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PET 8032 Video to MDA Monitor

dave_m

dave_m

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For my Commodore PET main board Tester, I'm looking at hooking up the 8032 PET video (80 column X 25 lines) to an external monitor. I tried composite video to an old Sony Watchman TV, but the Sony could not sync the the PET. I think the older 40 column PETS are closer to the TV bandwidth, but the the 8032 has a higher Horizontal rate. I was wondering if anyone has had success using an old PC Monochrome monitor. The PET has TTL level Horizontal, Vertical and video signals. The frequencies are 20 KHz and 60 Hz vs the MDA spec of 23 KHz and 50 Hz. Is this close enough?
 
It should be.

I'm actually kind of surprised the TV wouldn't work. Was the image horizontally stable but the wrong size? Or was the horizontal not syncing at all?

In my experience mismatching monitors, getting sync polarity correct is the hard part. Even that's not the end of the world. Years ago when I couldn't get my hands on TTL inverters I'd build inverting amplifiers out of ye olde 2N2222.

I'll soon have an 80 column PET and was planning on using a TV- standard "video" monitor. Now I'm anxious to see what I'll run into :)

I've done some pretty bizarre monitor mismatches in the distant past. When everyone else was using 14 and 15" VGA monitors, I was finding discarded 17-21" workstation monitors with every different frequency and sync under the Sun. I always managed to mate them to my Amigas. When I got the Cybervision 64/3D, that was a godsend for me. It would output any video configuration you wanted. Then I was adjusting both ends and had really crisp displays at oddball sizes like 1532x1106 or something.
 
When I say "it should be" I mean you should be able to adjust the horizontal and vertical hold and size controls far enough to get it to work, assuming the sync polarity is correct.

Also, TV signals in this region are 15.575kc by 60c. That's closer to 20kc by 60c than 23kc by 50c. The TV should work better. Unless there's something unusual about the Watchman which there probably is.
 
Last edited:
There's a Tandy VM-5 on eBay for $5 plus shipping that might fit the bill too.
 
KC9UDX said:
When I say "it should be" I mean you should be able to adjust the horizontal and vertical hold and size controls far enough to get it to work, assuming the sync polarity is correct.
Click to expand...

Matt,
That may be my problem, there are no controls for the Watchman TV. The PET Sync signals are negative going. I'll look for monitors with lots of controls. Typically do MDA monitors have such controls?

I looked at some newer multi sync monitors, but they seem to start at 30 KHz and go to 100 KHz, not like the old NEC Multisync that started below 20 KHz.
-Dave
 
Dave,

I don't recall much about the real MDA monitors. I'm pretty sure there were no user controls for that on the IBM ones, don't recall what was inside. Most monitors have all four necessary adjustments inside. I think I have an Amdek monitor that had MDA and composite inputs. That would have to have all the controls outside.

I do know there were a lot of general purpose monochrome composite monitors which had all the controls external. But I don't know if you want to combine the sync, or keep it separate for diagnostic purposes. I'm trying to think of a real advantage to that but I can't.

It's not terribly difficult to modify a monitor without controls to sync to something just outside its normal range, but I know I probably wouldn't bother anymore.
-Matt
 
dave_m said:
For my Commodore PET main board Tester, I'm looking at hooking up the 8032 PET video (80 column X 25 lines) to an external monitor. I tried composite video to an old Sony Watchman TV, but the Sony could not sync the the PET. I think the older 40 column PETS are closer to the TV bandwidth, but the the 8032 has a higher Horizontal rate. I was wondering if anyone has had success using an old PC Monochrome monitor. The PET has TTL level Horizontal, Vertical and video signals. The frequencies are 20 KHz and 60 Hz vs the MDA spec of 23 KHz and 50 Hz. Is this close enough?
Click to expand...

