RPi UART Connection

[Jeremy Doolin]

First of all, I'm quite excited about the PiDP-11. The kit was delivered a day early and the build went rather smoothly, so I'm already up and running. I'm currently waiting on some parts from Adafruit before attaching the back panel. 

In the meantime, I'd love to use a USB serial connection to the PiDP-11. I've done this with other RPi boards, clones and microcontrollers, so this is nothing new to me. I've enabled the UART on the Pi (It's a Pi3), disabled getty on the serial port, and even disabled bluetooth (though I don't think that's exactly required now). 

However, when attempting to connect with picocom, I get nothing. I'm using 115200, 8N1. I've tried two different USB serial cables (both are like these: https://www.adafruit.com/product/954)  and even swapped the RX/TX connectors just in case they were reversed (I've seen that on some boards). I've tried from macOS and Linux. 

Am I missing a step? Do I need to add a setting in simh in boot.ini for whatever system I'm trying to use? Should I be getting a Linux console or simh console, or just a serial connection to the OS? 

Thanks folks. 

[Johnny Billquist]

Depends on what you are trying to do.

By default, nothing will just happen automagically on a serial port.

getty is the normal process that would pop up a login prompt on a serial
port when something is connected. But that would lead you to be able to
login to the Linux/Unix system itself, and don't really have anything to
do with your PiDP-11.

So I guess what you want is to be able to connect to, and log in to your
PDP-11 system using the serial port.

For this you first need to configure a serial port controller in simh
for the PDP-11 itself. That would probably be either a DZ11 or a DHU-11
(if we're talking Unibus). Next you want to associate one of the lines
on this PDP-11 serial controller with an actual, physical serial port on
your host (your USB serial adapter). For that you need another couple of
commands in your simh ini-file.

Once you've come that far you are then good on the simh side. Next you
also need to have your PDP-11 system know that you have that serial
controller, and how you go from here depends on which OS you are running
on your PDP-11. But if you are using 2.11BSD, then in addition to
configuring the controller as such, you then need to enable getty on
that port on your 2.11BSD system in order to get a login prompt.

Johnny

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[Jeremy Doolin]

Ahh, I see, thank you for the information. I was thinking maybe there was something in the PiDP11 software/simh setup that handled that automatically. 

Although I suppose just a Linux login prompt would still grab the simh console/screen session anyway. So maybe I'll just re-enable getty on the the serial port. Maybe a little less realistic, but good enough for me. 

This is all really making me wish someone hadn't thrown out my Wyse 150 serial terminal years ago. 

[terri-...@glaver.org]

On Wednesday, February 18, 2026 at 11:33:45 AM UTC-5 jdo...@gmail.com wrote:

Although I suppose just a Linux login prompt would still grab the simh console/screen session anyway. So maybe I'll just re-enable getty on the the serial port. Maybe a little less realistic, but good enough for me. 

You'll need to run pdp.sh, or just screen -d -r which is all that script does. 

Rather than fiddling with the Pi's console UART, you could just plug a regular

USB/serial cable into one of the four USB connectors on the Pi. They don't

have getty running by default, so you don't have to deal with that. Or you can

get very fancy and populate the PCB locations for MAX232 serial converters

and have up to four genuine DB-25M connectors on the back of your PiDP-11.

Here's what I'm using on my PiDP-11's RSTS/E boot.ini. Not all operating sys-

tems support the DHU-11 and may require a SYSGEN or simply not suuppo rt it

at all, in which case change "vh" to "dz", get rid of "set vh dhu" as it won't apply,

and combine the "set vh hangup" and "attach vh 4000" to a single "attach -am

dz 4000". In either case, you'll have up to four actual serial ports on the back

of your PiDP-11 and the remaining 'ports' will be reachable by telnet to the

PiDP-11's IP address. Of course, if you have an OS that supports TCP/IP by

itself (like 2.11BSD or Johnny's RSX-11M+) you won't need the Telnet ability

here.

Thanks to Mark and Johnny for explaining the esoterica of combined physical

and telnet access to a multiplexor, and for explaining the oddity of "hangup"

vs. "-am", respectively.

