Manager - AB7RG
Manager Notes

Use That Old TU Again. It’s fun!

Created by on 2004-01-19
Use that old TU again

Use that old TU again. It's fun!

Remember that old Terminal Unit you had so much fun using on RTTY and ASCII?

It was useful even in CW mode as a keyer. Alas that old computer you used with it died or was replaced by a modern computer with no TTL ports. So off it went to the Davy Jones boat anchor closet and forgotten or sold at the local Hamfest. You can still see them for sell at a Hamfest or on Ebay for $5 and up. You might spend more on shipping the thing than what you paid for it! Well did you know that you could build a simple little level converter and still use it? There are still lots of RTTY operators out their and its as popular a mode than ever. Some of the old Terminal Unit's were inexpensive for there day but they did work. Others were very expensive and high performance like all the HAL units and are still used today with HAL keyboards and NTSC video display to print the text. Well on to the project part of this article! Not only can you still use it for RTTY but the free software to use with it supports Amtor and Pactor and not just in FEC mode but the fully error correcting ARQ modes as well!

First off we need to understand how these units work before we can setup the little level converter to properly use it with a RS-232 serial port and a Terminal Unit. As an example I will use a Kantronics “The Interface” Terminal Unit to show how it's done as there all pretty much alike in the way there controlled and data received from them. I will also cover the two most popular software programs used with a PC computer, which are Hamcomm ver 3.1, and Terman93. Below is a drawing of the converter.

0x01 graphic

Analog audio to TTL level data is the most important job of the Terminal Unit and it's the most simple to convert to RS-232 serial level data. A single chip does the entire job for not only TTL to RS-232 data conversion for the software but also software RS-232 to TTL control of the Terminal Unit. The TU receives the audio from the receiver and converts it to a TTL level signal where high and low signals represent Mark and Space tones. It will make no difference whether Mark is low and space is high or the other way around as the software can be changed to handle it either way. Of note here is that the level converter chip will invert the TTL signal. If Mark is low and Space is high then on the RS-232 side Mark will be high and Space will be low. This is critical to remember when we get to PTT and FSK control! The TU demodulated signal is connected to pin 11 of U2. R1 and C7 prevent the high and low levels from jittering in the software. I discovered that it was a problem in Terman93 and Hamcomm. Adding the resistor and capacitor improved the copy of data in both programs using the Kantronics TU. If you use a different TU you might not require R1 and C7. Terman93 expects to receive external converter Mark and Space (Low/High) levels on DB-9 pin 1 and Hamcomm expects the same type of signal on pin 8. It works just fine to tie both these line together so no manual switching of these two lines is required when switching software programs. When you get to the point that you can hook it up and try it out on receive tune in a RTTY station and try to copy some of the text being sent. If it's looks like a bunch of garbage showing up on the screen simply switch to the other side band mode on your receiver and retune the station. It should now start properly decoding text. In voice modes USB is used on 20 meters and up and LSB on 40 meters and down. This is not the case with FSK modes! LSB is the normal standard AFSK generated mode on all HF bands. You can use USB if you reverse your tones in the software. Also do not expect all stations to use the correct tones on all bands. At this point if you can properly copy a RTTY station in either side band mode or using the software to invert the received text then your ready to move on to the next stage of the project.

Push to talk and tone control. This sounds simple but can cause hair loss pretty quickly! Between U2 doing that nasty little inverting trick and the requirements of the TU and the way Hamcomm is setup on the software side it can get very confusing very quickly! I lost at least 2 years on my receding hairline with the Kantronics TU! First of all Kantronics called the PTT line CW-ID and FSK control RTTY. Having these lines high or low between them produces four different combinations and no jumpers are available to switch them around. Terman93 will allow you to set these lines either high or low depending on these requirements but Hamcomm does not and as it's such a good and powerful program we would like to be able use it. So we will start with the software requirements on Hamcomm. In receive pin 7 (RTS/PTT) is low and pin 4 (DTR/FSK) is high. Connected to the level converter on the TTL side these are now inverted, RTS is high and DTR is low. Now we look at the requirements of the old Kantronics TU. Looking at the manual we find that for the TU to be in receive mode both CW-ID and RTTY must both be at high TTL level. Oops, We have a problem! If we leave it this way when Hamcomm is loaded the TU goes into transmit mode and generates the space tone. To solve the problem we must add a PNP transistor and two 10K resistors to bring DTR high when in receive mode. Below is a picture of the TTL inverting circuit. The PNP transistor is a 2N3906.

