Angled View of Home Brew AM79101 300 / 1200 Baud Modem
Housed in a spacious Radio Shack project box, I added Switches and L.E.D.'s to fill the front. It looks neat when all lit up. The switch labeled 'AMP' selects two different transmit audio levels. I needed a big current boost (not voltage) for my deaf HF radio while my 2 Meter Packet radio need a low 10mV of audio input. The baud switch selects 300 or 1200 baud operation.

Rear View of 79101 Modem
I just used what ever I had on hand for parts. I had to completely isolate the radio push-to-talk jack (first one on left) due to ground looping and the heavy PTT current from the radio. I mounted the PTT jack on Plexiglas. Years later the DB-25 was added for RS-232 communications.

Inside the Home Brew 79101 Prototype

While not pretty, it worked right the first time and still in use today. On the left is the power supply board for the +5V and -5V. If you design a 7910 / 79101 modem you may use a lot less parts than I needed to suit my equipment. It was originally TTL data I/O. That's all you needed with with a Commodore 64. When I got a IBM clone, I had to upgrade it with a MAXIM chip for RS-232. There's still room enough to make an active filter some day.

More on what you are seeing here

What you might need
If you are using a +5 Volt regulator, you will need a small heat sink, anything will do. I used a pressure fit type as seen in the picture. Ferrite beads are always good to have to keep RF away. I have some right on the DB-25 connector and up to the radio jacks as seen on the back panel. Bypass capacitors where applicable couldn't hurt either.

What you probably won't need
In the picture above there are several parts shown that you most likely won't be need in your design. Being this was the prototype, I made enhancements over time while I just left the old stuff (but still active) in the box. I used a 7404 Hex Inverter IC as a protection buffer between the AM79101 and TTL computer. It was cheaper to blow up a 7404 if something happened. You won't need the 7404 if you have a PC. You can rely on a MAX232, 202, or 203 to handle your interface needs and protection. You won't need the series of keying transistors to key the medium current relay I had to use for one of my old radios. One good transistor, turned on fully will do for modern radios. With that said, you most likely won't need the relay for PTT. And you probably won't need to have a toggle switch to boost audio amplification out of the op-amp as I had to, again for my old radio. I omitted the AMP switch from the schematic. The 79101 data sheet recommends analog and digital grounds. As you can see from the board and schematic, I only have one common ground. It worked for me.