Attaching a VT-420 to the PiDP11

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Dwain Sims

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Jun 8, 2026, 8:18:14 PMJun 8
to [PiDP-11]
Folks:

I thought I knew a fair amount about RS232 but it is clear to me now that I know very little.

I should have never acquired this terminal.  It has the horrible MMJ connector and not a DB25 or DB9 like a normal VT terminal.  

I have tried to follow the nice write up on the Lammert Bies website, but I am still having no luck.  I even got the connector crimped with a trip to my vise.  I checked to make sure electrons were flowing with a Multimeter on the individual wires.


image.png

I am using a USB serial adapter and I can use an old laptop with a serial port running Minicom and that seems to work with no issues.  But I can get no action with same settings from the VT-420.

Any suggestions?

Thanks

Dwain

Adam Thornton

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Jun 9, 2026, 12:55:01 AMJun 9
to Dwain Sims, [PiDP-11]
It's probably not the serial interface?  I have a VT-420.  Works fine at 9600N81.

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Johnny Billquist

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Jun 9, 2026, 3:25:21 AMJun 9
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A VT420 should have one DB-25, and two MMJ, if I remember right.

Johnny

On 2026-06-09 02:17, Dwain Sims wrote:
> Folks:
>
> I thought I knew a fair amount about RS232 but it is clear to me now
> that I know very little.
>
> I should have never acquired this terminal.  It has the horrible MMJ
> connector and not a DB25 or DB9 like a normal VT terminal.
>
> I have tried to follow the nice write up on the Lammert Bies website,
> but I am still having no luck.  I even got the connector crimped with a
> trip to my vise.  I checked to make sure electrons were flowing with a
> Multimeter on the individual wires.
>
> https://www.lammertbies.nl/comm/cable/dec-mmj#h71j <https://
> www.lammertbies.nl/comm/cable/dec-mmj#h71j>
>
> image.png
>
> I am using a USB serial adapter and I can use an old laptop with a
> serial port running Minicom and that seems to work with no issues. But
> I can get no action with same settings from the VT-420.
>
> Any suggestions?
>
> Thanks
>
> Dwain
>
> --
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Dwain Sims

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Jun 9, 2026, 10:31:10 AMJun 9
to Adam Thornton, [PiDP-11]
Adam:

Thanks.  That is what I have both the terminal and Minicom settings on.  9600 8N1.  Works fine on Minicom but nothing on the terminal.  I am fairly certain that my homemade cable is not right..  I will keep messing with it.

Dwain

Dwain Sims

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Jun 9, 2026, 12:47:26 PMJun 9
to [PiDP-11]
Folks:

Thanks for all of the great suggestions.  They collectively helped.  Being fresh in the morning does not hurt either.

I got it working this morning.  Using better gear to make a DB9 connector, at least temporarily.  I will use this to make a more permanent style cable later.  
PXL_20260609_163640562.jpg

This is an ugly mess but much better than what I was doing yesterday.

I was super happy to see this pop up on my VT420!!

PXL_20260609_163731053.MP.jpg


Many thanks!!

Dwain

On Tue, Jun 9, 2026 at 12:06 PM Johnny Billquist <b...@softjar.se> wrote:
Hmm. Interesting. I didn't even know they made any such thing.

Ok. Let's start from the beginning then.

First of all, there are two MMJ connectors. And your terminal needs to
be set up to use one or the other (you can also run two sessions in
parallel, on on each port). If you are connecting your cable on one, but
the terminal is set to use the other, then obviously things won't work.

Second is DSR. You can set the terminal up to use or ignore DSR. If it's
set to use, then nothing will happen on the terminal until that signal
is active. I would generally say you want to have "data leads only".

Third. At the other end of your cable, short pins 2 and 3. That should
give up a electrical loopback. So anything you type should appear on the
screen. Until that works, there is no point to try anything else. But
this really should be very simple and straight forward. Obviously check
that transmit speed and receive speed match.

Once those things are all sorted, we can move on.

Oh, also... Note that MMJ have a balanced Tx and Rx. You should connect
Tx+ and Rx+ to pins 2 and 3 on the DB25. Tx- and Rx- should be tied
together, and goes to pin 7. (I hope you already got that, but just in
case...)

   Johnny

On 6/9/26 16:34, Dwain Sims wrote:
> Johnny:
>
> This particular VT-420 only has the two MMJ connectors.  There is a
> cover plate that says COMM on it but there is nothing under the cover. 
> That is where the DB25 should be, I am sure.  Probably a lower cost
> model that left off that connector.  (Gee having that connector would
> have made my life much easier.)
>
> Thanks
>
> Dwain

>
> On Tue, Jun 9, 2026 at 3:25 AM Johnny Billquist <b...@softjar.se
> <mailto:b...@softjar.se>> wrote:
>
>     A VT420 should have one DB-25, and two MMJ, if I remember right.
>
>         Johnny
>
>     On 2026-06-09 02:17, Dwain Sims wrote:
>      > Folks:
>      >
>      > I thought I knew a fair amount about RS232 but it is clear to menow
>      > that I know very little.
>      >
>      > I should have never acquired this terminal.  It has the horrible MMJ
>      > connector and not a DB25 or DB9 like a normal VT terminal.
>      >
>      > I have tried to follow the nice write up on the Lammert Bies
>     website,
>      > but I am still having no luck.  I even got the connector crimped
>     with a
>      > trip to my vise.  I checked to make sure electrons were flowing
>     with a
>      > Multimeter on the individual wires.
>      >
>      > https://www.lammertbies.nl/comm/cable/dec-mmj#h71j <https://