0x01 graphic

Now the FSK line is high in receive mode and we are ready to connect the converter to the Kantronics TU. In transmit mode with this setup the PTT or CW-ID line goes low and the FSK or RTTY line toggles high and low producing the mark and space tones with the software controlling these two lines. Remember, This is a worst-case conversion project! No onboard jumpers to switch settings and no straightforward PTT and FSK controls. The MFJ-1224 terminal unit (which they still sell) has jumpers and can be setup very easy. Other TU's may be easy or hard and the PTT and FSK lines may require a different setup than the Kantronics unit shown here. This article shows how things work with a real hair pulling setup so if you understand what is going on with this unit you can setup any other terminal unit you may find or have already. Also these things are not TNC's! A TNC is a computer with software stored in ROM chips. When you connect your PC computer to a TNC you use a simple terminal program to send commands to the TNC to tell it to do things for you and accept text for transmit and also receive text from the other station. The TNC is doing all the work. A terminal unit only does three things, convert audio tones to computer compatible levels, Produce the correct audio tones for the transceiver to send, and key the transmitter when required. The software you load on the computer turns the entire computer into a TNC with a built in terminal. Hamcomm and Terman93 are both DOS programs. These programs cannot be run from Windows. You must boot up in DOS and then run one or the other programs. There can't be any other programs running on the computer at the same time. It makes no difference how fast the computer is. These programs take total control of the CPU and timing. When running ARQ Amtor and Pactor modes any program accessing the CPU will cause timing errors and they will not work correctly. Computer requirements are very low in today's world, a 386 is plenty fast! Even a 286 should be able to handle these two programs. A trip to the thrift store should get you a computer suitable for use with your terminal in the $20 to $40 dollar range including a working monitor. Terman93 is a small program that can fit on a single floppy DOS boot disk so it can be run on an old floppy disk only Laptop computer even with old 720 KB disk drives. Hamcomm is a large program and must be run from a hard drive.

Here is the completed “The Interface” to PC RS-232 drawing.

0x01 graphic

When I converted the Kantronics TU I mounted the converter inside the case and ran the serial cable out the hole where the old TTL computer cable would have been connected. I soldered jumper wires from the converter board to the proper points on the TU board. I also connected the power input for the converter to the TU power switch so turning on the TU will also supply power to the converter and both the TU board and converter board share the power jack on the back. A cheap 300 mA DC power supply will supply plenty of power for both the TU board and the converter. U1 is a 7805 5-volt positive voltage regulator. U2 is a MAX 202 or LTC1383 chip. All capacitors should be rated at least 16 volts.

One glitch I found when setting the AFSK level was a hum in the transmit audio. I checked my cable between the TU and the transceiver and found no problems with the shield or soldered connections. I then removed the 115 to 6 volt power supply and connected a 12-volt gel-cell battery to the TU. The hum was gone! I suspected the 4.7 uF electrolytic capacitor in the power regulator circuit had dried out after so many years (1982 vintage). I removed it and installed a new 47-uF capacitor and tried transmitting into the dummy load again. This time the AFSK signal was clean and I had the level set just right with the ALC meter just moving off 0 just a tiny bit and had full power output.

So how do I like the old TU? It's fun! It has a built in tuning indicators and a 180 Hz active bandwidth filter so it's great in crowded conditions. I got it on the air and found Randy WB9WJK who helped me get it set up. I started out using the Terman93 software and found that copy was correct in USB mode as I am using the AFSK generator in the TU. After calling CQ a few times Randy was very helpful in letting me know that my transmit tones were reversed. I was split! Receiving upside down and transmitting correct. I shut down the program and brought up a text edit program and changed the allmode.ini software file command ComDCDInv=F to ComDCDInv=T which would fix the problem. I reloaded the program and switched to LSB mode and called Randy and he confirmed that everything was correct now. As I had the top off from having set the AFSK level I sent a thank you to Randy and shutdown so I could finish up the project. I screwed everything down including the cover and cleaned up. I then took care of some XYL work orders and took a supper break. Coming back to the radio room I found 20 meters to be dead so I switched to 40 meters and thought that this would be a real test of the old TU with all the noise and QRM on that band. I found Dave WB8P calling CQ so I returned with my call and Dave answered me right away! Dave gave me a good signal report and confirmed that the tones were clean and everything was working great. The built in 180 Hz filter was doing a splendid job for receive on my TU. We had a long talk and a great time chewing the rag. The only other thing I must do now is to set the timing correction for the computer I am going to use the Kantronics terminal unit with. It's an old 486. I plan on using ARQ Amtor and Pactor 1 with it and must set the proper timing correction so it will operate properly on those modes.

