Connecting to external RS-232 device via UART

[MaffooClock]

I realize this product is in its infancy and not all features are present, and not all code and schematics have been made open-source yet, so I'm probably getting ahead of things, but...

I'm tinkering with using the extension port to communicate with an external KVM (manufactured of questionable Chinese quality by KCEVE) that supports switching inputs via serial control. My ultimate goal is to use the JetKVM to effectively convert the "standard" KVM into an IPKVM. The final solution will offer buttons in the web UI that I can click to switch the input on the external KVM.

I built a simple serial cable with a 6P6C RJ-11 and a MAX3232 transceiver, using the 3.3V on pin 4 to drive the MAX3232.

Here's the wiring:

RJ-11 Pin	Function	Wire Color	MAX3232 Pin - TTL	MAX3232 Pin - RS-232	RS-232 Device
1	GND	Green/White	15	15	GND
2	UART_RX	Orange	12 (R1 Out)	13 (R1 In)	TX
3	UART_TX	Blue	11 (T1 In)	14 (T1 Out)	RX
4	3V3	Brown	16	-	-
5	(n/c)	-	-	-	-
6	VSYS (5V)	-	-	-	-

While poking around the OS on the JetKVM, I see three tty devices:

# ls /dev/*tty*
/dev/ttyS3    /dev/ttyFIQ0  /dev/tty

Note

22 May 2025: If you're new to this discussion, skip down to this comment.

I assume ttyS3 is the UART pins on the Extension Port. Running stty -F /dev/ttyS3 -a shows that it's already set to a typical 9600/8/n/1, so it appears that I don't need to set anything (except maybe the baud, but I have no clue what the external device expects, so this is a decent start).

The serial device on the other end expects commands like X3,1$ or XA,1$ to switch inputs (input 3 or 10, respectively, for example). Seems easy enough. So the actual command I'm sending, escaped, is:

# switch to input 2
echo "X2,1\$" > /dev/ttyS3 

Well, nothing happens. So, there must be a problem in one of these points: 1. Maybe `ttyS3` isn't the right device 2. I'm not formatting the message correctly 3. Wrong baud rate (this might be Try 'n' See™ until I figure out the right speed, as there is no documentation that I could find) 4. I got the TTL or RS-232 RX/TX backwards on the MAX3232 5. The manufacturer's command documentation is incorrect, or the TX/RX labels on the physical port are backwards 6. I'm missing something else and I'm not smart enough to think of it

Probably the first thing to do would be to test my custom TTL to RS-232 cable -- I could use a Raspberry Pi for that:

1. TTL end connected to the appropriate GPIO pins
2. RS-232 end connected to a USB-to-Serial dongle
3. Send some text into one port and see what comes out of the other

So, no point in messing with the JetKVM until I perform the test on my custom cable. But meanwhile, I figured I'd start this discussion in case anyone might have some insight, or for anyone else interested in this who might want to follow along.

[Nevexo]

Neat! I'm not sure which chip the official serial extension will use, but if it's the ever-so-common MAX3232, then the software for it will be merged in shortly.

To that end, the infrastructure around extension boards will be introduced fairly soon, so that'll tell us a lot more about how the JetKVM team expect us to interact with the UART port.

There hasn't been much investigation into the extension port and it's infrastructure yet, mostly waiting to see what the official implementation looks like.

[dseven]

I don't think the software would know or care if it's a MAXx232 or something else - it'd just be talking to a serial port, no? I don't disagree that it should become clearer when the software gets released, but if we're itchy in the meantime....

@MaffooClock, the other thing you could try (before or after verifying your adapter) would be to connect the RS232 side of it to a known-working RS232 port (could be a USB adapter) on a PC with a terminal emulator (minicom, PuTTY, etc). When you have it working, anything you send on the JetKVM side should appear in the terminal (and vice-versa)

[Nevexo]

MAXx232 or something else - it'd just be talking to a serial port, no

That really depends how they've implemented it. If it's a case of telling the JetKVM which module you've attached, then sure.

But if it's doing some magic to discover it automatically (an MCU on the board?) then there'll be a bit more to it than just banging serial data down the UART.

[MaffooClock]

I don't think the software would know or care if it's a MAXx232 or something else - it'd just be talking to a serial port, no?