>     www.lammertbies.nl/comm/cable/dec-mmj#h71j>>
>      >
>      > image.png
>      >
>      > I am using a USB serial adapter and I can use an old laptop witha
>      > serial port running Minicom and that seems to work with no
>     issues. But
>      > I can get no action with same settings from the VT-420.
>      >
>      > Any suggestions?
>      >
>      > Thanks
>      >
>      > Dwain
>      >
>      > --
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>
>     --
>     Johnny Billquist                  || "I'm on a bus
>                                         ||  on a psychedelic trip
>     email: b...@softjar.se <mailto:b...@softjar.se>             ||
>     Reading murder books
>     pdp is alive!                    ||  tryin' to stay hip" - B. Idol
>
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>

Anton Lavrentiev

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Jun 9, 2026, 1:01:25 PMJun 9
to Dwain Sims, [PiDP-11]
First, you can use DB9-DB25 adapter, they are still widely available (e.g. on Amazon).
Also, when I just started connecting the MMJs to my setup, I was using regular 6p6c-jacks, which you can "adjust" to fit into the MMJ slots with just a sharp knife (careful!) and a small file.
They are not as good as the real MMJs but actually work quite nicely.
Lastly, believe it or not, Amazon sellers still have the MMJ plastic jacks (less than $10 for 10), and also many crimpers support them (also in the range of $10).
So it's not a big deal to make your own DEC BC-whatever office connect cable.
IMG_20260609_125812488.jpg

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Johnny Billquist

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Jun 9, 2026, 1:06:43 PMJun 9
to pid...@googlegroups.com
To add a few more comments, then.

First of all my favorite nitpick: It's a DE9, not a DB9.
The second letter is the size of the connector. And you're talking about
an E-size connector.

Second - if someone gets a few MMJ connectors and a 6-wire cable, crimp
both ends and getting a MMJ to DE9 is also an option.
You can find them on Ebay, and probably other places as well. Makes for
a much easier setup than soldering pins.

There are also MMJ to DB25 adapters out there.

Johnny

On 6/9/26 19:00, Anton Lavrentiev wrote:
> First, you can use DB9-DB25 adapter, they are still widely available
> (e.g. on Amazon).
> Also, when I just started connecting the MMJs to my setup, I was using
> regular 6p6c-jacks, which you can "adjust" to fit into the MMJ slots
> with just a sharp knife (careful!) and a small file.
> They are not as good as the real MMJs but actually work quite nicely.
> Lastly, believe it or not, Amazon sellers still have the MMJ plastic
> jacks (less than $10 for 10), and also many crimpers support them (also
> in the range of $10).
> So it's not a big deal to make your own DEC BC-whatever office connect
> cable.
> IMG_20260609_125812488.jpg
>
> On Tue, Jun 9, 2026 at 12:47 PM Dwain Sims <ds...@bayleafnc.org
> <mailto:ds...@bayleafnc.org>> wrote:
>
> Folks:
>
> Thanks for all of the great suggestions.  They collectively helped.
> Being fresh in the morning does not hurt either.
>
> I got it working this morning.  Using better gear to make a DB9
> connector, at least temporarily.  I will use this to make a more
> permanent style cable later.
> PXL_20260609_163640562.jpg
>
> This is an ugly mess but much better than what I was doing yesterday.
>
> I was super happy to see this pop up on my VT420!!
>
> > <mailto:b...@softjar.se <mailto:b...@softjar.se>>> wrote:
> >
> >     A VT420 should have one DB-25, and two MMJ, if I remember
> right.
> >
> >         Johnny
> >
> >     On 2026-06-09 02:17, Dwain Sims wrote:
> >      > Folks:
> >      >
> >      > I thought I knew a fair amount about RS232 but it is
> clear to menow
> >      > that I know very little.
> >      >
> >      > I should have never acquired thisterminal.  It has
> the horrible MMJ
> >      > connector and not a DB25 or DB9 like anormal VT terminal.
> >      >
> >      > I have tried to follow the nice write up on the
> Lammert Bies
> >     website,
> >      > but I am still having no luck.  Ieven got the
> connector crimped
> >     with a
> >      > trip to my vise.  I checked to make sure electrons
> were flowing
> >     with a
> >      > Multimeter on the individual wires.
> >      >
> >      > https://www.lammertbies.nl/comm/cable/dec-mmj#h71j
> <https://www.lammertbies.nl/comm/cable/dec-mmj#h71j> <https://
> www.lammertbies.nl/comm/cable/dec-mmj#h71j> <http://
> > www.lammertbies.nl/comm/cable/dec-mmj#h71j <http://
> www.lammertbies.nl/comm/cable/dec-mmj#h71j>>>
> >      >
> >      > image.png
> >      >
> >      > I am using a USB serial adapter and I can use an old
> laptop witha
> >      > serial port running Minicom and that seems to work with no
> >     issues. But
> >      > I can get no action with same settingsfrom the VT-420.
> >      >
> >      > Any suggestions?
> >      >
> >      > Thanks
> >      >
> >      > Dwain
> >      >
> >      > --
> >      > You received this message because you are subscribed
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> >                                        ||  on a psychedelic trip
> >     email: b...@softjar.se <mailto:b...@softjar.se>
> <mailto:b...@softjar.se <mailto:b...@softjar.se>>             ||
> >     Reading murder books
> >     pdp is alive!                    ||  tryin' to stay hip"
> - B. Idol
> >
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