My Projects

Published 31 Mar 2021

TTL Displays


Early microcomputers used CRT displays driven by TTL level control signals. There was monochrome TTL, monochrome Hercules TTL (different sync frequencies, with bit mapped graphics), and CGA/EGA compatible TTL.

TTL from VGA port

Driving TTL from VGA from a RaspberryPi 3 1):

Wire VGA green to the video input and let the monitor chop off the values on it's own (it's a TTL display).

Wiring diagram:

HD15 6 → DE9 1 HD15 2 → DE9 7 HD15 13 → DE9 8 HD15 14 → DE9 9

Video settings from /boot/config.txt:

# hercules/ibm5155 crt hdmi_ignore_edid=0xa5000080 hdmi_group=2 hdmi_mode=87 hdmi_timings=720 1 16 130 16 350 0 16 16 16 0 0 0 50 0 16257000 0

Others say: combine RGB signals on the VGA connector together with, say, 330 ohm resistors. Connect H-sync on VGA to H-sync on MDA, same for V-sync, and connect the ground and the MDA intensity line to ground.

VGA outputs are supposed to be .7v (probably higher on some implementations without a proper 75 ohm load though) and MDA is TTL logic level, where >2.4V is considered a high, <.8 is considered a low, and in between is undefined….

Hardware description

References and Additional Resources