LEGO Interface A (9750/9771) – LEGO Technic Control 1 (TC1) reference/ideas thread

[evank]

Who you gonna call? @Toastie!

 

3 hours ago, Toastie said:

But there is more, there always is: @Bliss's LEGO Blockly (I love to control Interface A/B/CM/RCX/PF/VLL and more with it, using every time the exact same language, heck, I am trying to get behind the Compis logic using his software!), @Mr Hobbles' approach to so many legacy and modern LEGO electronics, @maehw's Lines program (and so much more, saw some of your videos!), @BrickTronic's knowledge, @amine's unbelievable hunting skills, @alexGS knowledge about vintage educational hard- and software, as well as his electronics skills, and many others not tagged, I can't recall as of writing; I am sorry!!!

 

That's why I joked a while back that I may be the only person here actually building things based on Interface A. 

[aFrInati0n]

For forum structure the mods or @Jim may be better @Toastie.

[vascolp]

9 hours ago, alexGS said:

I think I’ve explained before… when I post in a public group, anyone from my work colleagues, to occasional work acquaintances (tradespeople), to friends, not-so-friends, and family all get shown that post randomly in their feeds. Some will private-message me in confusion. Some will use it as evidence that I wasn’t working on their project at the time. Some will just laugh or post unrelated discussions in that group. I even had a phone call from a friend’s mother (whom I hadn’t seen in 25 years) worried that my identity had been stolen after I posted on a UK group.

A private group avoids all of these embarrassments, and I think you’d get more people sharing things if they knew they weren’t being ‘watched’.

Having said all that, and considering that Facebook is the easier way to upload photos and videos, I’m aware that important contributors here don’t use Facebook. And so, I support your idea of a sub-forum here - that seems a great way to organise things better.

I understand you. Facebook has a thing called profiles. I have my usual facebook profile and a Lego version, so that things don't get mixed. From AI:
Facebook allows you to create up to four additional personal profiles, giving you up to five total profiles on a single account. Each profile can have a unique name, custom Feed, friends list, and privacy settings. You can seamlessly switch between them using your primary login without logging out.

You have to subscribe all your groups on the Lego profile, friends, etc, but then you get separated notifications. In the begining it is very good, only lego! But soon the algorithm finds other subjects to bother you 

Edited July 7 by vascolp
Triplicated!

[amine]

Found this post on Facebook:.

"My husband inherited some old Lego

It concerns:

- Lego technic interface A (9750), the box is in very poor condition, but the contents look new.

- Lego Dacta (9700) - The box is broken, but contents in unbroken bags.

- Lego Dacta (1038) - The box is broken, but the contents are in unbroken bags.

-Lego Dacta (9760) + (9765) - activity pack

Additionally, 3 teacher's books for Lego Dacta.

We have no understanding of it in any way.

Is it worth anything and if so how much? "

https://www.facebook.com/groups/legoretrodk/posts/26451040977923544/

 

[alexGS]

3 hours ago, vascolp said:

I understand you. Facebook has a thing called profiles. I have my usual facebook profile and a Lego version, so that things don't get mixed.

Thank you @vascolp for your thoughts - you’re right - that’s really what I need to do 🥰 I use Facebook as a substitute for LinkedIn, lots of business contacts - they should really see a different profile. I think usage of Facebook varies around the world.

Anyway, instead of duplicating on Facebook, I strongly agree with Evan’s proposal that we should have a sub-forum here, so we can split all our various topics into threads within it.

Otherwise, discussion on this mega-thread becomes hard to follow - I’d like to see all the material related to the BBC in one thread, for example, so that we can pick up months later where we left off, or so that someone finding the thread in a search doesn’t feel obligated to read through 15 pages of other progress including the PC interface multiplexing, etc.

Thanks, @evank for the idea :)

 

Edited July 7 by alexGS

[evank]

Wasn't it @Toastie's idea?

Anyway, I personally have no problem using one Facebook profile for all of my stuff there :) but yes let's have a vintage Technic subforum!

Someone would need to be a volunteer moderator, because inevitably people of a certain age will find a Mindstorms set from 2005 and decide it's quite old. :)

[Toastie]

2 hours ago, evank said:

inevitably people of a certain age will find a Mindstorms set from 2005 and decide it's quite old. :)

Well such sets >are< quite old (and need repeated repair)!

Vintage Technic though means sets from a time, when global warming was called summer, and executing LDIR on a Z80 CPU considered "data transfer faster than electrons can move".    

We shall see, I'll ask around.

