by Harry Lythall

I have recently done quite a lot of work on RTTY, using the HamComm program and been so impressed that I made a PCB to support the program. I think that I will continue to use the software, so I have registered it with the programs author: W. F. Schroeder, DL5YEC has obviosly put so much work into it. The program will send and receive RTTY up to 300 bauds as well as sending and receiving CW. I am very impressed with the programs ability to resolve and display signals even in the presence of high levels of background noise, although I do give the program a fighting chance by using a 250Hz CW filter. The V3.1 HamComm program also has an on-screen tuning indicator, and will even decode weather information. I have not yet tried the weather or even the ASCII modes but my time has been a little limited in the shack.

There are many programs available for monitoring other modes all using the same simple modem/interface circuit. If you take a peek at my download page you will see some recent additions to the software list, all using the HamComm interface, denoted by the text "HC interface".

The program only needs a simple "infinite gain" circuit to square off the input tones so that they can be read by the serial port. This is achieved using a 741 operational amplifier. The basic circuit of the unit is given below. The RS232 pins are given for a 9-pin serial port. A 25-pin port is given in brackets.

I have taken the liberty of modifying DL5YEC's design a little by not using the two back-to-back diodes in the TX AF path. I found they created a square-wave into the microphone input of the rig. Without them you have a triangular-wave with the tops a bit rounded, more like a sine-wave!! I have also increased the value of the resistors in the AF filter to reduce the level a bit more. Even so there is more than enough AF drive for most transmitters. Switch OFF any audio compression and adjust your rig AF-GAIN control for the correct output power. I run the rig at 50% power since, in RTTY mode, the rig is delivering 100% output which can cause transmitters to overheat.

My prototype PCB is shown in the two photographs below but I have now tidied it up a bit. You will notice that it bears some resemblance to the Baycom PCB. The same interface cables and pin-outs are used so you can literaly substitute your Baycom modem for the HamComm interface. Convenient, uh??

Ok, so now have I convinced you that you too should get active on RTTY? Then it only remains to download the HAMCOMM program and give it a try. Note that this is NOT freeware. You may download and use it, but if you decide you want to continue using it beyond the 30-day trial period, then please be fair and send DL5YEC his registration fee. He has spent several years developing it and deserves something for all his efforts. My registered copy came within a week of sending him my donation.

If you would like to have my printed circuit board for yourself, then visit my kits'n things page. I do have some surplus PCBs complete with the all the board-mounted components including the two connectors for which the board was designed (same connectors as used on the BAYCOM board).

Have fun, de HARRY, Lunda, Sweden.

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