;
; Set up the emulated DHU11 multiplexor    
set vh lines=16
set vh dhu
set vh modem
set vh hangup
set vh enable
; Use Telnet port 4000 for non-physical ports
attach vh 4000
;
; Set up the USB-to-serial port(s) on the RPi
; Comment out any that are not physically present on the RPI
attach vh line=0,connect=/dev/ttyUSB0
;attach vh line=1,connect=/dev/ttyUSB1

;attach vh line=2,connect=/dev/ttyUSB2
;attach vh line=3,connect=/dev/ttyUSB3

Here are some pictures of a full 4-port setup:

[Anton Lavrentiev]

Since the Pi console (P3) is "/dev/ttyS0", you can actually use it (provided the getty service has been stopped in Linux):

attach dz line=0,connect=/dev/ttyS0

if you have the dz device configured for your OS (see RSX11+, for example), likely listening on some telnet

port (10001), but with the above Line 0 of it is going to be connected to the console serial port (other lines

are to remain networked via port 10001), so when you connect to the other end (e.g. PuTTY in Windows on your PC

COMn:), you'd see a guest OS prompt, like RSX11M+ ">".  Make sure COMn: is set to match the default comm settings,

9600 8N1 (because that's what simh changes it to when using the serial line).

HTH

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[Johnny Billquist]

simh really should set the serial port to whatever parameters the
emulated serial port is set to on the PDP-11.
I haven't checked that recently, but I think that is still the case. So
no need to set it to anything particular in Linux/Unix, or in simh.

But with that said, I'm not sure I can recommend grabbing the Linux/Unix
console port and use for this. If anything hits the fan, you might want
to use the serial port to get into the host.

However, you can certainly just use screen to attach to the existing
session if logged in to the host. But it's also not a big deal to just
configured an additional serial port to go into simh.

Johnny

On 18/02/2026 20.43, Anton Lavrentiev wrote:
> Since the Pi console (P3) is "/dev/ttyS0", you can actually use it
> (provided the getty service has been stopped in Linux):
>
> attach dz line=0,connect=/dev/ttyS0
>
> if you have the dz device configured for your OS (see RSX11+, for
> example), likely listening on some telnet
> port (10001), but with the above Line 0 of it is going to be connected
> to the console serial port (other lines
> are to remain networked via port 10001), so when you connect to the
> other end (e.g. PuTTY in Windows on your PC
> COMn:), you'd see a guest OS prompt, like RSX11M+ ">".  Make sure COMn:
> is set to match the default comm settings,
> 9600 8N1 (because that's what simh changes it to when using the serial
> line).
>
> HTH
>
> On Wed, Feb 18, 2026 at 2:23 PM terri-...@glaver.org

> <mailto:terri-...@glaver.org> <terri-...@glaver.org <mailto:terri-

> gro...@glaver.org>> wrote:
>
> On Wednesday, February 18, 2026 at 11:33:45 AM UTC-5

> PXL_20250720_204621103-deskew-s - Copy.jpg

>
>
> PXL_20250720_204842050-deskew-crop-s - Copy.jpg
>
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[Clem Cole]

On Wed, Feb 18, 2026 at 2:44 PM Anton Lavrentiev <anton.la...@gmail.com> wrote:

Since the Pi console (P3) is "/dev/ttyS0", you can actually use it (provided the getty service has been stopped in Linux):

attach dz line=0,connect=/dev/ttyS0

Hmm. This has not been my experience.   I just tried it on both my PiDP-11 (RPi3) and on my PiDP-10 (RPi5) [PiDP-8 (RPi2) is currently offline]. 

The following  is cut and pasted from my PiDP Serial Ports document:

... from a shell command prompt type:

ls -l /dev/tty[AU]*

On a RPi5 it should return something like:

crw-rw---- 1 root dialout 204, 64 Oct 26 17:21 /dev/ttyAMA0

crw-rw---- 1 root dialout 204, 74 Oct 26 17:21 /dev/ttyAMA10

crw-rw---- 1 root dialout 188, 0 Oct 26 17:21 /dev/ttyUSB0

Older RPi’s will likely be:

crw--w---- 1 root tty 204, 64 Oct 5 11:20 /dev/ttyAMA0

crw-rw---- 1 root dialout 188, 0 Oct 5 11:20 /dev/ttyUSB0

/dev/ttyAMA0 is the UART on the RPi 40 pin connector. The RPi5 has a GPIO-based second

UART on its AMA chip — look at the RPi5 docs for details on the other.

 

 

[Anton Lavrentiev]

> I haven't checked that recently, but I think that is still the case. So
> no need to set it to anything particular in Linux/Unix, or in simh.

