Since my HP 1350A and 1351A vector generators are not working, and I don’t have time to fix them for RetroChallenge, I’m building a simple vector generator from scratch. The design is a slightly updated version of Steve Ciarcia’s design in “Make Your Next Peripheral a Real Eye Opener” from the November 1976 issue of BYTE magazine, which was a simplified version of Hal Chamberlin’s design from the first three issues of The Computer Hobbyist in 1974-1975.The updated design uses two Microchip MCP4822 dual 12-bit SPI DACs, three DG201 quad analog switches, three TL084 quad opamps, and one LT1260 triple video amplifier (necessary to drive the 50 ohm input impedance of the HP 1338A monitor). I haven’t built it yet, so the schematic should just be considered a draft, and probably has many errors.
The original designs use one-shots to generate the timing pulses T1 and T2, and the pulse width of T2 has to be adjusted to make line endpoints meet. I will use GPIO outputs of the microcontroller running the show to drive the T1 and T2 signals, so the timing will be controlled by software.
A drawback of the basic design is that it takes a fixed time, approximately 100us, to draw all vectors, independent of length. This will cause intensity variation; short vectors will be brighter than long vectors. The velocity is not constant during the vector, providing another source of intensity variation. I will compensate for the former by adding a Z (intensity) axis with the same circuitry as the X and Y axes. A single DAC for Z would be sufficient for compensating for the vector length, but by using the full vector drawing circuit for Z, it will be possible to have vectors that intentionally ramp in intensity, useful for 3D.
I spent entirely too much time figuring out equations for resistor ratios for unipolar to bipolar conversion of the DAC output. Although the Microchip datasheet for the DAC has an example and equations, I found their presentation confusing. I used Wolfram Alpha to solve the system of equations for the general solution, then put the results in a spreadsheet to solve for specific cases.
After I started on updating the design, Richard Ottosen told me that he built the Ciarcia design for use with his 6502-based Digital Group microcomputer back in the day. John Doran pointed out that he use an updated design, very similar to what I’ve done, for his Electronic Vector Clock (scope clock).