Real RS232 For Laptop/Notebook PCs

Many Metex DVMs feature a serial interface (RS232) which enables measured values to be copied to a PC for processing. Although this works just fine with most desktop PCs, problems may arise — as the author found out the hard way — when a laptop PC like the IBM Thinkpad 370C is hooked up to the Metex DVM. The cause of the problems is the limited voltage swing of just ±5V on the 370C’s serial interface. This is simply not enough for the Metex DVM, which will appear ‘deaf’ to the laptop.

Two Keyboards on one PC

This circuit does the exact opposite of what most of this type of changeover switches attempt to do. Usually a changeover switch is used with two PCs and one keyboard. This version however makes it possible to operate one PC with two keyboards.

K1 is connected to the PC for this purpose while K2 and K3 are connected to two keyboards. The data outputs of the keyboards are high in the idle state. As soon as a key is pressed the keyboard will serially transmit the data. The data line will now also be low from time to time.