Best
Thorsten     

On 7/3/2026 at 12:19 AM, alexGS said:

I think it’s time for us to start new threads perhaps, for these different configurations - it’s a lot to try and keep up with

It was Alex' idea, see above - I just threw in the sub-forum approach, because then it floats together with the Technic forum ...

 

[evank]

Lol so Alex blamed me, I blamed you, and you blamed Alex. Teamwork! 😁

Regardless, we all agree it's a good idea. A whole vintage Technic section for our retro creations, vintage software, and modern hacks!

[alexGS]

7 hours ago, evank said:

Lol so Alex blamed me, I blamed you, and you blamed Alex. Teamwork! 😁

Regardless, we all agree it's a good idea. A whole vintage Technic section for our retro creations, vintage software, and modern hacks!

LOL… Thanks all 😁 Toastie’s idea was the sub-forum which I now think is best and we all agree.

Personally I think we could have a 20-year cutoff for our ‘Vintage LEGO robotics’ sub-forum. We could have threads about RCX and even NXT in our new sub-group without interfering with our Interface A or Control Lab content. Or perhaps we could specify “pre-NXT’. The old Mindstorms threads would just drop to the bottom if no-one’s interested (it’s a fairly small group collecting and using original RCX software on old hardware as I do). It’s a similar principle after all; ways to get the old hardware working again.

Edited July 7 by alexGS

[evank]

I think it would help make our case to the admins if we can say that someone is willing to be the subforum moderator. Otherwise we're creating more work for them.

I'll raise my hand because I am masochist with no extra time. 🙄

[alexGS]

33 minutes ago, evank said:

I think it would help make our case to the admins if we can say that someone is willing to be the subforum moderator. Otherwise we're creating more work for them.

I'll raise my hand because I am masochist with no extra time. 🙄

You can put me forward, Evan. Although my rules might be different to yours :)

[evank]

It's good, since we live on opposite sides of Earth.

[Toastie]

8 hours ago, alexGS said:

Personally I think we could have a 20-year cutoff for our ‘Vintage LEGO robotics’ sub-forum. We could have threads about RCX and even NXT in our new sub-group without interfering with our Interface A or Control Lab content. Or perhaps we could specify “pre-NXT’.

Well, this seems to be one thing (in addition to suggesting an admin for the sub-forum), that should be clearly defined and in an educated way: The cutoff year. Or maybe better the cut-off theme. Or cut-off tech ... 

Personally, I am madly in love with the RCX, but I also admire the Cybermaster, Scout, MicroScout, CodePilot and Spybot PBricks. All can be programmed using, e.g., NQC (maybe using the BricxCC IDE), provided you have a USB tower and "compatible computer" (32 bit): The USB tower can emit IR as well as visible light byte codes, all the PBricks above speak. You'd need and RF tower for the Cybermaster, but that's it. The programming approach is always the same though.

In addition to that, we have a current programming environment developed by @Bliss (LEGO Blockly), that is tailored toward non-BASIC/COMAL/C gurus and can be used to operate (not program) some the above PBricks, IntA, IntB, etc. @Mr Hobbles is also actively developing such an environment. 

For me, one possible cut-off could be: The 4.5V/9V (i.e., the 9V 2x2 electric connector) world. A further argument would be: "The time prior to TLG leaving the philosophy of integrating their electronic devices as much as possible". A MicroScout knows VLL which the Scout speaks natively. A Scout can directly interact with an RCX, Interface B, and so on. The USB tower knows how to interact with the CodePilot, and so on and so forth. Heck, I used one output LED of InterfaceA to program the CodePilot and MicroScout using BASIC. 

The Technic Control world is an entire universe created for the minds before the storm - during the wild 8-bit frenzy - which per definition - qualifies as true vintage :D and connects to the 9V world as well.

With the NXT the "new incompatible plug" frenzy started (and yes, I know, it could not work otherwise), and thus new motors, new sensors, etc. were required.

Which creates a nice opportunity to define a cut-off on the tech-level, rather than a year (which is essentially the same, but sounds cooler).

What do you guys think?

Best
Thorsten      

[evank]

I know very little about the Mindstorms sets, so I can't comment on which ones should or should not be welcome in our proposed subforum.

What I do know, after leading a non-profit (Vintage Computer Federation) in the retrocomputing hobby for 15 years (2004-2019), is that year-based definitions of "what's vintage?" are a BAD IDEA because the definition depends entirely on when each person was born and when they discovered the hobby. Some of the older, less-clueful people were sticklers for "20 years!" and they felt quite foolish when 2021 arrived, meaning they had to accept kids talking about Windows XP. :)

What always worked better -- and this supports @Toastie's perspective -- is defining our scope via technology generations. Try hard to avoid subjectivity, or anything that'll come back to bite you later.