The original 2001 machines (non-CRTC based) had a 15kHz scan rate that matched standard TV frequencies. The "Fat40" and 8000 series with 12" monitor have 20kHz rate. I believe the video signal is also inverted. I have one of those monitor adapters you can get on ebay for $25 (which is really just a single chip and a few components worth about $2) and it would not work on my 8032 as-is. However the CRTC-based PETs can be easily re-programmed to modify the output to match normal NTSC TV frequencies. I was able to get the output to work. Here are the values you need:

Code: 
; PET/CBM EDIT ROM - CRTC Setup Tables
; ================   80 Column x 25 Lines, 60 Hz, 15.75 kHz (NTSC) for external monitor/TV
;
; These tables are used to program the CRTC chip to set the screen to the proper rows, columns, and timing.
; Note: These seem to be the same as the 40 column values. Must verify!

;--------- Text Mode - 80 Column, 60 Hz, NTSC

CRT_CONFIG_TEXT
    		!byte $3f,$28,$32,$12,$1e,$06,$19,$1C
    		!byte $00,$07,$00,$00,$10,$00,$00,$00
    		!byte $00,$00 

;--------- Graphics Mode - 80 Column, 60 Hz, NTSC

CRT_CONFIG_GRAPHICS
    		!byte $3f,$28,$32,$12,$1e,$06,$19,$1C
    		!byte $00,$07,$00,$00,$10,$00,$00,$00
    		!byte $00,$00

This is from my CBM Editor ROM project:
http://www.6502.org/users/sjgray/projects/editrom/index.html
And on GitHub:
https://github.com/sjgray/cbm-edit-rom

You need to poke the values into the CRTC register with a basic program like this:

Code: 
10 rem pet crtc setup program for NTSC video output
20 rem ---------------------------------------------------
30 for a=0 to 17
40 read n
50 poke 59520,a:poke 59521,n
60 next a
100 data 63,40,50,18,30,6,25,28
110 data 0,7,0,0,16,0,0,0,0,0

If you want your machine to be permanently set for NTSC you can build a new editor rom from my source, or you can patch an existing ROM.
Sorry, I've never tried to connect an MDA monitor so I don't know if the CRTC chip in the pet can output sync that high. I did write a small utility that allows you to play with the CRTC registers in real time. You might be able to use it to find the correct values to give a stable output for your MDA monitor.

Steve
 
Well ntsc lcds with composite are plentiful and a simple editor rom replacement would be pretty easy for quick testing. At least you could verify proper operation. The pet composite adapter would be easy to build and the signals are available on the user port for quick attachment.

You might still be able to find some vintage nec projectors that can go down to 15khz. I picked up an mt-1000 for under $100. I also found a NEC 1550m lcd monitor that can handle 15khz as well. These will probably accept 20khz signals.

Steve
 
sjgray said:
MDA is 18.432 khz Horizontal and 50 Hz Vertical.
I would think that would be close enough to the PET output, although the PET vertical is probably 60 Hz.
Click to expand...

I agree. Even the 50/60Hz should easily be within range of internal adjustment. It may even work without adjustment.
 
sjgray said:
However the CRTC-based PETs can be easily re-programmed to modify the output to match normal NTSC TV frequencies. I was able to get the output to work. Here are the values you need:

Code: 
; PET/CBM EDIT ROM - CRTC Setup Tables
; ================   80 Column x 25 Lines, 60 Hz, 15.75 kHz (NTSC) for external monitor/TV
;
; These tables are used to program the CRTC chip to set the screen to the proper rows, columns, and timing.
; Note: These seem to be the same as the 40 column values. Must verify!

;--------- Text Mode - 80 Column, 60 Hz, NTSC

CRT_CONFIG_TEXT
            !byte $3f,$28,$32,$12,$1e,$06,$19,$1C
            !byte $00,$07,$00,$00,$10,$00,$00,$00
            !byte $00,$00
Click to expand...

