Power R Video Adapter

[Bunsen]

It's a MaxView Phase II CRT I picked up in an auction lot years ago and haven't had the heart to throw out just yet. TBH I don't know what the specs are, but it has the old 9 pin D-sub connector which would seem to indicate it's CGA or EGA.

And yes, I still have the Mac-specific LCD overhead projector panel from the other thread, as well as a mono 640x200 (CGA?) one.

That Infocus panel looks like not a bad buy actually. Note I'm not recommending it for this specific project

 

[Mac128]

but it has the old 9 pin D-sub connector which would seem to indicate it's CGA or EGA.

... or TTL variant, MDA, Hurcules, or ... my head swims at lack of video standards before 1990.

Seems like you could best use the adapter. I'm starting to think the best thing for me to do is look into building a custom TTL adapter and D/A converter to get it up to a VGA standard, which is the ultimate goal to use contemporary monitors.

 

[Bunsen]

/me goes dizzy at the thought of a 22" SE/30

 

[Bunsen]

Specifications:

Resolution........................................ 512 x 342

Horizontal scan rate.......................... 22.3KHZ, (4µs on, 40µs off)

Vertical scan rate ............................. 60HZ (180µs on, 16.4ms off)

Taking another look at that, I am starting to wonder if a B&W TV monitor could work. 60Hz is the NTSC vertical scan rate.

I'm starting to think the best thing for me to do is look into building a custom TTL adapter and D/A converter to get it up to a VGA standard

Well, it wouldn't have to be a D/A converter per se. As you're only doing 1-bit colour, you'll only need the one voltage level and zero. I suppose you could call it a 1-bit D/A

I had this other thought: if you double the horizontal and vertical resolution, you get 1024x684 (1024x768 with a few scan lines thrown away). Ok, so I'm not quite sure where I'm going with this ... I'll just jot down what comes to me ...

Random thoughts

• 768 - 684 = 84 (42 lines top and bottom)
  
• What's the blanking interval on the classic CRT?
  
• 60HZ is a common refresh rate for LCD monitors
  
• A 512 bit buffer that stores each horizontal scan line and dumps it out to the monitor at the required scan rate, repeating it if doubled
  
• Buffer the entire screen and draw it one frame later at whatever scan rates and resolutions you like
  
• Tap the VRAM directly
  
• -edits to come-

 

[tomlee59]

From my experience, ordinary TV monitors won't sync properly -- the Mac's horizontal sweep rate is nearly 50% too high -- well outside the lock-in range of conventional TV sweep circuits. The vertical rate is fine, as you note, but the horizontal frequency is very far off, unfortunately.

 

[Bunsen]

Results 1 - 10 of about 66,700 for 1024x684.

:?:

For some reason that seems to be a popular size for JPEGs

 

[Mac128]

[*]What's the blanking interval on the classic CRT?

Looks like 28 lines

http://developer.apple.com/documentation/Hardware/Developer_Notes/Macintosh_CPUs-68K_Desktop/Mac_Classic_II.pdf

http://www.stockly.com/manuals/MacintoshFamilyHardwareReference/020305-Page1-14.jpg

 

[Mac128]

So, let me put my thoughts out there. Based on everyone's input above:

If the Compact Mac horiz. sync rate is 22 KHz and even minor deviations will cause the monitor to fail and monitor manufacturers adapted their existing monitors to create custom Power R video adapter Mac versions, why then does the Power R use the unique 22.3 KHz? Did that ensure compatibility with only custom Mac specific monitors designed to work with it and no other adapters (Like a home-brew 22 KHz TTL)? I mean if the industry standard Max EGA horiz. sync rate was 21.8 KHz and the Mac was 22 KHz, if the Power R could increase the rate to 22.3, couldn't it have come down to the industry standard too? Clearly, those adapters designed to display the Mac image on a standard TV had to drop the rate down into the 15.7 KHz video signal.

Which brings me to another question: Do CGA & EGA both require a specific pixel resolution, meaning a 512 horiz. resolution can never be displayed on a CGA or EGA monitor which require 640 pixels across? Also, just what is a TTL monitor? Is this different from and MDA or Hercules based monitor? How can these monitors display graphics at all if they have no pixel control? How can they accommodate a Mac signal at all if they are designed to display a significantly wider horiz. image than the Mac?

More brain-ticklers to come.