This is what I'm thinking. But the documentation also says "Using a standard RJ-11 connector, it provides power, I2C, and GPIO pins...", but then immediately after only shows UART pins. Like a lot of modern CPUs/MCUs, GPIO pins can have multiple purpose (although I'm not an expert on the Rockchip implementation in the JetJVM), so perhaps what Nevexo might be thinking was the pins being used as I2C, and the official serial extension will have an I2C-to-UART device ahead of the MAXx232 (or whatever the actual UART-to-RS232 device will be).

the other thing you could try would be to connect the RS232 side of it to a known-working RS232 port...

Yes, that's basically identical to what I outlined in my "verifying your adapter" steps (just different words).

[Nevexo]

Yeah the pins can be configured to be UART, GPIO, I2C etc.

We've just seen the ATX board has an RP2040, so will be interesting to see the protocol that is using to talk to the JetKVM.

[dseven]

Yes, that's basically identical to what I outlined in my "verifying your adapter" steps (just different words).

Using the UART pins on a RPi will verify that the adapter works... but assuming it does, putting it back on the UART pins of the JetKVM extension and using a terminal might help you find the right device etc on the JetKVM, without trying to also get the Chinesium KVM to respond.

[spawnflagger]

For testing your custom cable, if you just jumper the TX and RX pins together (on the RS232 side), do you see anything?

Whatever terminal app you're using will have to disable hardware flow control. (there's no CTS/RTS/etc pins).
Keep in mind that actual RS-232 uses negative voltages (-3 to -15) for logical 1, so your KVM device might be labeled as "RS232" but in reality it's just looking for 3.3V, so you might not even need the max232 chip.

[MaffooClock]

Welp, tested my custom TTL to RS-232 cable -- worked perfectly. One of the tests included the JetKVM, of course, and I can confirm that ttyS3 is, indeed, the UART on the Extension Port.

The problem: the TX and RX pins on the "Chinesium KVM" are labeled backwards 🙄. Oh, and the baud rate is 115200, for anyone who cares.

And now, sending X1,1\$...XA,1\$ makes bespoke KVM switch inputs. 🎉

Next step: learning some Go and adding a way to dispatch these command from the web UI. If I build this right, it would be compatible with the plugin system discussed in #9 (and related pull request #10), and be configurable so the user can specify how many inputs are available and what the associated serial command would be.

[MaffooClock]

Well, fun fact... every time I run this:

echo "X2,1\$\r\n" > /dev/ttyS3

...the JetKVM reboots 😳

Incidentally, found that I don't need the newline bits, nor even to escape the $. So this works:

echo "X1,1$" > /dev/ttyS3

...but it still causes JetKVM to shidd its pants 🧐

[MaffooClock]

D'oh, I think I know what's happening...

When the external KVM switches its input, it must be dropping the USB temporarily, which causes power loss on the JetKVM. Derp.

[spawnflagger]

Congrats on getting it working. Do you mind sharing what MAX3232 mini PCB you used in the picture above? (I've seen many variants, but not one with the pins brought out to the short edges like yours)

As far as losing power when switching - does the JetKVM also reset if you aren't using the Extension port at all and simply press the button(s) on your external KVM to change source?

[MaffooClock]

Do you mind sharing what MAX3232 mini PCB you used?

Sure, it's this one from Amazon. Hope you're skilled with a soldering iron because the data pads aren't through-hole like the power pads, so it can be a bit of a challenge (to a novice, maybe) to get the wires on. And it's not obvious just how tiny these are until you get one in your hands!

does the JetKVM also reset if you... simply press the button(s) on your external KVM?

I'm certain that the switching action internally is the same regardless of how it is triggered. However, I am a bit curious why the external KVM would be designed to bounce power on the USB ports when switching (perhaps it's merely an oversight, as might be expected for a low-cost Chineseum product). But we're venturing outside of JetKVM territory at this point.

[MaffooClock]

For what it's worth to anyone that might want it, I've whipped up a simple script to make it easy. I'm going to guess that this will work for any serial-connected external KVM manufactured by KCEVE (just set the NUM_INPUTS as required).

