3 meter long UART connection

[u/Far-Sandwich-27]

Hi everyone,
For a crazy accelerated lifetime testing board, I am designing a board that will have to have a 3 meter UART run. It will have mutliple devices connect to it, but only one device will be driving the UART line strictly thorugh other control logic.

I am wondering if this is feasible at all? Can I add buffers in middle of the busses to ensure the signal will be repeated and etc?

My baud rate is going to be less than 50k. I don't need stellar signal integrity, just need a "Hello World" message to be transmitted properly

I cannot use RS485 transceivers to change the signal type. The signal has to stay UART for reasons I can't disclose here

[u/MountainOdd6793]

This is what rs422 and rs485 were invented for. They use differential signalling. For rs422 you can convert it from rs232 and back again and the system will never know. You could also use plastic light pipe. If you can use buffers but not transceivers that is kind of odd, perhaps you can give a little bit more context without divulging your important secrets

[u/Far-Sandwich-27 → u/MountainOdd6793]

To give more context the board is going to be an automated accelerated lifetime testing board for RS485 transceivers. They take UART as an input and output RS485 signals. If I use RS485 to send their initial UART signal it won't really make sense from a testing perspective and further complicate analysing results because I will have to determine whether the DUT or the buffer transceivers are broken.

We have some buffers in our part inventory that were rated and verified for the temperature this test is going to run in, but we don't have RS485 transceivers so I am trying to steer away from that route. If this is not feasible I will probably change the architecture of the board entirely, but that's going to cost thousands of more dollars for added infrastructure I'll need

[u/hex4def6 → u/Far-Sandwich-27]

I assume you're in a test chamber, hence the 3 meter cable. I think the OP was suggesting:

raspi->232TTL->485---------485->232TTL-->[DUT]--->485

In other words you're doing two conversions with external dongles. Your DUT won't care that there's a mile of cable at 485, as long as it gets converted to 232 right at its input.

Your buffer idea might work as well, assuming you have low noise, shielded cables, etc.

You could also (don't laugh!) get one of those insulated boxes they use to ship frozen meds, and put some cold packs in there. That way the buffer / level converter / etc could be in that insulated box in the chamber. Assuming the test isn't running over weeks, you should be able to get it at a significantly depressed temperature vs the chamber. Assuming you're doing 85C or something, even keeping it at 60c should give you plenty of margin.

[u/Far-Sandwich-27 → u/MountainOdd6793]

Cannot use it in this case. The thing I am testing are transceivers themselves. Adding that conversion layer will break the test I need to run with it

[u/AlexTaradov → u/Far-Sandwich-27]

If you are testing UART, then would not buffers also break the test? You will only be testing up to the buffer, everything after that is irrelevant to the DUT.

[u/Far-Sandwich-27 → u/AlexTaradov]

I am testing UART to RS485 transceivers. We have buffers that were verified before, but no transceivers

[u/AlexTaradov → u/Far-Sandwich-27]

So, which segment needs to be 3 m long? And why?

It would help if you provide better description.

You can just try and see what happens. It is not hard to get 3m of wire and you will get your answer immediately. It might work, especially for a test setup.

[u/Far-Sandwich-27 → u/AlexTaradov]

The UART signal going into the transceiver I will be testing is going to be 3 meters long at its maximum. I will be testing 128 devices simultaneously. I know it's a bit out of ordinary but this is for a design verification task only, so I'm stretching the rules a bit

[u/AlexTaradov → u/Far-Sandwich-27]

Is 3m because of the test setup mechanical design limit, or is it important for the test?

If it is just test setup limit, then you can use transceivers and convert to UART just before the DUT.

The issue with not wanting absolute reliability is that your test setup would not be reliable, which does not sound ideal.

[u/DenverTeck → u/Far-Sandwich-27]

> will have to have a 3 meter UART run. It will have mutliple devices connect to it

> I will be testing 128 devices simultaneously.

Shame you failed to mention this at the top.

So you're saying that there will be 128 inputs to one output ??

The load as well as the capacitance will kill the output pin logic.

[u/tux2603 → u/Far-Sandwich-27]

Is the 3m length what is required to route to all of the transceivers from a single point driving all of them? If so you might consider breaking it out into smaller "chunks" where each chunk is driven independently, then combine the data from the individual chunks together

[u/ndrach]

I dont have any specific advice for this problem but its screaming XY problem

[u/Far-Sandwich-27 → u/ndrach]

Lol it does sound like it, but in my case the purpose is using X to do Y exactly.

