TRS80 Model 1 Horizontal gaps (lines)

[SnapCraft]

Does anyone know how to eliminate or minimize the prevalent blank (thus, black) horizontal lines that appear on Model I's? Is it exclusive to Model I's?

 

Mine seem to scroll up slowly and happen only during motion. I've tried two different TRS monitors. Unfortunately, I only have one Model I and one Model III to compare.

 

Several non-emulated YouTube videos seem to have this problem (again, just Model I's) and Wikipedia mentions something about it. It's highly irritating, do I just have to live with it? Is this how they acted when they were new?

 

Thanks...

 

 

[+sm3]

I've never really seen this issue myself (or noticed it). I think it probably happens so quickly that unless you take a picture of the monitor, you don't really see it. The screen has always sort of flickered with the graphics. I just chalk it up to being an over 38 year-old design.

 

I'll need to set up my Model 1 to see if mine looks like that as well. I don't have enough room to keep all these old computers ready to use

 

I use a device called the "MISE" so most of the time I don't even use the real TRS-80 monitor: http://home.comcast.net/~bartlett.p/MISE/

 

Here he briefly talks about the black line: http://home.comcast.net/~bartlett.p/MISE/doc/vga.html

[SnapCraft]

Oh, no, you definitely can see it. Highly irritating. If it's normal--it is what it is. I just need others to verify.

[JamesD]

It looks like some bits aren't responding in the video RAM.
Not sure if it's bad RAM or some other part of the circuit though.

[+sm3]

That was my first guess (and post) as well but I figured I'd check mine first.

 

There is a diagnostic program that is listed (BASIC) in the Sams Computerfacts for the Model 1 to check the video memory.

 

There are other diagnostic programs as well available on the internet if you have the capability to transfer them to a disk or use them with a disk drive emulator.

 

Here is where you can find the Sams Computerfacts among other things: http://www.classiccmp.org/cpmarchives/trs80/Library/Manuals/Hardware/

[JamesD]

It doesn't look like bad RAM, parts of the graphics characters are still visible on all the lines. That might indicate the video out circuit has a bad chip.

[+sm3]

My Model 1 does not produce the output shown above. What I see on mine is an occasional flicker (best I can describe it) around the border, but that is about it.

[Bruce Tomlin]

There is no video synchronization with the CPU, so the CPU is given priority over video memory access. You would get those black "snow" lines all the time whenever stuff was constantly writing to the video memory.

 

So it should be normal, BUT that looks worse than normal. I suspect something in the video generation circuits has gone a little flaky.

 

It has nothing to do with the monitor.

[John_L]

It's a tearing issue, still happens today on modern computers. It's when the video offset switches in the middle of raster, not much you can do about it.