I would like to send a big Thank You to Chris KC0ATC for sending me the Kantronics “The Interface” terminal unit. It's just starting a second life! I would also like to Thank John KC4KLM and Sam W4KUM at The posted information on TTL to RS-232 and the information on AEA CP-1 and CP-100 really helped get the Kantronics TU on the air. Here are links to the Terman93 and Hamcomm 3.1 software. this link will start the download of this program. this link takes you to the Unofficial Hamcomm Homepage where you can download this program.

WA9HCZ 2004-03-09
RE: Use That Old TU Again. It’s fun!
No ARQ with sound cards comment. ARQ is very touchy with timing so I think the comment on the common soundcards not working in ARQ is correct, so dont hold your breath. There are a couple of AMTOR DOS programs that will work with the TU, so dont give the TU away. TOR308 works with Baudot, ASCII, and AMTOR through a common TU.

The EVM 56000 works with the EVMPAC14 software program in AMTOR and PACTOR. But the EVM 56000
is not a plug and play device so is not suitable for all.

I ran a head to head test with a TU, MMTTY, BTL154E, Multipsk, and MIXW with 850 hz shift 100 wpm baudot.
There were 1150 characters and spaces in the test and they finished as follows:
TU 3 errors
MMTTY 28 errors
BTL154E 140 errors
Multipsk 237 errors
MIXW 739 errors

Hard for me to agree that a soundcard will replace a TU
Yes, you can tell me you get perfect copy with the soundcard under perfect conditions, but wait till the band falls apart and see who wins.

Remember some TU's use AM detection and threshold correction. This is recognized as the most desirable method of FSK detection on HF (ST-6 etc). The common limiter/two coil units will not do as well. The PLL usually is not a high performance design. If in doubt run them head to head with poor signals if you wish to evaluate them yourself.

Jerome, WA9HCZ
K1CJS 2004-01-26
Use That Old TU Again. It’s fun!
Everybody who has gotten a newer computer with no serial or parallel ports available on it, there are add-ons called Port Replicators available that will give the newer machines the port needed for use with your old TNC.

The one I have is from Dell and works just fine.
N4ZOU 2004-01-23
Use That Old TU Again. It’s fun!
I got a ton of e-mail requesting setting up a TTL level TU like the one above for use with WF1B contest software. Well I got it working today! Pretty simple really. You must use diffrent serial port pins than used with Hamcomm and Terman93. RD is connected to Demod out. TD is a FSK control signal for the RTTY circuit. RTS is used for the CW-ID or PTT circuit. I also added a PN2222A transistor, 1N914 diode, and 1K resistor to the TD serial pin so that the TU could be run with the radio in RTTY or DATA mode and the software keying the transceiver FSK line. This is done so the narrow filters in RTTY of DATA mode may be used. I will upload a new Article about it soon and then the wait for to post it. Also a simple Hamcomm or Volksrtty modem will NOT work!
WB3CTC 2004-01-21
Use That Old TU Again. It’s fun!
Mechanical RTTY was always my favorite mode.
Had a model 15 on the Harrisburg Pa. rtty repeater.
that was great fun. Then packet came along.
In the mid 90's had the bright idea of setting up
a mod 28ASR someone gave me on HF. Using my T-368
with a TMC rtty exciter for transmitt and a R 390-a
ST-6000 for recive. Worked great wonderful station.
But by that time 99%of the stations on were using computers. And never sent a carrage return or
line feed. The printer would go to the end of the line
and make a black block. Or I could put in the cr-lf
locally. That would more often then not tear the paper
when the hammer hit when the carrage was halfway back.
Would plead with people to put cr-lf in at the end of line but they didn't know what I was talking about.
Oh well it was fun getting it up and running.
But about 10 to 15 years to late.
K5CEY 2004-01-21
RE: Use That Old TU Again. It’s fun!
When I saw the title of your article, I thought perhaps that you might be referring to a TU that I built up in 1969. It uses a couple of 12AT7's and a 5763 to drive the selector magnets in an old WU teleprinter that sits on the floor and looks like one of those old green, rounded top postal mail drop boxes.
It's set up for 850 cycle shift (2125~ and 2975~) and 60 WPM. I used to copy commercial text (a lot of it comming out of South America) using my National NC 240D receiver on H.F. A fast way to use up a roll of paper. Especially when they would keep hitting the line feed at the end of a message!
Never did rig it to my transmitter. I still have the old girl and fire it up on local loop occasionly just to watch it work.
There's something mystical about an old clatter box jumping around like an old washing machine.
John K5CEY
N4ZOU 2004-01-20
RE: Use That Old TU Again. It’s fun!
The problem with using modern sound cards with ARQ modes is the new DSP chip sets have a delay in the internal processing of the audio. It makes no diffrence which operating system you use or how fast the computer. You can see this for yourself if you connect both a TNC and a sound card up with a free program called Winwarbler. This is a very good program as you get dual copy from both the TNC and the sound card with an interface that splits the audio from the transceiver and feeds it to both the TNC audio input and the sound card line in jack. The TNC will put the decoded text on the screen a few moments ahead of the sound card. It's not much but for Amtor and Pactor in ARQ it will prevent proper operation. Back a few years ago there were PSA type sound cards available and they would do ARQ Amtor and Pactor 1 modes. There are programs available for doing ARQ and other modes using these type sound cards and are still available and free. The only problem is that not very many of the PSA sound cards were produced. They cost more than a new Pk-232MBX or KAM at the time they were being made. An old Terminal Unit will do ARQ Amtor and Pactor using Terman93 as the old Terminal units used analog audio to computer level data conversion with almost no time delay. Hamcomm will do ARQ Amtor but not Pactor. Also almost all the OLD terminal units do have a loop control jack for hooking up the old mechanical RTTY or Green key machines, even the old Kantronics "The Interface" has one! I would love to have one in my shack! The ARQ modes are far from being dead. If you havent noticed the upper HF bands are really getting in poor condition. When you move down to 40 meters with the high noise level and DX SSB stations in our digital part of the band you find that ARQ Amtor will allow you 100 percent copy even when a DX SSB station gets tired of hearing your digital noise and puts a carrier on top of you trying to run you off "his" frequency. ARQ Amtor will simply wait him out and continue sending text when he gives up. If you listen around the bands now you will find more stations using ARQ modes now. I spent last Saturday making ARQ Amtor contacts with 9 diffrent stations during the day along with many RTTY contacts.
AD7DB 2004-01-20
RE: Use That Old TU Again. It’s fun!
I've still got a PK-232MBX, I guess I'll have to dust it off and see what it can still do!