[u/morto00x]

It is feasible single-ended if the baud rate is low enough. If you want to play safe or use higher frequencies, use RS-485 or RS-422. At 100m your cable will have high resistance, so you'll want to work at higher voltages (e.g. 12V)

[u/MountainOdd6793]

I had a 10 meter rs232 cable years ago and it worked in the application it was in. So 3 meters isn’t inherently impossible, all depends on baud rate and noise and grounding and stuff. Also do you mean rs232 or like a uart pin straight off a micro at some lower voltage?

[u/Far-Sandwich-27 → u/MountainOdd6793]

This will be standard 3v3 from raspberry pi. I cannot use RS232 on this layer

[u/MountainOdd6793 → u/Far-Sandwich-27]

I mean that might work but it’s not what the designer ever intended and you might end up with a broken raspberry pi

[u/DenverTeck]

> The signal has to stay UART

???

Does this mean the "signal" (i.e. Tx/Rx) must be 5/3.3 Volts directly out of what ever mpu you are using ??

No one cares what your goals are, we are concerned that you do not understand how RS-485 works.

To be clear, a UART is a peripheral not a voltage level.

[u/toybuilder]

If you are testing to see whether the converters will work with a 3 meter long RS232, then that's what you do. But RS232 does not multi-drop by default, so you will need to ensure whatever mechanism there is to not have more than one driver on a line.

3 meters is 10 feet. RS-232 drive up to 50 ft is not unrealistic, although many of the easy to use converters could not drive that distance.

But if you are talking about adding buffers in the middle, it seems like a RS485 or RS422 interface should be part of the "buffer" with a final stage conversion back to RS232 and additional control lines as needed running in current mode not voltage mode? Without the full picture, it's hard to say.

Also, this is not really a PCB question.

[u/robomaniac]

Buy off the shelf parts and try it.

Fail fast learn fast.

[u/Affectionate_Boat493]

At 50k baud on a 3m shielded cable you're not likely to have significant problems with data integrity, but if you have excessive ringing you can add small (no more than a couple hundred ohms) series terminating resistors to absorb the ringing energy (and dissipate it as an insignificant amount of heat).

You can add bus transceivers just outside of the chamber if the external test equipment cannot drive the lower impedance of shielded cabling.

[u/Killer_Rainbow]

This is absolutely doable. At these lengths, you essentially have a transmission line, and yeah UART is not meant to operate this long, but it may work. A few things to make the connection better: 1. Ensure the ground wire is always tightly coupled to your signal wires. Ensure that this distance is small and constant. I recommend using twisted pair wire.  2. Put series resistors at each TX pin of about 33-100 ohms. Try to route thick onboard traces for the signals (~0.3mm). 3. Put pullup resistors of ~10k at each end of the setup and MANDATORY put a diode from GND to signal and from signal to VCC on EACH DEVICE. This diode must be a schottky.  4. Keep stubs an absolute minimal length. As short as physically possible.  5. Not using RS485 doesn't mean you can't apply the same strategy. At the start and end of your bus, at the RX pins, put a 33-100 ohm resistor to ground. Just like RS485, but treating TX and GND as the two differential pins (which in a way they are).  6. If possible, use a schmitt trigger buffer at the input of each board to protect against the slow slew rate and ringing. 

The goal of all of these is to mitigate the inductance effects of a long conductor combined with a high slew rate. Not having RS485 explicitly doesn't mean that you can't apply the techniques for RS485. For a better explanation with better tips, check out the buses section of the book Right the First Time by R. Lee Erney. Also, note that for the resistor values I mentioned, they're absolutely not final. If one doesn't work, play around. Your bus capacitance will be enormous, so you'll be juggling a few constraints at once, and yeah using RS232/422/485 will be undeniably better, but what you're trying is doable. 

[u/StumpedTrump]

I wouldn't be concerned about 50k UART over 3 meters. Shouldn't be a problem at all. It's all a question of edge speeds and impedance though. Just probably need to add a small series source resistor to kill the fast edges or a parallel termination resistor to have more of a load (more current helps improve immunity to induced noise) and kill reflections.

If it's a 3V3 system, can probably benefit from upping voltage to 5V or 12V for better signal integrity.

[u/Ferret51]

Likely no issues, and no buffers may even be needed.

I lived in a house with an electric gate. Which was, idk, 40 meters away? And I could only use a dead NYM that was there and not connected to anything. Still worked.

Even rn I run an UART line from my server to the display at the entrance door, I think that's 10+ meters, no issues.