Steve,
Thanks for this. I'll give it a try. As Matt says, it may not be a perfect solution for a 'Hot Bench Tester' having to swap an Edit ROM in the unit under test, but at least I may be able to use my tiny Watchman TV instead of a clunky MDA CRT as space on my bench is at a premium. If it works, perhaps I will create a PETVet configuration with a mapping of the custom Edit ROM in PETVet and the other ROMs and RAM mapped to the PET and troubleshoot that way.
-Dave
 
dave_m said:
Steve,
Thanks for this. I'll give it a try. As Matt says, it may not be a perfect solution for a 'Hot Bench Tester' having to swap an Edit ROM in the unit under test, but at least I may be able to use my tiny Watchman TV instead of a clunky MDA CRT as space on my bench is at a premium. If it works, perhaps I will create a PETVet configuration with a mapping of the custom Edit ROM in PETVet and the other ROMs and RAM mapped to the PET and troubleshoot that way.
-Dave
Click to expand...

Let me know how it goes! If your screen is inverted change register 12 ($0C) from 16 ($10) to 0. This controls the "invert" line TA12 on the CRTC chip. I thought about getting a PETVET. Do you find it useful?

Steve
 
I bought one but fixed my PET whilst it was in transit so didn't need it. I kept it thinking it would be fun to run earlier versions of BASIC, but for some reason the one I have won't do that.
 
sjgray said:
I thought about getting a PETVET. Do you find it useful?
Click to expand...

It is a very useful gadget. For novice guys with 2001 PETs with problems in the unavailable 6540 ROMs and 6550 RAMs, it is about the only feasible quick fix. For more experienced guys, it is a great troubleshooting tool to sneak up on the problem. By default all memory is mapped to the PETVet and if that fixes the problem, then selectively enable RAM on the PET or ROM on the PET in 4K chunks if wanted. Changing from the default configurations does take using a PHP script file to create a download binary for the gadget, but a RS-232 boot loader is built into the gadget and it is not difficult to understand using the instructions provided. No Atmel chip programmer is needed although that can be done also with a PROM burner or using an in-circuit AVR programmer through the built-in ISP interface (better than PROM Burner as chip does not have to be removed from the board).

One problem is that there always seems to be a waiting list. Mike (bitfixer.com) builds them in small batches as he finds time from his busy Silicon Valley real life.

There is another board now available from Great Britain that is an improved 'Nicolas Welte' board if you remember that one. A simple RAM/ROM replacement that can hold several 'DIP Switch' selections of ROM configurations (BASIC 2, BASIC 4, custom, etc.) in a 512K or 1M EPROM. It has a single 32K byte RAM chip to replace all RAM. With the default equations in the GAL chip (16V8 ), it is all or nothing i.e, you choose all PET RAM or all replacement RAM, and all PET ROM or all replacement EPROM. However if one is handy with PAL equations, you can create custom mappings. http://blog.tynemouthsoftware.co.uk/2015/05/commodore-pet-romram-replacement-boards.html
 
My shiny new (give or take) 80 column Porsche will not display on a 1702 and without measuring anything it sure looks like inverted sync.
 
Oddly a cheapy TV I have will sync it well enough.

It's an Orion brand 12" colour TV that I bought at a thrift shop for £5.

I just discovered I don't like the Porsche. The 80 column display is beautiful, but the double quote is on the wrong key. There's a commercial at in its place!
 
My BMC Data Display model BM-12AUW displays it every bit as well as an attached monitor should.

All that is, is a late 70s monochrome composite monitor which I normally use with an Apple ][. Actually that's what I've used it for since it was brand new.

I'm kind of puzzled about the 1702 now. I would surely expect any CRT based "NTSC" monochrome TV or monitor to work.

O.k. it was my fault. The 1702 works very well, I had a bad connection.
 
I've pondered this several times myself since I started fixing up all my stored PETs back in October. There's several mono monitors in my collection so it would be great if I could just make up a cable to connect an 8032 to say Apple][ or Sanyo MBC555 monitor.

I found a home-made TV-out converter in a box of PET bits in the garage recently, haven't plugged it in yet...

Cheers
 
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