As I understand it, the sound card interfaces can't do ARQ AMTOR. They can do FEC AMTOR though. I don't know all the reasons for it. The gripping hand of course is that everyone's gone to PSK31, leaving AMTOR nearly extinct.
KE5GK 2004-01-20
Use That Old TU Again. It’s fun!
Thanks for helping me to remember why I got rid of my $400 + TNC for $50 at a hamfest.

Nothing wrong with finding new ways to use old gear. But I guess I am stuck on using the sound card and a ton of free software.
WB8YMV 2004-01-20
Use That Old TU Again. It’s fun!
a nice addition to the circuit would be a level converter to also interface a 20ma loop for hard copy print out on a model 19 or model 28. Nothing like the smell of teletype oil and the clatter of all those moving parts! Its sort of like the "Harley" of RTTY equipment.
N5NW 2004-01-19
RE: Use That Old TU Again. It’s fun!
Slight correction -- sound cards will not do ARQ (full error correction). AMTOR and PacTOR are designed to be error-free, and require (at this point) a TNC to make them work.

The MARS programs use AMTOR and PacTOR for their HF mailbox systems. The TNC is a requirement. I'm looking forward to the day there is some Linux software that can run the sound card and work around the timing issues in Windows that prevent ARQ modes from being feasable, but until then the trusty MFJ-1278 and AEA PK-232MBX continue to get a daily workout.

de N5NW (Marty)
Bellbrook, Ohio
NNN0RBB (Navy-Marine Corps MARS)
AE7I 2004-01-19
RE: Use That Old TU Again. It’s fun!
Thanks for the nice article. I also have a PK232MBX down in the basement and was thinking about getting it hooked up for AMTOR, etc. I'll keep your article handy in case I need to build any additional interfaces down the road.
OLDFART13 2004-01-19
RE: Use That Old TU Again. It’s fun!
ARRL to Propose New Entry-Level License, Code-Free HF Access:
OBSERVER11 2004-01-19
Use That Old TU Again. It’s fun!
OK.... but now you can do all that and more with your sound card... and a whole lot easier.

I used to use my PK232MBX with a Brother EP-44 typewriter or a TI Silent 700, or a Teletype M43, or a DECwriter LA36. I have sold or given away all the dumb terminals but I still have the PK232MBX - somewhere.

K0BG 2004-01-19
Use That Old TU Again. It?s fun!
I guess I should have kept the thing instead of giving it away. Good article.

Alan, KØBG