The User Port

Abstract

The User Port on the BBC microcomputer (figure 2.1) is situated underneath the computer keyboard in the middle of a row of multi-way sockets. It is an 8-bit, two-way digital port. That is to say it can handle eight separate digital signals at any one time. These signals can be configured so that information is sent into the computer from external devices, or can be configured to send signals out from the computer to other digital devices. The User Port is connected inside the computer to a very complex chip, a 6522 Versatile Interface Adaptor (VIA) chip. The 6522 VIA not only handles the eight separate data lines of the User Port, but also controls the parallel Printer Port as well. It has two timers built into it. These can be used for timing very short time-intervals. The use of these is outside the scope of this book. There are two 6522 VIA chips inside the BBC machine, VIA A and VIA B. VIA A handles some functions from the Analogue Port, the keyboard input, and the serial input/output through the speech processor. VIA B handles the User and Printer Ports.