 

[tomlee59]

The adapter is a very low-tech device. It doesn't generate or convert sync signals at all. It merely buffers what is generated internally by the Mac. It's that the Mac didn't evolve to accommodate external monitors, and so the internal sync signals are what they needed to be to support 512x384 at 60fps.

 

[Mac128]

The adapter is a very low-tech device. It doesn't generate or convert sync signals at all. It merely buffers what is generated internally by the Mac.

That's what I mean ... the Mac generates 22 KHz and the adapter changes it to 22.3 KHz. What is the adapter doing to change the sync rate?

 

[tomlee59]

Again, there is no change. You are perhaps attributing too much precision to "22kHz" and treating it as different from 22.3kHz. They are the same, with the former simply an approximation of the latter (which itself is not exact, either). The adaptor, again, is not changing any frequencies at all.

And even if it did, a 1.5% difference is too small to matter.

 

[Bunsen]

What tomlee59 said. The adapter is just there to get the signals outside the box. It doesn't do any conversion at all. It was left up to the monitor or a second adapter to cope with what it was given.

Remember, this is what the Mac market was like in the day. It wasn't that long since there had been several major forces in the personal computer space, each with their own lines of software and accessories, and their own standards. One by one they fell to the Wintel juggernaut. Apple only started to admit defeat, if you will, or get with the program, if you'd rather, around the mid to late '90s when they moved from Nubus to PCI architecture. Even then, it was several years before the proprietary Apple monitor plug fell by the wayside.

Don't mistake the dominant norm for a standard. A standard is agreed upon by several independant parties and/or an engineering committee, not enforced by market power and illegal bullying.

 

[Mac128]

Again, there is no change. You are perhaps attributing too much precision to "22kHz" and treating it as different from 22.3kHz. They are the same, with the former simply an approximation of the latter (which itself is not exact, either). The adaptor, again, is not changing any frequencies at all.
And even if it did, a 1.5% difference is too small to matter.

Ummm ... now I'm really confused, wasn't it sort of emphatically emphasized above that 21.8 KHz was not compatible with 22.3 KHz? Or is that a big enough hair to split.

With that in mind, the ONLY difference I see between EGA & this adapter is that the output is 512x342, rather than 640x350. Is THIS the reason it is being called totally incompatible? I'm also a little confused about those TTL monitors, which were designed for what, MDA/Hercules cards? In which case we're talking 720x350 ... so VERY, very confused. It is a wonder we ever got out of 80s without blowing ourselves up.

 

[tomlee59]

Well, I don't recall having said that a 5% difference will definitely matter, so I'm confused as to why you're confused. If I said as much, I must've "mistyped" (it's election season here -- no candidate ever makes a mistake now, they only "misspeak").

Here's what you need to make it sync:

1) The right polarities for sync and video

2) "Close enough" sweep rates (BOTH vertical AND horizontal)

What's "close enough?" There's no hard and fast rule. There are no guarantees when operating equipment outside of original design targets. Monitors differ in their tolerance of off-spec signals. That said, if you're within 5%, that's usually not a problem. In some cases, even 10-20% might work. As you go beyond about 15%, the population of monitors that will sync drops off rapidly.

I have plenty of experience interfacing compact macs to TTL monitors of the type used in the early IBM PCs. Almost none of those sync natively; they generally require internal mods to their horizontal sweep circuits to work with the Mac sweep rates. The vertical sweep presents no problem that I recall.

The necessary mods are easy to implement, but difficult to deduce without a schematic. In many monitors of that era, the horizontal sync circuits were implemented with the venerable 555 timer chip. The sweep rates were set by precision resistors and capacitors (typically of 1% and 5% tolerance, respectively). So, if you're able to identify the horizontal circuitry, and additionally can zero in on the timing components, you can rapidly calculate the value of resistor to put in parallel with the existing one to push the frequency upward by the necessary amount.

If you want to use a potentiometer instead of a fixed resistor (not recommended), be sure to put a resistance in series with it, to prevent ever going to ~0 ohms at one extreme of the pot's adjustment. The series resistance should be chosen to cause the sweep frequency to exceed the target by no more than 10-15% when the pot is at its minimum resistance.

