Videx Videoterm jumpers

You probably won't like this:

• I couldn't find figure 9 by leafing through the manual
• The Table of Contents says Fig. 9 "VIDEOTERM Solder Points" should appear on page 6-7
• The contents of page 6-7 does not include any figures or illustrations at all.

Videoterm ToC.png

Videoterm p6-7.png

We might be able to find those solder points in the schematic, but my scan is too blurry at the hinge.

I will re-scan the schematic and try again...

https://www.applefritter.com/files/2022/08/24/VideoTerm%20schematic.pdf

Attached is a PDF scan of the Videx VIDEOTERM schematic.  You may need to download the file and zoom-in to see the fine details.

From what I can see in the schematic:

• X5 chooses whether pin 20 (chip select) on the 2716 EPROM at position U20 connects to pin 19 of the chip at U18, which could be either an LS273 or LS374.  Pin 19 also connects to X6.
• X6 chooses whether pin 15 of the LS175 at U22 connects to pin 12 vs X5.  By default it connects pin 15 to pin 12 (not X5).
• X7 chooses whether pin 10 vs pin 11 on the same LS175 at U22 connects to a NOR gate that feeds a NAND gate that feeds a NOR gate that sums the H and V sync signals through a bipolar transistor to the video-and-light-pen connector.  LS175 data sheet says pin 10 and pin 11 are inverting vs non-inverting outputs from the same flip-flop, so X7 undoubtedly flips the polarity of that particular signal...whatever it is. 

Sorry if that's a bit muddy, but the schematic is a bit muddy.

My best-guesses:

• The 2716 EPROM at U20 has all its data pins tied to the 2708/2716 EPROM at U17, which contains the base character set.  So I would guess X5 is used along with X4 to configure/install character set EPROMs like "inverse characters" and "APL characters".  Perhaps X7 chooses whether VIDEOTERM generates extended characters from the second character EPROM or just generates inverse characters from normal characters by inverting polarity? (like the earliest Apple ][ motherboards did)

Please share your guesses and observations!

bradleyb wrote:

Yeah, the jumper symbols on that schematic sure are vague! but your guesses sound pretty close to what I was thinking.

That chart looks better than anything I could come up with.  I had never even heard of those last two, X9 and X10.

Those two jumpers are especially surprising because my second-hand Videx VIDEOTERM contains an Intel 2716 at U3, but it doesn't have those X9 and X10 jumpers to enable it.  Someone bodged wires to the back of the 24-pin EPROM socket instead, connecting pin 21 (VPP) to pin 24 (VCC), and pin 18 (/CE) to the +12v supply...but also cutting the 12v supply at the edge connector.  (Unlike a 2708, most 2716s don't require a 12v supply.)

IMG_0859.JPG

I'm guessing my board was manufactured in 1982 because that's the date on most of the components.  (Except my own 2716 character EPROM)

Did Videx bodged the 2716 into this board themselves?  That seems unlikely to me, as the pin 18 revision is naively done:

• It uses an unnecessarily long route through the +12v supply to send the chip-select from pin 18 to pin 19.  A simpler revision would just isolate pin 19 from +12v and simply link it to pin 18.  That would be a better revision, and easier.
• It replaces the whole board's +12v power supply with the firmware EPROM's chip-select signal, so it's no longer feasible to install a 2708 character EPROM.  Not a huge sacrifice, but an unnecessary sacrifice.
• It prevents a light-pen from working, too.  A light-pen needs +12, but the light-pen power supply pin is now bridged to pins 18 and 19 of the firmware EPROM.

I verified all that with a continuity meter.  Pin 18 and 19 are indeed connected, but through a ridiculously long route.  And they're both connected to the (former) +12 supply on the light-pen port, pointlessly sacrificing light-pen capability.

Very odd...but what an interesting surprise!