Electronic – How to incorporate output and input impedances connected to the circuit

circuit analysisimpedanceinput-impedancepc

I'm designing a TTL to VGA DAC for a vintage PC, and I'm confused about the significance of the output and input impedances of my video card and monitor, respectively. The VGA spec states 75 ohms at both ends, if I understand it correctly. I couldn't find mention of an output impedance for the TTL video cards (I have an IBM CGA card and an IBM MDA card). My first inclination is to treat both the card and the monitor as 75 ohm equivalent resistors, and design accordingly. Specifically, my diagram would look like this:

schematic

simulate this circuit – Schematic created using CircuitLab

But on further consideration, it occurs to me that the video card is TTL, so there should be 5V on the signal lines, and I can just ignore its output impedance and treat it like a straight 0V/5V signal source. In that case, my diagram would be the same, except it would omit R1.

The contents of the solid box is just a resistor network, to combine the digital rgbi lines into an analog signal and drop the voltage down from 5V to .7V max, as per the VGA spec. No active components, transistors, diodes or other fancy stuff.

Am I thinking about this correctly, or am I going to set fire to something?

Best Answer

it occurs to me that the video card is TTL, so there should be 5V on the signal lines,

You cannot assume the outputs will go up to 5 V, only that they will meet the TTL minimum output levels of 2.4 V high and 0.8 V low, when loaded with TTL inputs. CGA and MDA cards often used bipolar TTL chips that pull up weakly to ~3.5 V, with exact values depending on the particular logic family and IC.

To guarantee a stable output voltage and impedance you should buffer the TTL signals with TTL compatible high speed CMOS Logic, preferably a line driver such as 74HCT244 which has strong output drive (need ~9 mA to drive 75 Ω to 0.7 V).

With normal cable length an exact impedance match is not required, but getting the correct level is. A single series resistor with value adjusted to get 0.7 V with 75 Ω termination should be sufficient. For a closer impedance match you could use an 'L' pad, like this:-

schematic

simulate this circuit – Schematic created using CircuitLab

Related Topic