The XR2206 and XR2211 are a versatile pair when it comes to modem building. Their advantage is that they may be set to what ever Mark and Space tones you like, creating the tone shift desired. On this page are two schematics for XR chip modems. One is suited for RTTY and 'TOR modes. The other one is for a 300 and 1200 baud packet modem. Detailed instructions for the XR-2206 and XR-2211 tone alignment follow the schematics. After alignment comes the discussion of how to wire up the modem to a serial port for use with a PC. And last, what popular programs work with the XR chip modem. Links to data sheets, for the IC's and where to buy them are found at the bottom of this page.

XR Chip Modem for RTTY and 'TOR Modes With RS-232 for the PC
The schematic is designed for RTTY and the 'TOR modes. It has somewhat of a RTTY squelch. The RTTY squelch action is handled by pin 6 being tied to pin 7. When there is no valid (locked) data pin 6 forces the data out line (pin 7) low. In case your contact replies to you "upside-down" just flip the data polarity switch. For tone alignment procedures and wiring for a PC see the rest of this page.

A note on the push-to-talk circuit
Like the schematic says: "DESIGN PTT AS REQUIRED", means just that. Push-to-talk varies from radio to radio. Some require a positive keying on the PTT. Some require grounding the PTT. And some older radios pull a lot of current through the PTT circuit in the radio, where a single keying transistor may not handle the current. A prime example of this is my ICOM IC-730. The push-to-talk line actually switches an 8 volt power supply to power up the transmit section of the radio and pulls 500mA of current through the PTT line! I opted for using a transistor to key a relay. At the time, not knowing how much current was being pulled, I used a reed relay. That was a mistake. The contacts welded shut and I was stuck in transmit. So for those older rigs out there I would recommend a transistor driving a relay for your push-to-talk line as shown in the schematic. If you have a modern radio, you can eliminate the relay and maybe a transistor as well.

XR Chip Modem for 300 Baud and 1200 Baud Packet With RS-232 for a PC

The first modem I ever built was based on this design with the exception of the RS-232 circuit. I used it for years with my C=64. One good thing about packet is that there is no data polarity to go wrong when communicating with the computer. This schematic does not have the RTTY squelch as it is not needed for packet. See the rest of this page for details on tone alignment and connecting the modem to a PC.

Assembly notes
Being the XR IC's are not crystal controlled, the tones may drift about over time. To reduce the chance of drift, use high stability components in the timing, loop detect, and comparator circuits.

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Tools for tuning up your XR chip modem
Back in 1988 when I built my first modem I did not have a tone generator. I had to do a lot of tuning for the XR-2211 decoder IC right over the air and assumed my radio was on the same freqency as the other guy. (Big asumptiuon!) It was a horrible way to go. Aligning hte XR-2206 tone Generator IC was just as difficult. But today, even if you do not have a frequency counter and tone generator, but you do have a PC, you probably have all the tools you need to get both IC's aligned quickly and correctly. More on this later.

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What tones to use for Mark and Space
Traditionally 2125 Hertz 'Mark' and 2295 Hertz 'Space' tones were used to obtain the 170 Hertz (cycle) shift. That is what I set all my modems for. The AEA CP-1 and CP-100, and HAL, and many other modems use these tones as well. But then along came the TNC. They use a different tone pair and shift only because the modem chips (are not XR chips) TNC manufacturers use are designed for land line use. These IC's have predefined tone pairs and shifts and referenced from a crystal. A typical tone pair and shift these IC's use are 2025 Hertz (mark) and 2225 Hertz (space) to obtain a 200 Hertz shift. There is no problem with copying a station with a different tone pair using a XR2211 decoder IC because it only detects one tone. The only difference would be on your VFO frequency readout.

1200 baud packet tones have always been the same. 1200 Hertz for mark and 2200 Hertz for space to obtain a 1,000 Hertz (cycle) shift.

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Alignment tools needed for the XR2206 dual tone encoder
There are two general ways to get your XR2206 tuned up. One is by using a stable tone generator with speaker attached, and zero beating it against the tone coming out of the XR2206 via another speaker. You will probably need to use an amplified speaker or signal tracer to pick up the tone from the XR2206. The other way to tune up the XR2206 is to use a frequency counter that works in the audio range.

Using a tone generator
The tone generator route will work unless you have a drifty tone generator, lacking an oscilloscope, or bad hearing! What if you don't have a tone generator or your generator is way to drifty? You have a computer right? If you have access to a Windows based PC with a sound card, then you have a tone generator simply by downloading WaveTools. Not only does it have a tone generator, it also has a dual trace oscilloscope, spectrum analyzer, and VU meter!

