Alternate, cheaper, simpler designs?
The SIO2PC hardware is pretty simple with just one chip and a handful of other components. But it can be made simpler and cheaper. The reason this is possible is that an RS-232 input will usually recoginze TTL voltage levels as two discrete logic levels. The problem does remain that the higher (more positive) voltage represents logic 1 in TTL logic and logic 0 in RS-232 logic. So it will always be necessary to invert the levels. But TTL inverters are common and cheap. My first design used a 7404 hex inverter. You can run the line from the TTL source (Atari) thru one of these and straight into the PC’s serial port and it will usually work OK. There’s a slight problem in the other direction, though. Both voltage levels of RS-232 are potentially damaging to a TTL input, so it’s necessary to clamp the positive level to 5 volts or less and block the negative voltage such that the TTL gate sees about 0 volts instead. Both of these actions are simply accomplished with a zener diode plus maybe