 

[Charlieman]

With that in mind, the ONLY difference I see between EGA & this adapter is that the output is 512x342, rather than 640x350. Is THIS the reason it is being called totally incompatible? I'm also a little confused about those TTL monitors, which were designed for what, MDA/Hercules cards? In which case we're talking 720x350 ... so VERY, very confused. It is a wonder we ever got out of 80s without blowing ourselves up.

Ignore the proprietary Hercules PC video standard. The Power R was designed to work with a monitor closer to EGA. All the same, it would require modification, possibly excepting the first generation Sony and NEC multisync monitors.

Yes, we did blow stuff up in the 1980s by connecting the wrong monitor to the wrong card... Fortunately, we didn't have an awful kit to blow up and the kit was designed to be repaired.

Even Apple got it wrong a few times. That 19/20 pin floppy drive connector still causes problems, as you know Mac128. The first Mac II fixed frequency monitors used a mixture of D-15 and 13W3 connectors, and it took Apple a while to come up with sense codes and plug and play monitors. In the PC world, things were naturally worse, even when the HD15 VGA connector was adopted.

And your Power R adapter definitely comes from the pre-plug and play world. In that world, computer dealers worked on 50% profit margins, hence the quality support that they could deliver, and made sure that they sold you a monitor that provenly worked with your video adapter. So whilst we don't have that dealer support today, we rely on forums like this.

For information about old monitors, have you browsed net.micro.mac on Google groups yet?

 

[Mac128]

Yes, we did blow stuff up in the 1980s by connecting the wrong monitor to the wrong card... Fortunately, we didn't have an awful kit to blow up and the kit was designed to be repaired.

Head ... so close ... to exploding ...

I'm just gonna go scan the External Disk Drives manual now and put this little project on hold. ;-)

 

[Bunsen]

As promised, here's the table comparing the Mac sync rates (through the Power R) and EGA

				Power R		  EGA			% off (from EGA)
=======================================================
Reso H			512			640			- 20.0
Reso V			342			350			-  2.3
Scan H	kHz	 22.3 		 21.8		 +  2.3
Scan V	 Hz	 60   		 60			   0.0
=======================================================

So yes, it does look as though the horizontal resolution is the only serious impediment. If you stick to greyscale CRTs (which have a continuous phosphor coating) rather than colour CRTs, or any LCDs, (which have a fixed number of dots) it may not matter at all.

If I'm thinking properly, the vertical resolution shouldn't be a problem either. Once the required number of lines has been drawn, the V sync signal should send the raster back to the beginning of the frame. A little vertical position and size adjustment may be all that's required.

However I bow to the wisdom of those who have posted here with more experience than I as to what might or might not work. It sounds like it's hit and miss whether an individual monitor would sync up or not. Bear in mind that any of these monitors are going to be quite old themselves, and may not like to be strained so.

To them I ask: do things really explode or do they just fail to sync up? If they do sync, is a mere 2% going to add any appreciable strain? And is there a difference in strain between undersync and oversync (+ and -)?

 

[tomlee59]

To explode or not to explode, that is the question...

Many (but not all) older sweep circuits are designed to free-run at a slightly lower-than-normal frequency in the absence of a sync signal. The job of the sync signal in that case is to cause a premature termination of the sweep, thereby effecting synchronization. From that description, you can see that synchronization will fail if the sync frequency is lower than the free-run frequency.

If the sync frequency is too high, the sweep oscillator frequency might not be adjustable upward by a sufficient amount, again causing synchronization failure.

In both cases of failure, applied sync pulses will still attempt to truncate the sweep. Because the latter's frequency is no longer related to the sync pulse frequency, the result is a chaotic mess of "sweep/don't sweep" signals. Transistor currents and voltages can exceed device limits, with the typical result being a blown horizontal drive transistor (and less often, a cooked flyback).

Modern monitors tend to use PLLs to support multisync operation. Synchronization failure with these types of circuits tends not to be catastrophic (it's still possible, but the engineer had to have been somewhat incompetent to allow that to happen).

 

[Bunsen]

So perhaps a modern multisync monitor with published sync rates of a range that takes in the Mac rates above are the best bet?

As far as LCDs go, if you had one with a native resolution of 1024 x something, you might be in with a fighting chance

 

[tomlee59]

So perhaps a modern multisync monitor with published sync rates of a range that takes in the Mac rates above are the best bet?

That would be my expectation as well. However, I only have square tuits (I got them on sale; seemed like a good deal at the time), so someone else would have to perform the confirming experiment.