[alexGS]

You’ve both made very good points; Toastie has thoroughly dissected the idea, and I agree with what he’s found.

I think there is a case for including Mindstorms RCX (1998-2004) out of practicality, as the 9V sensors/wires etc. are the same as used in Control Lab. Plus, anything built with ‘studded Technic’ is more familiar to us than the studless used in NXT (2005) and later EV3, Robot Inventor, Control+ etc.

So I’m going to propose that it should be Vintage LEGO Robotics 1984-2004 (4.5V and 9V pre-NXT)…

Edited July 9 by alexGS

[amine]

4 hours ago, alexGS said:

You’ve both made very good points; Toastie has thoroughly dissected the idea, and I agree with what he’s found.

I think there is a case for including Mindstorms RCX (1998-2004) out of practicality, as the 9V sensors/wires etc. are the same as used in Control Lab. Plus, anything built with ‘studded Technic’ is more familiar to us than the studless used in NXT (2005) and later EV3, Robot Inventor, Control+ etc.

So I’m going to propose that it should be Vintage LEGO Robotics 1984-2004 (4.5V and 9V pre-NXT)…

Not AI.

Edited July 9 by amine

[Toastie]

IT WORKS  I mean one possible Compis J5 <-> IntA 2/4 wire “solution”.

Just to reiterate/get you back on track: The Swedish school computer "Compis" controlled Interface A via a cable with a 6 pin DIN plug on one side that - well - plugged into a 6 pin DIN socket named “J5” on the Compis. On the other side was the usual 20 pin ribbon connector for Interface A, however, as new footage proves, with a rather bulky box attached. Paul Findley (@amine and @evank also know) has taken some photographs of the Compis cable for the 9750:

 

 

According to the Compis schematics, the DIN socket contacts are wired internally to +5V, GND, in_0, in_1, out_0 and out_1. The in/out lines are connected to an 8255 PIO. Its ports are programmable, but I left that out of the equation and assumed they are (directionally) “fixed”.

So how do 2 (TTL/PIO) outputs control 6 Interface A outputs? Obviously, this must be serial in some way. Recall that we are in Big Brother’s year 1984 or even before - 1984 was the year when the Compis computer was officially rolled out to schools, as far as I know.

Serial-to-parallel is as old as serial and parallel are. So a quick check on my beloved 74XXX TTLs and CD4XXX CMOS chippies told me: It's all there, even before 1980 ... My storage facility  here had a 74LS164 (8-bit parallel out serial shift register (SR) in stock. However, these do shift = clock-through the 0/1 data coming in, so the outputs flicker accordingly, until all 8 bits are in. No problem, when you do that on the micro seconds temporal scale. This needs a microsecond clock of course, and I believe a decent machine code program can do so using the 8255 ports. So that could be all what is needed: A 74LS164. Plus maybe a 100 nF blocking capacitor.

But to demonstrate this approach, I didn't have a Compis, nor a computer equipped with a 8255 for I/O. And: I wanted to go full educational (i.e., teaching myself basic digital electronics). So the computer is my Dell laptop, the interface mimicking an 8255 is an Arduino Uno loaded with the USB2Parallel software @Bliss provided, and the programming language is LEGO Blockly, again @Bliss' work - available freely and discussed here on EB in its own thread.

This is the plan:

• On the Arduino's 20 pin Interface A compatible output, I am using only 4 lines: +5V (pins 1+3), GND (pins 5 to 19), out_0 (pin 6), and out_1 (pin 8), for mimicking the two output ports of the 8255 in the Compis.
• I wanted this to be fully independent of the clock rate for the shift register, be it 1 Hz so I can actually observe what is happening, or as fast as Blockly can go. Or even in situations, when clock rates change during 8-bit data submission, or running at full-speed MHz machine code rates. This rules out a constantly running fixed frequency clock.
• I didn’t want any output flicker on Interface A – at low clock rates, this could be nerve and model wrecking. In other words, a latch is required. 74LS273 is my friend, a (pos. edge) triggered 8-bit D latch.

Sounded good, but turned out to be a little tricky: When 6 bits are in the SR, the latch needs to be triggered. But there are only 2 lines available: Data and clock. Solution: Use SR output #7 (G) to trigger the latch. This requires the introduction of a small delay though, as all 6 relevant SR outputs have to fully settle before the latch is triggered, otherwise wrong readings are latched in. As the latch wants a positive slope, output G needs to transition from L to H upon the 7th clock pule – at that moment the 6 SR outputs A-F are also clocked in and then subsequently latched in.