I said to set that on the "other end" (which was suggested to be
Windows COMn:, using an UART2USB cable)

> I'm not sure I can recommend grabbing the Linux/Unix
> console port and use for this.

And why not? You can always connect to Pi via ssh, but as an
alternative to using Windows on the other end (e.g. COMn:) you can
also connect to a real terminal (using MX232 adapter, that is, to
shift the signals to the RS232 levels). It's just yet another serial
port, after all. Most of the time, "/dev/ttyS0" remains unused in
PiDP11, AFAIK, even though there's a "getty" session gets fired off
there by default.

It's just another connectivity option, it's neither superior nor it is
inferior to anything else that can be done with Pi.

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[Johnny Billquist]

On 18/02/2026 21.05, Anton Lavrentiev wrote:
>> I haven't checked that recently, but I think that is still the case. So
>> no need to set it to anything particular in Linux/Unix, or in simh.
>
> I said to set that on the "other end" (which was suggested to be
> Windows COMn:, using an UART2USB cable)

Ah. Misunderstood you then. And what sort of interface are on the other
end is obviously not that important. It should lead out to a RS-232
port, on a DE9 or DB25 or something. You then need a simple serial
null-modem cable. And that bit is important. :)

>> I'm not sure I can recommend grabbing the Linux/Unix
>> console port and use for this.
>
> And why not? You can always connect to Pi via ssh, but as an
> alternative to using Windows on the other end (e.g. COMn:) you can
> also connect to a real terminal (using MX232 adapter, that is, to
> shift the signals to the RS232 levels). It's just yet another serial
> port, after all. Most of the time, "/dev/ttyS0" remains unused in
> PiDP11, AFAIK, even though there's a "getty" session gets fired off
> there by default.
>
> It's just another connectivity option, it's neither superior nor it is
> inferior to anything else that can be done with Pi.

Like I said - if something goes wrong in your RPi, you might want/need
that serial port. ssh will not be of much use if the network isn't
coming up. Similar story with other serial ports. Initially, only your
console can be expected to always be usable. Everything else depends on
some things working in the starting up.

Johnny

[Anton Lavrentiev]

> /dev/ttyAMA0 is the UART on the RPi 40 pin connector.

While it might be so, I'm using Pi3B+ and /dev/ttyS0 is definitely
whatever the P5 header in PiDP11 makes available.
It worked both through simh for me, as I described, as well as it
works as a Linux console for Pi (as the alternative).

The AMA devices are modems, BTW. They are not "plain serial lines"
(like ttyS0).

[terri-...@glaver.org]

On Wednesday, February 18, 2026 at 3:03:40 PM UTC-5 cl...@ccc.com wrote:

Hmm. This has not been my experience.   I just tried it on both my PiDP-11 (RPi3) and on my PiDP-10 (RPi5) [PiDP-8 (RPi2) is currently offline]. 

This is one of those seemingly-inexplicable changes between Pi models.

I recommend one of those USB-A to DE-9 adapters - this seems to be the least-

expensive ships-from-US on Amazon: https://www.amazon.com/dp/B0759HSLP1

Then you don't have to wrestle with turning off getty, dealing with odd pinouts,

and so forth.

As far as remote access to the Pi, I recommend enabling SSH and VNC (if it is

on your private network and not reachable by random people from the outside

world). And of course there's always the USB keyboard+mouse and HDMI cable

if you get it completely borked. I've never used the built-in Pi serial console - I

set up my Pi systems with SSH enabled using the SD imager, then log in and en-

able VNC, and finish "moving in" with all the stuff the imager doesn't do.

[Johnny Billquist]

/dev/ttyAMA0 is (on my system) owned by root.dialout, which would
suggest that it's a device for outgoing connections.
So not something you use for incoming things like answering modems, or
terminals. But otherwise usually for modems, yes. It's about how the
modem signals are handled and treated...

Johnny

[Johnny Billquist]

And of course, the serial console is the only thing I've ever used on my
RPi. Works just fine. :-)

Yes, I usually also access the RPi Linux system using ssh, but for the
initial setup, I obviously needed the console. But yes, display keyboard
and mouse is obviously an equally good alternative.

Johnny

[Jeremy Doolin]

Thanks for the information everyone. I just fired up getty on the serial port and all is well. I'll experiment with attaching it to an OS in simh at some point as well, which sounds like it could be cool. 

This will be fun.