View Full Version : MEK6800D1 Output Interface for 60mA Teletype Current Loop

March 28th, 2015, 03:05 PM
In 1979 I interfaced my MEK6800D1-based microcomputer to a Kleinschmidt AN/FGC-25 Teletypewriter set that I repaired and rebuilt in 1978. Over the past several months, off and on, I got the old teletype set back in operation after having been in basement storage for the past 20 years. The most significant problem I had was failure of two electrolytic capacitors in the selector magnet bias circuit for the printer. I posted a demonstration of the working printer here; http://www.zippcast.com/video/a722081e609549512fa


Below is a diagram of the computer interface that I made back in those days. The output transistor is a high voltage type for operating the 60mA current loop.


The M6800 uses the 1 millisecond timer for timing the mark/space signaling; 22 milliseconds for the start bit and five character bits, and 31 milliseconds for the minimum stop bit period.


This interface worked fine, but almost as soon as I got it working I abandoned my hobby activity when I went back to graduate school.

I recently developed a modified interface design that uses a diode bridge and an opto-isolator instead of the transistor, so that the computer and the teletype set are electrically separate. With the bridge, the current loop polarity does not matter. After I get the entire hookup going again, I plan to upgrade the interface with the bridge and opto-isolator.

I have completed the hardware design for an input interface for the current loop, also using an opto-isolator.

I plan to update this post as I make progress, just like DaveH and his 6809 Flex progress, which I am interested to read about.


April 11th, 2015, 05:52 AM
Demonstration video is at http://www.zippcast.com/video/31acf5aa666f99256a9
I use MacWrite on the Mac512K to create ASCII text file. Then I use MacTerminal 2.0 to send it to my M6800 microcomputer. The microcomputer utility program I wrote performs the ASCII-to-BAUDOT code conversion and drives the TTY current loop. I built the TTY output card in 1978. During the present effort I had to replace the 7406 chip and the 2N2222A transistor. Both had gone bad, presumably with age.

June 20th, 2015, 12:02 PM
I completed the modifications to the TTY interface card of my MC6800/MIKBUG microcomputer. I can now send data to and receive data from my Kleinschmidt AN/FGC-25 teletype set. Demonstration video is here http://www.zippcast.com/video/8a0f40e3806bff93b64

Here is the schematic of the upgraded interface card

Here is the card
This completes my project of the past year, to resurrect my old MC6800 computer, the Mac512K, and my teletype set, connect them together, and create a handy software development environment for MC6800 assembly code.

Can anyone suggest some other forum where teletype machines or MC6800 enthusiasts might congregate? I have not had much luck finding it. Maybe there aren't any.


June 21st, 2015, 01:17 AM
Impressive! Can't see the ear muffs anywhere though?

Not certain what the diode in the 2N2222 base is supposed to do. Stops the transistor turning cleanly off because back biased.

I haven't found anything to do with telex. I have a number of old Creed teleprinters to get running one day, a couple came from a museum that was getting rid of them so can't be much interest. There was a Strowger and telephone museum in mid-Wales somewhere, but not heard about it for years.

Do you have the manuals for the Kleinschmit printer and paper tape reader/punch? Presumably all mechanical like the Creed machines. Might be worth putting them on bitsavers if you have the scanning equipment.

Search for the 6809 as well, tend to be grouped together. Also the Exorciser development system of which there are quite a number of threads on this forum. Also SWTP (South West Technical Products) who made an early 6800 computer.

June 21st, 2015, 01:57 PM
Not certain what the diode in the 2N2222 base is supposed to do. Stops the transistor turning cleanly off because back biased.

Yeah, I'm not sure either, but seems to be working OK;
1. When the 7406 output is ON (low), the voltage across the diode and 2N2222 is less than 0.3 volts and the 2N2222 is OFF.

2. When the 7406 output is OFF (high) the voltage across the diode and 2N2222 is about 1.4 volts, and the 2N2222 is ON.

I measured these logic levels during testing.

Do you have the manuals for the Kleinschmidt printer and paper tape reader/punch?

Yes I have the manual, TM 11-2246 dated 26 June 1956. I acquired it with the set, back in 1977. There is quite a bit of electrical circuitry in the reperforator unit. The set is very capable, supplying up to two separate lines (external current loops) as well as a local loop. You can make tape locally while the printer is receiving messages from an external line.

Thank you for your interest! Hope you can get to your Creed sets soon!

June 25th, 2015, 03:01 PM
Well done, and nicely documented. Thanks!

June 25th, 2015, 03:35 PM
The diode drops the voltage at the base.

June 26th, 2015, 12:06 AM
Teletype folks meet on the "greenKeys" e-mail list....

June 26th, 2015, 06:11 AM
Ah, thanks, seen the Baudot.com site but not noticed the Greenkeys forum? Just registered so here goes.

When the 7406 is on, low, the diode isn't pulling the base low, just relying on leakage, remove the diode.

June 26th, 2015, 12:08 PM
I suspect that the diode is a left over from an earlier design. The ST-5 used to have a similar circuit but it used an OP-Amp buffer in front of the transistor which could pull the base negative.

June 28th, 2015, 02:21 AM
I whizzed off and joined the Greenkeys ...... thing? But what is it? I can't really find any posts, a forum, discussion, anything? Just get the heading page with no links to anything other than how to sign up, done that?

Anyone else a member? What is the secret?

Did start to get some posts about scientific experiments, not really my thing, turning of the global posting stopped them?


June 28th, 2015, 06:33 AM
its a mail list.... so stuff appears in your in-box...

July 2nd, 2015, 01:53 PM
If you want to read (or search) the mailing list archive, that is possible too: http://www.rtty.com/greenkeys.htm

July 3rd, 2015, 06:17 PM
Anyone else a member? What is the secret?

Sorry about the delay in responding. I only have internet access about 4 days out of every 14 days.

I discovered and joined the Greenkeys email forum shortly after my post to this thread a couple of weeks ago where I wondered if such folks were even around.

It is email based. You can find the thread I started in the June section of the archive. My inbox started to fill up quickly so before I lost internet access 10 days ago I turned off the emailing option. Today when I got back online I just checked the archive to see if any more contributions to my thread which starts at http://mailman.qth.net/pipermail/greenkeys/2015-June/033082.html

The Greenkeys forum is great. I found the community of TTY enthusiasts I was looking for. They are called CommCenter-1 group.

I plan to be putting up a new posting on GreenKeys later this weekend. It will be about using my MEK6800D1 as a distortion tester, of sorts, for the teletype set. The data that I took was very interesting, and showed that there appears to be a problem in the keyboard transmitter and in the tape reader transmitter that I need to track down. I will reference a report that I am going to upload to scribd.com sometime this weekend.

I think I understand about the diode, thanks! I will clip lead around it to see if it matters.


June 20th, 2017, 04:44 AM
Nice restoration on the Kleinschmidt. Do you have your video on a different host ? Zippcast.com just comes back not available.