Delay solution: Exploiting signal propagation delay. Six 74LS04 NOT gates cascaded (resulting in an about 70 ns delay) did do the trick, but there is more, there always is: When using the SR to provide the pos. slope latch clock, the SR needs to be reset for the next serial data loading, as arbitrary pos/neg bits of the last loading are still present in the SR and will trigger the latch on each pos. slope, i.e., every 0/1 sequence coming through upon next 8 bit load. As there is no additional clear line available, I used the SR again:

Solution: Connect SR output #8 (H) via a further NOT gate to input “clear” on the SR.

Composing an 8-bit serial data packet accordingly, assuming the SR was cleared before (outputs A-H = 0), looks like this:

• Bit #1 is always 1 and loaded into SR upon clock tick 1. This will show up on output G upon clock tick 7, is then running through the delay line and fires the latch.
• Bits #2 to #7 = data bits. Clock ticks 2 to 7. All 6 data bits are present, and the latch triggers after delay.
• Bit #8 may be L or H: It is just used to shift bit#1 into SR output H, which is inverted via a NOT gate and resets the SR. The latch outputs remain unaffected and thus stable Interface A outputs are guaranteed.

Here is a schematic (I know, I’ll get flak from @BrickTronic for the wrong a’s in the IC numbering - I am still too dumb to get it right in Target 3001). I am also absolutely sure that this can be done much more elegantly, I am thinking about 74LS595, but don’t have one. There are for sure better ways of doing this. @BrickTronic pointed me already to CD4094 - that is a nice one as well. The LEDs are purely for optical inspection of the 8 outputs of the SR and the 6 latch outputs. In addition, I used the LEDs for serious debugging, particularly software ^^ ...

Note about the cascaded NOT gates for establishing the 60-100 ns delay between the SR clock and latch clock: 6 TTL NOT gates (74LS04) were sufficient, but inverting the SR H output for resetting the SR would need another TTL IC. Well, the good old CD4000 CMOS chips have much longer propagation delays, and some are fully compatible with the TTL world: CD4069 (6 NOT gates) is. So two NOT gates of CD4069 in series provide an about 100 ns delay, safely ensuring proper timing. And reducing the IC count required to running this setup by one, as 4 NOT gates are still available, one used for the SR output H – SR clear connection. Alternatively there is 74LS123, (two mono-flops) will definitely work as well.

Also note  that I did not use the HC version but the LS version of 74164; but Target had only a "poor" schematic for the LS version.

Further note  that the InterfaceA inputs (pins 18, 20) may also be latched by connecting them to the D7+8 inputs of the latch and routing the outputs Q7+8 of the latch to the Compis side inputs. However, this would require an output write cycle (whether outputs changed or not) to update the latch for updated input reading. This is exactly what TC Logo does at a 1 ms rate on an IBM XT. But its optional; once your computer is talking/listening to 9750 through a PIO (or any other peripheral I/O device separating the data bus from the outside world), parallel input latching is essentially luxury.

The setup shown above runs really stable with the LEGO Blockly program shown below; it counts from 0 to 63, creating all possible Interface A output states. Really fast with no delay, sometimes apparently yielding swiftly to other Windows processes. With delay, one can study the SR vs. latch clocking. The left red LED never lights up visibly, as the reset process is too fast. Here is a photograph -  as you can see, I am using only 4 lines of the 20 pin connector: +5, GND, out_0, out_1:

I am fully aware, that @Bliss will tear my decimal to binary conversion to shreds, I am simply a novice regarding LEGO Blockly. This is what I came up with to count from 0 to 63, convert each decimal number into binary and wrap a leading and trailing bit around the binary number:

  

I have learned a lot about shift registers, timing of such devices, and handling latches in such an environment -  particularly when you don't have more than two control lines. As @BrickTronic says: There are many ways to ROM, uhm Rome.

Finally, here is another of my crappy videos showing the dynamics of SR shifting/latch loading. First 5 seconds: Max Blockly speed shifting/loading of numbers 0 to 63. Remaining plus 60 seconds: ASMR version of shift register loading and latch clocking for the same cycle, delayed by one second for each clock step in the Blockly program:

https://bricksafe.com/files/Toastie/compis-and-9750/Compis Serial to parallel.mp4

It is on BrickSafe - as it is too lame for YT. When I hover over the link the video starts.

