CommodoreZ
Experienced Member
Greetings. I've been reading up on the usage of the MC6845 CRT Controller, and I was amazed at the wide variety of machines that put this chip to use. Most deployments generate simple composite video output for use with generic monitors and televisions. Some tend to use them with TTL CGA monitors, and a few others have integrated video hardware.
Each deployment uses a different set of timing values that are pulled into the 6845's registers and usually left alone (Sanyo MBC-555 not withstanding). However, I haven't seen most of these systems in action long enough to know how well they generate NTSC composite, so I need your collective experience here. Given the chance, I would sit down with an oscilloscope and take notes on the timing of at least a dozen different example machines, but that isn't exactly in the cards. Two questions:
Which machine used it in a way you think looked "best"? I realize this is subjective, but I'm fine with that.
Which machine conformed best to NTSC in terms of timing and frame size?
My goal is to have a good reference point of which composite video signal played nice with a wide variety of display hardware. I've been playing around with providing composite video output on a Heathkit H-19 terminal, but the timing was designed to work with Heathkit's own video hardware, which isn't exactly NTSC compliant. I've been looking up the timing information from documentation of the common ones with the goal of programming my own custom set onto EPROMs. Here are the hex values used on a handful that I've read up on:
Edit: The H19's 6845 uses a clock of 1.536MHz
Each deployment uses a different set of timing values that are pulled into the 6845's registers and usually left alone (Sanyo MBC-555 not withstanding). However, I haven't seen most of these systems in action long enough to know how well they generate NTSC composite, so I need your collective experience here. Given the chance, I would sit down with an oscilloscope and take notes on the timing of at least a dozen different example machines, but that isn't exactly in the cards. Two questions:
Which machine used it in a way you think looked "best"? I realize this is subjective, but I'm fine with that.
Which machine conformed best to NTSC in terms of timing and frame size?
My goal is to have a good reference point of which composite video signal played nice with a wide variety of display hardware. I've been playing around with providing composite video output on a Heathkit H-19 terminal, but the timing was designed to work with Heathkit's own video hardware, which isn't exactly NTSC compliant. I've been looking up the timing information from documentation of the common ones with the goal of programming my own custom set onto EPROMs. Here are the hex values used on a handful that I've read up on:
Code:
Register H19 MDA Videx Hercules
R0 Horizontal Total 60 61 82 61
R1 Horizontal Displayed 50 50 50 50
R2 H-Sync Position 54 52 64 52
R3 H-Sync Width 8 F 29 F
R4 Vertical Total 19 19 1B 19
R5 Vertical Scan Line Adjust 4 6 8 6
R6 Vertical Displayed 18 19 18 19
R7 V-Sync Position 19 19 1A 19
R8 Interlace Mode 0 2 0 2
R9 Max Scan Line Address 9 D 8 D
R10 Cursor Start 48 B E0 B
R11 Cursor End 8 C 8 C
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