents this pin and the USB TX pin from being damaged. If you think about it, you would have two output pins connected to each other if the 1k was not there.






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DrAzzy

Beyond that, they allow another serial device, connected without resistors and communicate via serial. Otherwise you would need to physically disconnect the onboard serial adapter to use the serial to talk to anything else.
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noob314

#3
Dec 05, 2019, 08:28 pm Last Edit: Dec 05, 2019, 08:33 pm by noob314
Thank you very much for your replies.

I do not have any special equipment to deal with SMD desolderting, then it is much simpler to order dozen of Pro Mini, instead. It would be a bit more complicated for programming, but anyway it is clean, elementary board.

DrAzzy

You do realize that there are series resistors in most commercial serial adapters too, right?

Is there something that leads you to believe that those resistors will cause a problem at relevant baud rates? Have you tested and found problems that were fixed by bypassing the resistors?
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noob314

#5
Dec 05, 2019, 09:43 pm Last Edit: Dec 05, 2019, 10:05 pm by noob314
What I need is 1 Mbaud reliable transfer. ATmega328p and 2560 are capable to accomplish that. Through a Digitus 70156 USB to Serial that was possible. Anything placed between will interfere. Long time ago, there was many fake products claim that works and  I have read quite detail documentations from https://www.ftdichip.com in order to buy proper adapter.

Digitus 70156 USB to Serial is based on FT232RL (chip itself is able to support up to 3 Mbaud, I believe) and now it is possible to find schematic of it - as long as I see there is no serial resistor on RX/TX. Thus extra 1K will probably make quite a mess.

Transfer large amount of data through serial interface with 10x factor than standard (115200), can't be ignored.

westfw

Quote
extra 1K will probably make quite a mess.

Transfer large amount of data through serial interface with 10x factor than standard (115200), can't be ignored.
I doubt it.   There have been reports of people uploading (and verifying) their (stock) Arduino hardware (with 1k resistors) at 1Mbps, with no problems.  1k at say 7pF pin capacitance makes for a ~7ns difference in rise time; pretty insignificant at 1Mbps, and resistors are not very "detrimental" components - in fact, series resistors are frequently added to reduce high-frequency issues (usually ringing.)
Unless you already have hard data from scope measurements or test software, I'd say they're worth getting.
Quote
Through a Digitus 70156 USB to Serial [1Mbps Serial] was possible.
Isn't that an actual rs232 cable?  Surely a 1k resistor is less invasive that 2x rs232 level conversion!


At 1Mbps, you're pretty likely to have software performance problems.  That's only about 150 instructions per byte...

noob314

#7
Dec 06, 2019, 10:14 am Last Edit: Dec 06, 2019, 01:07 pm by noob314
I'd say they're worth getting.Isn't that an actual rs232 cable?
It have no serial cable, it is cleanly just adapter. No additional cables and no additional lost.

https://www.digitus.info/en/products/computer-components/computer-peripherals/serial-parallel-adapter/da-70156/

Quote
I doubt it.   There have been reports of people...
You have no personal experience to confirm that, just assumption based on what other talks? Not likely relevant to be accepted as a fact. I have heard people manage to do up to 250 000 baud with UNO, but they are not mentioned what is the power source - datasheet of 328p claim it can reliably transfer 1Mbaud, which is the fact. I have manage to get 1M on my own version of barebone 328p board (clean battery source, well decoupled ), with no serial resistor. Anyway, will try again to test with 1K resistors...

westfw

#8
Dec 07, 2019, 02:54 am Last Edit: Dec 07, 2019, 02:55 am by westfw
Quote
Quote
I doubt it.   There have been reports of people...
You have no personal experience to confirm that, just assumption based on what other talks?
Perhaps I should have said "reports from long-term respected forum members who sell Arduino hardware boards"?

In any case, I set up a version of ASCIITable outputting at 1Mbps, and ran it on my freeduino (with through-hole 1k resistors on the serial lines), and it seems to work fine...

zoomkat

"According to schematic, Arduino Nano have series resistors of 1K on Rx and Tx. What for?"

I think if you dig into the subject you may find the resistors on tx/rx pins are to keep the pins from floating.
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DrAzzy

"According to schematic, Arduino Nano have series resistors of 1K on Rx and Tx. What for?"

I think if you dig into the subject you may find the resistors on tx/rx pins are to keep the pins from floating.
No, they are connected between the TX and RX pins of the serial adapter chip and the RX and TX pins of the '328p.

See my initial post for a description of why they are there.
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gilshultz

The resistors solve one problem and are a convenience for the user.  You can connect directly to the pins and over drive them with your hardware.  This allows you to use the serial ports in your application, just remember it has to be disconnected with using the serial monitor.  This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil

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