Best wishes
Thorsten
 

Edited July 9 by Toastie

[BrickTronic]

22 hours ago, Toastie said:

...

Hello,

A possible improvement to generate a Strobe out of Clock & Data Signals :

 

A puls on upper Line shifts in a "1",
a puls on lower Line shifts in a "0"
and at last puls also 2.nd Line set to "H" issue a Strobe

No need of additional Strobe-Line or
to add 7th and 8th Puls to be use d as Strobe and Reset

Jo

[Toastie]

Hi Jo,

oh, this looks so elegant ... but I am slow - and need more "input":

• With pulse you mean an L/H slope, right? So upon startup, both inputs are L? 
• The propagation delay through the OR/AND gates needs to be sufficient for the correct clocking of data, right?
• This circuit means that I need two shift registers (SR), as I need access to two separate clock lines, correct?
• And finally: This gives me three outputs on SR 2, so I'd need two SRs for 6 outputs, correct?   

I am sorry for not getting it right away. 

All the best
Thorsten 

[BrickTronic]

50 minutes ago, Toastie said:

Hi Jo,

oh, this looks so elegant ... but I am slow - and need more "input":

• With pulse you mean an L/H slope, right? So upon startup, both inputs are L? 
• The propagation delay through the OR/AND gates needs to be sufficient for the correct clocking of data, right?
• This circuit means that I need two shift registers (SR), as I need access to two separate clock lines, correct?
• And finally: This gives me three outputs on SR 2, so I'd need two SRs for 6 outputs, correct?   

I am sorry for not getting it right away. 

All the best
Thorsten 

Hello,

Assumptions are :

Inactive :           both lines low
shift in "1" :       Set upper Line from "0" to "1" and then back to "0" while lower Line remain "0"
shift in "0" :       Set lower Line from "0" to "1" and then back to "0" while upper Line remain "0"
Strobe :             at last Bit set upper or lower Line from "0" to "1" while lower upper is still "0"
                           but then set the 2nd Line also to "1"
At end of sequence set both Lines back to "0"

The CLK of the Shift-Register is delayed by the OR-Gate. so that Data-In of Shift will be stable
If necessary 2nd OR/AND Gate (with 2nd Pin wired to GND or VCC) secures sufficient propagation Delay

If you use 74LS595 or CD4094, then the OR is wired to Shift-Clock and the AND to the Strobe-Clock
When using 74LS164 with 74LS273 then the OR clocks the 164 and the AND the LS273
Both circuits can handle 8-Bit, in drawing I was lazy and draw only first 3 Bit

Jo

[Toastie]

16 minutes ago, BrickTronic said:

Both circuits can handle 8-Bit, in drawing I was lazy and draw only first 3 Bit

Any chance, that we can meet in person? 

This is so cool. I need to digest this, order chips and put it together. Just for the fun of it!  

Thanks a million (again) for teaching these things. In your courses, I would be a student, reading your lips, even at 8 o'clock in the morning. I have to admit that I skipped 8 o'clock 1st year PChem classes regularly - now I teach these     

Yours
Thorsten

Edited July 10 by Toastie

[maehw]

Hi everyone, 

I am sorry but I can't keep up with the speed new posts arrive here. Great to see that everyone around is quite active and motivated what I've seen from my little email snippets. I am unfollowing this thread for a while and hopefully will get myself updated...

So long, and thanks for all the fish. 

[Toastie]

5 minutes ago, maehw said:

So long, and thanks for all the fish

Wait - any Vogons close by? I shall get my towel and 5 pints of stout (don't like lager that much) ready.

All the best
Thorsten

[alexGS]

2 hours ago, maehw said:

Hi everyone, 

I am sorry but I can't keep up with the speed new posts arrive here. Great to see that everyone around is quite active and motivated what I've seen from my little email snippets. I am unfollowing this thread for a while and hopefully will get myself updated...

So long, and thanks for all the fish. 

I think a subforum can’t come soon enough - Toastie’s post about the serial to parallel conversion is a fantastic post and deserves its own thread. I hope we’re able to move things out of here into separate threads soon. If that means I have to be the administrator to do so, then yes please… :)

[amine]

9 hours ago, alexGS said:

I think a subforum can’t come soon enough - Toastie’s post about the serial to parallel conversion is a fantastic post and deserves its own thread. I hope we’re able to move things out of here into separate threads soon. If that means I have to be the administrator to do so, then yes please… :)

I was in contact with the guy behind the 9760 manual blog.

He  announced me that he might be able to add the Piccoline legolines manual and the BBC micro 9760 to it's blog soon. 

 

Edited July 11 by amine