How can I easily convert rs-232 serial to TTL levels ?

Submitted ⁨⁨1⁩ ⁨day⁩ ago⁩ by ⁨interdimensionalmeme@lemmy.ml⁩ to ⁨askelectronics@discuss.tchncs.de⁩

Hi,

I just spent a lot of last week trying to access the console of an old datacenter switch. I made a cable from the pinout of a similar switch.

Image

My switch is a Dell S3048-ON this was for a C9010, it turned out to be correct.

Image

So I tried over and over to make it work, but kept getting this garbled mess, but the mess was consistent with the rythm of a booting switch, and it kind of always looked the same

With many of these, I have many spares I thought they were broken, but they were not

Image

I checked the signals with oscilloscope they were, dirty

Image

So I made the signals cleaner by tidying it up

Image

But still, it is a mess of garbled text !

Here I am using the other kind of serial adapter, this one serial to ethernet

Here is two runs, still garbled

Image

As a last ditch attempt, I remembered one my old server has a real serial port !!

Image

And it worked !

message continued in comment

source

Comments

Sort:hotnew top

litchralee@sh.itjust.works ⁨1⁩ ⁨day⁩ ago
I suppose the first question is whether you had the baud rate set correctly. The photo of the “cleaned up signals” (not entirely sure what you did, compared to the prior photo) seems to show a baud rate of 38400, given that each bit seems to take about 15 microseconds.

As for the voltage levels, the same photo seems to show 5v TTL. So it doesn’t seem like you would need a level converter from 15v RS-232 levels. This is one of the few times where the distinction between a “serial port” and an RS-233 port makes a difference, but a lot of data center switches will deal using 5v TTL, because the signals aren’t having to travel more than maybe 5 meters

source
SpaceNoodle@lemmy.world ⁨1⁩ ⁨day⁩ ago
MAX232

source
- interdimensionalmeme@lemmy.ml ⁨1⁩ ⁨day⁩ ago
  Thanks yes ! I think that’s what I was looking for !
  
  Image
  
  source
  - SpaceNoodle@lemmy.world ⁨1⁩ ⁨day⁩ ago
    I just soldered one of those up for myself the other week, I’ve got an old stash of those chips and was trying to resurrect a classic LEGO Mindstorms brain. Figures that you can get a cheapo adapter on Alibaba.
    
    source
BigDanishGuy@sh.itjust.works ⁨1⁩ ⁨day⁩ ago
Is the terminal emulator set up correctly in regards to baudrate and stopbits? I would try playing around with these settings. Maybe even go as far as to RTFM.

I haven’t used Dell gear before, but you are sure that it’s not some proprietary protocol that’s running over serial? As in it requires some special software instead of a console?

I’m not sure what you mean by the signals being dirty. If it’s the slope after the initial fast rise you mean, I guess it could be remedied with an opamp, a push-pull pair, an optocoupler, or some other switching circuit with a suitable power capacity. But as long as it gets above the hysteresis threshold for long enough it should be fine. Whether that’s happens you should be able to tell from the scope (which I can’t see while writing in my client)

source
interdimensionalmeme@lemmy.ml ⁨1⁩ ⁨day⁩ ago
I reached the limit in the post body so I can continuing it here

[I am no longer able to post pictures , imagine here the text, it is no longer garbled]

So the “real” serial port worked, but not my adapters !

So my question is this

What doesn’t the serial output work with these adapters ?

I imagine it is because the TTL voltage levels versus the serial voltages levels.

How do I do level shiting, preferably passively so that my adapters work ?

Especially my serial to ethernet adapter, which I don’t know any economical alternatives for.

These are WCH CH9121 by the way !

source