300 and 1200 Baud AM79101 Modem
The AM7910 and AM79101 are great IC's to design a packet modem around. They are capable of 300 baud and 1200 baud operation, making them perfect for Amateur Radio use. The only special parts you probably do not have on hand apart from the IC, is the crystal which is readily available. These integrated circuits operate on a dual power supply of +5 and -5 Volts. Being crystal controlled, the tones will not drift out of tolerance, a potential problem with other modems that rely on resistor/capacitor values to achieve tone frequency. They are easy to build and worthwhile if you have a need for both baud rates.		(Right) A typical dual baud rate modem using the AM7910 or 79101, sold in the 80's for pre-PC computers.

AM79101 / AM7910 Modem Schematic With RS-232 Interface for PC

This modem schematic started out in the pre-PC era and since has been updated with an RS-232 interface (MAX 203). I used what parts I had. A MAX 232 will do and is cheaper. A relay for the PTT circuit was used because of my Icom IC-730 that pulled a solid half amp through the PTT circuit. Modern radios are not built that way and so you might be able to use less parts in your design. See Pictures below. Be sure to see the rest of this page for tips.

The minor difference between the AM7910 and AM79101
The difference between the two IC's are pin 6 and 7 of both chips. With the AMD79101 you must add a 910 ohm resistor in series with the .002uF Mylar capacitor that goes between pin 6 and 7. If the AMD7910 chip is being used, omit the 910 ohm resistor so that only the .002uF capacitor goes between pins 6 and 7 of the AMD7910. The schematic above reflects this minor change.

AM79101 / 7910 Modem Dual Power Supply Requirements
This will satisfy the needs of the modem schematic above. All of these parts, mainly the 7905, used to be available at Radio Shack but not anymore. Instead of having a dual supply, you can use a switched capacitor voltage converter IC like the MAXIM MAX660 to generate the -5 Volts from a +5 Volt source.

Homebrew AM79101 300/1200 Baud Packet Modem for the PC
Made as a prototype in the 90's, it has undergone several revisions inside and out over the years. The prototype worked so well that it was the only one built. See Schematic.      More Pictures

Programs for use with the AM7910 / AM79101 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 AM7910 / 79101 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 below 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 AM79101 / 7910 and Maxim Data Sheets

Download AM7910, AM79101, AM7911 Tech Manual

DOS programs for the TCM3105 modem

Want more information on TTL to RS-232 interfacing?

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Notice...
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