Electronic SPDT controls two PCs

Luis Angulo, Barakaldo, Spain -- EDN, 9/2/1999

The circuit in Figure 1 is a switch that allows you to use one VGA monitor, one mouse, and one keyboard with two PCs. The switch has four pushbuttons. One switches the monitor between the two PCs, a second switches the mouse, a third switches the keyboard, and a fourth connects all the devices to the PC that's inactive at the moment. The active portion of the circuit uses a PIC16F84 µC, IC₁, from Microchip Technology (www.microchip.com) and MAX395 (IC₂, IC₃, and IC₄) and MAX392 (IC₅) electronic switches from Maxim Integrated Systems (www.maxim.com). The µC is the core of the system; with a simple program, it checks the pushbuttons and selects the proper switches to implement the desired connection. The µC also drives three LEDs that indicate the PC each peripheral device connects to.

The circuit derives its power from the keyboard connector. If both PCs are initially off, and one switches on, the circuit connects all devices to that one. R₁ detects the switched-on PC and communicates the switched-on status to the RB2 input of IC₁. The circuit receives its power through D₁ and D₂. These diodes decrease the 5V supply by approximately 0.7V. The MAX682 step-up regulator, IC₆, restores the 5V for the more critical parts of the circuit. The MAX238, IC₇, provides TTL-to-RS-232C conversion and vice versa for the serial ports. The program initializes the µC's registers and connects the devices to one of the computers, depending on the level at the RB2 input. It then executes a loop, which ends when you press a pushbutton.

The system detects the newly pressed key and changes the corresponding bit or bits of a register, PC_SEL (PC Select) in the µC (Table 1). The routine then changes the signals, if necessary, to the MAX392, IC₅, for the keyboard selection, and issues a group of bits to configure the MAX395s for the monitor and mouse control. Finally, before returning to the main program loop, the software updates the LEDs, which indicate which PC each device connects to. (DI #2375).