Another tone generator (DOS only)
If all you have is a DOS computer with a real PC speaker inside (not a Piezo element) you can download this small DOS PC speaker tone generator program and you have your self a tone generator! No sound card is needed as it uses the little speaker inside of your computer. The program is self explanatory. It can be used to align any modem that is adjustable such as an AEA CP-1 and CP-100, HAL, etc. There are two files to this program. 'bbl150.exe' and 'tonegen.exe'. 'tonegen.exe' is what starts the program. Both files have been zipped with PKZip204G (DOS). The zipped file is called tg.zip. Most windows based programs will also handle the zip files such as WinZip. Additionally, this tone generator program for DOS, will only work in DOS, and not through a Windows DOS box.

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Adjustment procedure for the XR2206 dual tone generator IC

Adjusting the mark tone
If you are using a tone generator, connect a speaker to the tone generator. You will be zero-beating the tone out of the XR2206 against the tone generator by ear. Not everyone has an Oscilloscope, but everyone has a speaker or two laying around, so I figured that I would explain the procedure in this way. The instructions below refer to the old 170 Hz shift. If you wish to align your modem for 200 Hz shift with the 200 Hz tone pairs, you can do that as well.

Set the tone generator to the mark tone of 2125 Hertz. This frequency is for 300 baud data or slower. If for 1200 baud packet, set the mark tone frequency to 1200 Hertz.

Connect a speaker, amplified speaker, or signal tracer to the AFSK output (on pin 2) of the XR2206. Adjust volume by using the potentiometer on pin 3 of the XR2206 so you can hear the tone.

(Referring to schematic above) Remove the connection between pin 9 of the MAX202 IC and pin 9 of the XR2206. Now ground pin 9 of the XR-2206. By doing so you have selected to generate what will become the mark tone.

If you have made the XR chip modem (like in the schematic above) that is selectable for 300 and 1200 baud packet, make sure the tones select (baud) switch is in the right position for the appropriate tones being tuned.

After turning the mark potentiometer fully counterclockwise (starting point), begin turning the potentiometer clockwise while listening to the tone coming from the XR2206 IC and your tone generator. You want to zero beat what you hear from the 2206 and tone generator. (zero beat = phasing between the tone sources stops. When you actually hear the tones oscillating between each source, you are close.)

To set the space tone
It's the same as above except for two things. Change your tone generator frequency to the space tone of 2295 Hertz. If you are tuning for 1200 baud set the tone generator for the space tone of 2200 Hertz. Be sure to un-ground pin 9 of the XR2206. Put pin 9 to source (+V) using a 10k resistor. You should hear the tone change coming from the XR2206 IC. Now adjust the space potentiometer until you are able to zero beat the space tone from your generator with the space tone from the XR2206.

You're done. Don't forget to remove the 10k resistor on Pin 9 of the XR2206. (Referring to schematic above) Reconnect pin 9 of the XR-2206 to pin 9 of the MAX202 IC. Reset the audio output potentiometer on the XR-2206 to meet the level required to properly drive the microphone input of your transceiver.

XR2206 alignment using a frequency counter
CAUTION: You need to protect your frequency counter. Some counters are super sensitive and can not handle much on the input. Use a .1 microFarad capacitor in series with a 100K resistor to make a probe for your counter. The XR2206 can produce up to about a 100mV signal.

Make sure your frequency counter is able to read well down into the audio frequency range. If not, you will have to use the tone generator method instead as described above.

Basically follow the directions for using a tone generator except you will only connect your frequency counter probe to the AFSK output (pin 2) of the XR2206 instead of a speaker.

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Alignment for the XR2211 tone decoder
Alignment of the XR2211 like the XR2206, is quite easy these days. One of the easiest ways is to use a tone generator program if you have a Windows based computer and a sound card. The traditional way is to actually use a stable tone generator. Usually it takes a frequency counter to check the tone generator. Many folks do not have a tone generator or frequency counter, so the PC tone generator is the way to go. Once the tone generator or the computers' sound card with WaveTools program is set up properly, the method of adjustment for the XR2211 is the same for each method..

Only one tone to adjust
The XR2211 is a single tone decoder. Therefore it can not detect both mark and space tones. When a mark tone is not detected, the logic in the XR2211 changes states and reports the data as a space tone. The software does the rest. The XR2211, as simple as it is, works really great. In reference to the schematic above, the lock L.E.D. will light up on the mark tone only when you have adjusted the XR2211 for 2125 Hertz. The lock L.E.D. is a fair indicator for tuning, but it does not give the best response for accurate alignment of the XR2211. You will need to connect a Volt meter or an oscilloscope to pin 7 of the XR2211 to see what's really going on. Make sure the 10 turn VCO potentiometer is fully counterclockwise. You may want to verify that it is so with an ohm meter. Some times you can hear the 10 turn potentiometer faintly 'tick' when they are at the end of their adjustment range.

