Pr0f wrote:One quick question - as this could be a showstopper for your project...
What monitor or TV do you intend to use?
Light pen will only work correctly with a CRT based TV or Monitor, as the LCD doesn't have a 'light spot'.
Well, not only it needs a CRT monitor, but it needs the specific high resolution monitor that was made for this computer. Moreover, it needs the light pen controller board option installed in the monitor. I have both obviously
What I don't have is the light pen itself and the electronic inside the light pen body. The good news is that the light pen electronic is quite simple. Not like the light pen controller board with it's 4 layer PCB and 50+ chips.
For those interested, the light pen is for the HP-9845, a very powerful workstation first released in 1978. With dual 16bit processors, integrated 80 col printer and up to 1.5Meg of memory, it had 80286 computing power years before the Intel equivalent. The light pen however only works with the enhanced version released in 1980. The monitors were not dummy monitor but were taking care of vector and other graphic primitive generation, alpha character generation with flashing and underscore, light pen handling etc. In fact, when HP released the color version of the monitor in 1980, the monitor had more computing power then the computer itself.
But back to the light pen schematic. It should be attached to this post.
In this schematic, as in all schematics of that era I guess, connections are symbolized by big fat dots. When a wire pass over another without being connected, the line kind of "bridges over" the other as can be seen at the right of diode CR3. But in this schematic, there are 3 locations where lines just go over each other, without fat dot that would signify contact nor the little speed bump that would imply no contact.
The first one is easy I guess. I you look at the transistor circled in blue, do I need to understand that both the anode of CR1 (the photocell) and the common point of C1-R1 are attached to the base of the transistor ??
For the area circled in green, is there a contact where the lines cross or not ? As there is no fat dot, I would assume no contact but there is no bump either. I am sure that for the electronic experts, the circuit makes sense for just one of these options. But asking around I got answers like : "The wires are not connected otherwise it does not make any sens" as well as "The wire are obviously connected, otherwise it does not make any sens"...
Finally, for the transistor circle in red: the HP number is 4-392 and an equivalent is not listed in the lower left corner of the drawing. Using the HP cross reference files, I was able to determined the type as being a NPN #2N5088. However, the arrow is missing so I do not know where the collector is. I dug up the original blue print which has a better quality and there is definitely no arrow. Again I hope some electronic expert would be able to correct that missing information by looking at the schematic logic. Finally, still in the red circle, there is also some wire cross over without dot. Some friend of mine told me there should be a dot there but I don't know if I can thrust him. Does the base is only attached to the NC end of the switch with the two resistance (R15 & R17) connecting the emitter and collector with a combined value of 11.96k or does the transistor base connect to both resistances and the switch.
Something that confusing things a lot in my mind, is that the circuit is powered by -12V. Also, I probed the "SW" socket hole on the monitor (without light pen plugged in obviously) and there is = -0.5V on that pin. The OUT and RETURN pins show only low noise values (0 +/- 0.015 Volts)
Going back at the switch logic part of the schematic, I have problem understanding what is going on here, whatever the 2N5088 transistor orientation might be. Everything ends up tied to ground, so I don't see the difference between the switch at the N.O. and at N.C. position. Also the diode orientation does not seems to match the fact that the controller put a minus 0.5 Volt on the SW pin. For information, the pen controller board is working as there are a couple of self diagnostics that the controller board can run on itself and they all come ok. It can even emulate the light pen, without any light pen attached.
Finally, I wrote a small program that monitored the light pen switch button. There is no "button HIT" if nothing is connected to the light pen socket. But as soon as I connect the SW pin to the Ground PIN (a bit like the switch in the N.O. position, but without diode) the program report that the button has been pressed.
Anyway, any information or tip is welcome. I don't want to put together the circuit wrong. Not because I am afraid of damaging some of the light pen component, but because I really don't want to blow up the controller card.
Thank you all for your patience.
François