it even used a full 25-pin connector at all. In fact, there were very few pins that were guaranteed to work on all computers, mostly the data in and out, "ready" indications that said whether the modem or computer was operational, and sometimes flow-control pins. While it would have been possible to use some of these pins for the sort of command-switching they needed, this may not have been universally supported across all machines. Heatherington instead came up with the idea of using a rarely seen sequence of characters for this duty. Since these characters could be sent to the modem using the same two data pins that the port would need anyway, they could be sure that such a system would work on every computer. The sequence he decided on was (three plus signs). When this was received from the computer, the modem would switch from data to command mode. Of course, it was possible that the computer would send this sequence for other reasons, for example, the sequence is contained within the text on this page, and likely would be in any document referring to modems. In order to filter out these "accidental" sequences, Heatherington's design only switched to command mode if the sequence was led and followed by a one-second pause, the guard time, in which no other data was sent. In this case it could be safely assumed that the sequence was being sent deliberately by a user, as opposed to being buried in the middle of a data stream. With the basic idea outlined, Hayes and Sirkis gave Heatherington the go-ahead to build a prototype by adding a microcontroller to an otherwise lightly modified version of their existing 300 bit/s hardware. Sirkis was particularly interested in using the 1 MHz PIC microcontrollers, which were available for only US$1 a piece. After six months of trying to get the modem working with the PIC, Heatherington gave up and demanded they use the 8 MHz Zilog Z8 instead, a US$10 part. Sirkis acquiesced, and a working prototype was soon complete. Hayes added a requirement of his own, that the modem be able to automatically detect what speed the computer's serial port was set to when first powered on. This was not simple unless the modem "knew" what data were initially being sent, allowing it to time the bits and thereby guess the speed. Heatherington eventually suggested the