Hi, if your switching video use a 4066 cmos switch, driven straight from the output of the 84, discrete semis seems a little hard and you will still have 2 lines left, maybe foe switching monitors etc... Regards Bruce Kerr ----- Original Message ----- From: Gennette Bruce To: Sent: Friday, July 09, 1999 12:42 PM Subject: Multiplexing Questions > Hi all. > > I have a switching circuit in mind that will drive a MAX-4256 video buffer > as a previewing distribution amplifier (initially for use in a class room > that has several video sources and several monitors, but could become an 8 > input security monitoring system with manual over-ride later on). > > I don't want to stray from what I already know (16C84 only!) so I'm limiting > myself to 13 I/O pins. I need 3 for the serial interface in the 4256, > leaving only 10 free. > > I need to select one-of-eight inputs and display (with an LED) which input > is currently selected. This is pretty easy, isolate the inputs with diodes > and rapidly swap the port between input (from the switches) and output (to > the LEDs). But I need to do this twice, once for the video going out to the > class room monitors and once for the pre-view monitor. > > Using an '84 I think I have a solution - parallel the switches AFTER the > isolating diodes and toggle the power lines for each set of pull ups (just > before the isolating diodes). The cheapest way to toggle the power lines is > to use an output from PortA to switch both a PNP transistor (ON when port = > Lo) and a NPN transistor (ON when port = Hi), but I don't know if this will > work. Any help here will be appreciated. > > For the 2 sets of LEDs another pair of transistors (controlled from the same > PortA pin) can switch the 2 common cathode lines. > > So, here are the questions - > * Will the PNP & NPN trick actually multiplex 2 sets of inputs/outputs? > * Can a PortA pin drive 4 transistors at once? > * Do the transistors need a resistor at their base, or will the TTL level > from the PIC drive them directly? > * If the PortA pin is set to input can it be put into high z to turn both > transistors off? > * If both transistors can be turned off, could more sets of switches/LEDs be > multiplexed onto PortB by using a second PortA pin to switch another pair of > transistors? (4256s can be cascaded for more video inputs/outputs). > > Bye.