Videx Videoterm jumpers

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Videx Videoterm jumpers

Does anyone have a copy of the Videx Videoterm manual which includes Figure 9, which is supposed to document the solder jumpers?

The only scanned editions I've been able to find do not include that figure.

 

Barring that, does anyone know what jumpers X5, X6, and X7 are supposed to do?

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You probably won't like this

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.

 

 

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...

 

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Here's the schematic and my best [worst] guesses

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!

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S.Elliott wrote:You probably
S.Elliott wrote:

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.

 

thanks for checking. Which edition do you have? I wonder if it was in the first edition and removed after that.

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Yeah, the jumper symbols on

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

 

Also I just found this paragraph which I hadn't noticed before on page 1-9:

 

Which really helps confirm a lot. (again, nothing regarding the jumpers on pages 6-4 to 6-8 in any scanned edition)

 

So here's my educated guess as to all the jumpers: 

 description1-22-3notes
X1Type of chip used in U5*U5 is a 74LS245, apply ground to pin 10 (GND)U5 is an AM8304. apply signal to pin 9 (CD)The same hole is used for pin 10 of the 74LS245 and pin 9 of the AM8304. Not used for a 74LS373
X2Type of chip used in U5*U5 is a 74LS245, apply signal to pin 19 (/CE)U5 is an AM8304. apply power to pin 20 (VCC)The same hole is used for pin 19 of the 74LS245 and pin 20 of the AM8304. Not used for a 74LS373
X3Type of chip used in U17*U17 is a 2758 or 2716, apply signal to pin 19 (A10/Aref)U17 is a 2708, apply +12V to pin 19 (VDD) 
X4Type of chip used in U17*U17 is a 2758 or 2716, apply +5V to pin 21 (VPP)U17 is a 2708, apply -5V to pin 21 (VBB) 
X5enable optional char set ROM*Optional ROM enabled, apply high data bit to U20 pin 20 (/OE)Optional ROM disabled, apply ground to U20 pin 20 so the main ROM is always enabledsignal comes from pin 19 of U18. high data bit is used to select the optional ROM
X6character invertenable character invert, apply high data bit to U22 pin 12*enable hardware cursor, apply cursor signal to U22 pin 12high data bit is used to invert characters. hardware cursor display is lost when enabled
X7invert entire screenenable whole screen invert, U22 pin 11 (/Q2) is XORed with video signal*disable whole screen invert, U22 pin 10 (Q2) is XORed with video signal 
X8Type of chip used in U18*U18 is a 74LS283, apply 5V to pin 1 (/CLR)U18 is a 74LS374, apply ground to pin 1 (/OC) 
X9Type of chip used in U3*U3 is a 2708, apply -5V to pin 21 (VBB)U3 is a 2758 or 2716, apply +5V to pin 21 (VPP)only present on clone cards?
X10Type of chip used in U3*U3 is a 2708, apply +12V to pin 19 (VBB)U3 is a 2758 or 2716, apply ground to pin 19 (ARef)only present on clone cards?

 

* = default option

I number the pads on the pie-shaped jumpers going around counterclockwise with pad 1 being the first one after the larger gap.

 

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All 2716s on my circa-1982 Videx VIDEOTERM
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.)

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!

 

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That's interesting. I'm

That's interesting. I'm guessing someone did that because they wanted to update to a later firmware revision? Does the chip have a label or anything to make it look like it came from Videx?

 X9 and X10 are names I invented for the "ideal" version of these jumpers, after seeing them on photos of clone cards, which more or less recreate in copper traces the bodges on your board.

 

-5V is "cut" at the connector, but with pads to reconnect if desired. U3 pin 21 (VPP) is probably just being left unconnected which is probably fine (?) but it's possible there's a trace connecting it to VCC under the U3 socket.

 

+12V is also "cut" at the edge connector, disabling all +12V power, and a connection is made from pin 19 to pin 18 of U3:

 

 

Why didn't they cut it on the other side of that via to preserve +12V for the light pen? who knows!

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I've seen at least two board

I've seen at least two board revisions of actual Videx cards, and yours does appear to be a later one. On the early revision (also on that clone photo above) there was a trace connecting CR1 to pin 5 of U9 and pin 10 of U24 (as in the schematic) but it's missing on yours:

 

 

From other photos I've seen, I think maybe CR1 now connects to U6 pin 11 and U9 pin 14, but is there any chance you could find out what they actually did with it?

 

Also, from the schematic, U17 pin 18 (/CE, PGM) is supposed  to be grounded, but I've see no traces making that connection. I'm guessing they connected it together with pin 20 (/CS,WE,/OE) - is that the case?

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Another very weird thing. U24

Another very weird thing. U24 is a tri-state inverter. But only two gates are used, and pin 1 appears to be connected directly to ground, permanently enabling them. Could have easily just used a 74x04 or possibly just rearrange some gates to eliminate it altogether...

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