powerful, the internal modem was commercially impractical. Not only did it require special driver software that often meant it could only be used with a single terminal emulator, but a different hardware design was needed for every computer bus, including Apple II, S-100, TRS-80, and others. As modems became popular, users on these platforms began asking for designs as well. An obvious solution to the cross-platform connection was to use the RS-232 serial port instead of the internal data bus; modems were serial devices in the end, and most computers included an RS-232 port or some variant. The trick would be how to send commands over the same connection as the data. A few external modems already offered the ability to dial the phone by entering a phone number when the modem was first started, based on the idea that it could not be connected to a remote system when first powered up, so anything sent from the computer could (optionally) be interpreted as a command. The problem was sending a command to hang up, while the modem was already connected. There needed to be some way to indicate that the characters flowing out from the computer to the modem were not simply additional data to be sent to the far end, but commands to be acted on. Hayes and the company's marketing manager Glenn Sirkis approached Heatherington with an outline for a new command-driven external modem. Several solutions to the command problem were studied, and in the end, Heatherington decided the only practical one was to have the modem operate in two modes. In one, data mode, all data forwarded from the computer was modulated and sent over the connected telephone line as it was with any other modem. In the other, command mode, data forwarded from the computer would be interpreted as commands. In this way, the modem could be instructed by the computer to perform various operations, such as hang up the phone or dial a number. The modem would normally start-up in command mode. The problem was how to move from mode to mode. One option would be to signal this using one of the many pins in the RS-232 cable. However, while the 25-pin connector on the modem side had more than enough pins for this purpose (even some meant for this purpose), the computer side often had far fewer pins connected and controllable, if