420C and 440's bought at VCF East XVIII

[CommodoreZ]

Howdy, it's been awhile!

This past weekend at VCF East XVIII, I purchased a pair of OSI boards from Glitch. I know the C1P typically uses a similar board to the 440, but I wasn't sure about what sort of system builds you might typically see this board installed. He mentioned that the 420C can be used to supplement the memory of the 440, and the manual seems to indicate that it allows the 440 to handle pixel graphics, not just 32x32 text. Also, it can handle color? That bit surprised me, but not as much as having to directly wire the output signals to the color gun drivers of a CRT to implement it. That's quite extreme.

I'm curious what various 440 board builds look like within a system, both stock and utilizing a supplemental 420C. I've yet to find a photo of the ribbon cables from the 420C connecting to the 3 empty sockets on a 440 to get an idea of how that might look. Are there any examples out there of the video output, either in text or graphics modes?

I also noted in the manual that 3 kinds of keyboard can be used: OSI, SWTP, or a Sanders 720 (I happen to have an example of the latter). I'm also curious if anyone has seen such a keyboard installed with this or any other OSI video board before. Anyway, here are photos I took of my new boards:

420C:

440:

[Mark]

The 440 board is similar in many ways to the C1P video display. It could be used with the OSI400 CPU & 430 I/O, or with the OSI500 CPU along with ASCII or serial keyboards using various ROM configurations. I can't help you with too many of the specifics of the 440 boards, having never had one, but utilizing the schematics from BillDrom I have emulated the board in WinOSI, both color and 128x128 dot graphics using my best guesses. (Color utilizes the upper 2 bits of character RAM, and 128x128 graphics uses 2K of 420C @ D400+.)
Of course on a real display, lines would be lost due to overscan.
Wiring directly to RGB CRT guns is scary, but I think you could drive an old RGB NTSC monitor instead with a few modifications (?)

Possible color + graphics display

screen0001.gif (377.33 KiB) Viewed 4681 times

And the silly BASIC program I used to test these features:

Code: Select all

 10 I=54272: S=13*4096
 15 FOR J=I TO I+2047:POKE J,0:NEXT
 20 DIM P(100): DIM C(4):C(1)=0:C(2)=64:C(3)=128:C(4)=192
 25 FOR J=S TO S+1023:N=(INT(RND(99)*4))+1:POKE J,C(N)+32:NEXT
 30 FOR N=0 TO 99
 35 P(N) = (INT(RND(N)*2048)) + (INT(RND(8)))/10 + I
 40 NEXT N
 50 N=INT(RND(100)*100)
 60 J = INT(P(N))
 70 X = (P(N)-J)*10
 81 IF X=0THEN T=128
 82 IF X=1THEN T=64
 83 IF X=2THEN T=32
 84 IF X=3THEN T=16
 85 IF X=4THEN T=8
 86 IF X=5THEN T=4
 87 IF X=6THEN T=2
 88 IF X=7THEN T=1
 89 IF (PEEK(J) > 0) THEN POKE(J), 0:GOTO 100
 90 POKE(J), T
 100 FOR T=1 TO 100: NEXT
 110 GOTO 50

Your board looks like it has six 2102s installed, and with the 2513 CHARGEN present will give you a 64 character uppercase ASCII display.
If you look on vintagecomputer.net, they have a picture of a 440 that looks like it has been modified with 8 2102s for an 8 bit charset with perhaps an OSI chargen ROM?

I do have this 440 picture that looks like it had an ASCII keyboard attached. Does your Sanders 720 present an ASCII output with strobe?

OSI440 old ebay

OSI440-05.jpg (296.64 KiB) Viewed 4681 times

The picture from billdrom has a 440 board with an RF modulator or some sort of video adjustment going on, yours looks to be a composite output.

OSI440 billdrom

CIMG1774.jpg (181.7 KiB) Viewed 4681 times

Since your board has the sockets installed, was it once used with graphics options, or was the builder just filling all the IC positions? I once came across this one which also has those positions filled with sockets.

OSI440 with sockets

OSI440-06.png (3.68 MiB) Viewed 4681 times

The 420C 4K board was used as general RAM expansion. My first C2P with 500 board used it to get to 8K of RAM as the 500 board had 4K to start with. I did install the switch and battery backup to hold programs in memory for a while when powered down. I've also seen them in floppy based C3 systems using CPM where CPM expects RAM at $D000-$D7FF

Cheers,
-Mark

[CommodoreZ]

Thank you Mark! These are great photos, some of which did not come up in my hunt for pictures of the 440 in action. I see that in all of the examples you've posted, the additional clock circuit has been added, which mine lacks.

The Sanders 720 and how to interface with it are described in Byte Magazine's inaugural issue from September 1975:
https://archive.org/details/byte-magazi ... ew=theater
I believe it does in fact do a slightly modified ASCII with strobe based on the documents I've been given, but I have yet to test my 720.

I'm not sure if the previous owner of my 440 had actually used the external RAM interconnects, or just added sockets for future use, but I'd like to connect my 420C to that port. I wish the manual had been more specific about what goes where on the 420C board.