#!/bin/sh

NUM_INPUTS=10
PORT=/dev/ttyS3
BAUD=115200


### Make sure the input number was supplied
if [ $# -ne 1 ]
then
  echo "Usage: $0 <input>"
  exit 1
fi

### Check that the argument is an integer
echo "$1" | grep -Eq '^-?[0-9]+$'
if [ $? = 1 ]
then
  echo "Invalid argument: $1"
  exit 1
fi

### Check that the argument is within range
if [ "$1" -lt 1 ] || [ "$1" -gt "$NUM_INPUTS" ]
then
  echo "Input out of range: $1"
  exit 1
fi

### Check if the serial port is already set to the correct baud (and set it if not)
stty -F $PORT | grep -q $BAUD
if [ $? = 1 ]
then
  stty -F $PORT $BAUD
fi

### Convert integer to hex (in case we have more than 9 inputs)
I=$(printf "%X\n" $1)

### Send it!
echo "X${I},1\$" > $PORT

For my purposes, having an SSH terminal open while I access the JetKVM web UI will satisfy my requirements to remotely view and control any of the 9 hosts connected to my external KVM. But I would like to see if I can add a component in the web UI to make it even easier...

[TorranceTM]

Hi MaffooClock - Just confirming you are running this script on the JetKVM - I am having lots of issues running it on the JetKVM, I know the syntax is good, but it is complaining about syntax when I try to run /bin/sh scriptname.sh 1 after doing a chmod +x scriptname.sh - Do you have to run it with any other parameters or special Path statement?

[dseven]

I don't have a KVM switch to test with, but running the script on my JetKVM produces no errors. Perhaps something went wrong with your copying of the code to your JetKVM? I just ssh'ed in and pasted it into vi

[TorranceTM]

Thanks dseven - I was being stupid and editing the file on the PC and copying it over to the JetKVM using the CAT via SSH and then trying to run it - The line formatting/ending was getting mucked up during that process. Now to learn vi :-(

[compulov]

Thanks for doing the legwork on this. I actually bought the parts to put together a UART to RS232 cable a few weeks ago but I've been too busy to actually put it all together. Glad it worked for you since this is what I wanted to do myself.

[Dune4]

Hi @MaffooClock

Can you post the final table with the actual working pin out please?

[MaffooClock]

See the first post

[MaffooClock]

Update:

I haven't tinkered with my JetKVM device in a while (I've used it and my Bash script via SSH, but no development), but I have been installing dev-channel updates as they become available.

I wanted to report that the built-in Serial Console functionality in the web UI has been working for sending commands to my external KVM (connected with the custom cable I built). Not sure which one of the updates I've installed over the past few months gave me this ability, I only noticed it today.

I just click the >_ Open Console button, then type the KVM command into the console that appears: X1,1$ for input 1, X4,1$ for input 4, and so on (the $ character acts like Enter). It's not as easy as using the Bash script via SSH, but it's simpler since I don't have to fire up an SSH session every time I need to use the JetKVM and want to switch inputs on the external KVM.

So for anyone wanting to use JetKVM with an external KVM, and you have a serial connection between them, you can use this built-in feature to send the serial commands.

The only issue I've had so far (app v0.4.1, sys v0.2.4) is sometimes the serial console will not take keyboard focus no matter what I click on (the WebRTC interface has it and is sending all keystrokes to the connected host). I have to click the Hide button to dismiss the serial console, re-open it, then I can use it again. But I know this is all still under heavy development, so it's not a complaint.

[Dune4]

Thanks for your work on this @MaffooClock

I am having a terrible time with a particular network appliance. I guess you have updated the table to reflect your comments, has the photo been updated too? The reason I ask is it looks like there are no cables in pin 1 & 2? clip facing down and going from left to right?

I also have the same chip and ordered from Amazon UK so wondering if I have the same issue, I order the same chip from a couple other sources just to test. Although my end goal might be different, but what im trying to do is make a RJ12 > MAX3232 > RJ45 Console connection.

I have confirmed that on my network device from testing with a multimeter, pins 1 and 2 are at 9.8v and pin 3 at -9.8v. 4 and 5 are GND. To me these are certainly RS232 voltages. So im suspecting 3 is TX. This is the diagram provided by the vendor attached

. Let me know if you have any thoughts on any of the above?

I have an eczoo and TEsmart switch and will also be giving these a go with the jetKVM.