The TCM3105 Used in Amateur Radio

The TCM3105 is a single chip Frequency Shift Keying (FSK) modem. Even though it is capable of multiple baud rates and tone pairs, only the 1200 baud - 1200Hz and 2200Hz tone pair is useable for Amateur Radio. The 3105 was not designed for 300 baud operation unfortunately. The use of this IC remains popular, for hams anyway, since the mid 80's. It has found it's way into TNC's, and home brew modems. With a crystal and about a dozen other components, a TCM3105 modem can be built in a small package. Tuning up the modem for operation is simple. If using a PC, you will need to add a TTL to RS-232 interface of sorts. The logic level interface can be built in many ways. One of the easiest ways is to use a MAXIM MAX203. One chip and you have RS-232 levels.

(Right) This compact modem was designed by John Krohn / KJ4GP and Craig Raider / N4PLK. Later on, a Watch Dog Time Out Timer, was added by Sam Baine / W4KUM. With the TCM3105 being TTL it was perfect for the Commodore 64/128 and Digicom (the best) software. Download modem Schematic (For C=64/128)

TCM 3105 1200 Baud Modem Schematic With RS-232 for the PC
This schematic is designed for use with a PC serial port. The chart below will show you what pins to use if you are using a DB-25 or DB-9 serial port. The schematic and chart assume you are going to use DOS programs such as Baycom and Eskay Packet (SP) which have the same serial port pin out requirements.

Programs for use with the TCM3105 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 TCM3105 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 TCM3105 and Maxim Data Sheets

DOS programs for the TCM3105 modem

Want information on TTL to RS-232 interfacing?

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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!

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