Connect your stable tone generator, whether it be a real tone generator or sound card tone generator program, to the input of the XR2211 (receiver audio in). Be absolutely sure you are using a very VERY low level audio input (just a few millivolts) to the XR2211 as it is quite sensitive. Too high of an audio level may damage the IC. The lower the audio level the XR2211 can attempt to lock onto the better the alignment will be. Use a method of attenuation if need be. Set your tone generator to 2125 Hertz. (mark tone) If you are aligning your modem for 1200 baud tones, set your generator for mark tone of 1200 Hertz.

Apply power to your XR modem. Measuring pin 7 on the XR2211 should read near 0 Volts. Now turn the 10 turn potentiometer clockwise until the Volt meter reads as close to source voltage as possible which is approximately 12 Volts. The lock L.E.D. should be light solid. This indicates the XR2211 is in 'lock' on the mark tone. Some how make a note of where the potentiometer is in it's travel. Now slowly turn the potentiometer in the same clockwise direction until the Volt meter reads approximately 0 Volts again. The lock L.E.D. will not be lit. The XR2211 has now gone 'out of lock' and the logic has changed states. Now that you have found the general area you should be in, turn the potentiometer counter-clockwise so that you find the voltage that is as close to source as possible again. Peak for maximum voltage. Once you have done this, try to lower the amount of audio input to the XR2211 and check alignment again. That's it. You're tuned up. And the lock L.E.D. for the XR2211 should be lit up solid. Remember the XR-2211 is sensitive, so very little audio is needed from your radio, it can pick up on a level that you probably can not even hear yourself!

If the XR2211 or XR2206 will not tune up
Occasionally you will get an IC that is way out of range and you simply run out of adjustment range on the potentiometer. It's happened to me before. This is easily correctable. Change the value of the resistor that is in series with the potentiometer for the tone you are having trouble with. Try adding or subtracting about 2.2K to 4.7K of the existing value, and then attempt alignment again.

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Interfacing the XR chip modem with a PC
If you plan to use your modem with DOS modem programs such as BMK Multy, Terman (AN-93), SP (Eskay Packet), BayCom, or HamComm, you may take advantage of the table below. Some of these programs use the same pin outs. The program Terman ( AN-93 ), deviates just a bit and will use a pin that the other programs do not. As you will see in the chart below, some pins have been tied together. This has worked for me on every program mentioned here with no problems.

In reference to the XR chip modem schematics above, you will note on the MAXIM RS-232 interface chips there are three lines labeled: To Computer RX, To Computer TX, and To Computer PTT. Use the chart below showing what data lines from the modem are to be connected to a DB-25 Connector. After you wire up the DB-25 connector, you may use any standard (shielded would be nice) serial cable.

Programs for use with the XR chip modem

• BayCom - Packet - Free
• Eskay Packet ( SP ) - Free
• HamComm - RTTY, AMTOR, CW - Free
• Terman ( AN-93 ) - RTTY, AMTOR, PACTOR - Free
• BMK MULTY - RTTY, AMTOR, PACTOR, CW, FAX - Expensive, Y2K bug!

And there are other programs out there. Note: Baycom and SP are packet programs. Both programs are capable of 300 and 1200 baud operation.

Why are there no Windows based modem programs for the XR chip modem?
There are serial port timing issues with the non-syncronized data arriving from the modem. There have been modem programs for Windows in the past but never worked right. The DOS programs do not use the TXD and RXD lines of the serial port for data which cause the timing problem. Instead other serial data lines that do not have to be synchronized as such are used to send and receive the data to and from the modem. The chart above shows where these DOS modem programs expect to see data on a PC serial port. Luckily, they are all the same serial pins for each program!

What to do if you have a non-DOS system
Windows 95 and 98 are DOS based and you can exit out of windows to DOS or just boot to DOS to use a DOS modem program. However Windows XP, NT, and Linux are not DOS based. So what to do? Dust off your old DOS boot disk, and copy the modem program onto it. I have a boot disk that has several modem programs on it with disk space left over. Pop the disk in the floppy drive and turn the computer on. You now have a DOS system.

Technical Data

Download XR2211 and XR2206 data sheet

Download MAXIM MAX202, MAX203 data sheet

Where to purchase the IC's

Download DOS only tone generator program

Download WaveTools tone generator and more, for Windows

Simple XR2211 decoder for stuff like EMWIN

DOS programs for the XR chip modem

Notice...
You are encouraged to check all of the pages on this web site for the information you seek, starting at the main page. Each page is possibly related to another. The information on each page is "as-is", try at your own risk, and so on. We can help you out some what, as time permits. Try to stay on topic. Please, do not ask to "engineer" or "re-engineer" a design to suit your special project. We do not sell parts or kits of any kind. All said, we are open to any suggestions on adding more material, as long as it is on topic with a particular web page of the Technicothica web site. An example of things that are not "on topic" are: Support for APRS, EMWIN, TNC's and sound card stuff.    ~Thanks!