Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
MHVLUG meetings end around 8 pm and, depending on this-and-that, the bell atop Old Main on the Vassar College campus will be tolling the hour as we emerge. Here’s a scene-setting photo from Wikimedia, taken from about where I parked the car:
Vassar College Old Main Building
Although the bell didn’t have its usual steady rhythm after the most recent meeting, I didn’t expect this:
Bell Ringers atop Vassar Old Main
The tree grows in the near foreground, not over Old Main.
Two of them realized the risk of permanent hearing damage, but do you see the real hazard?
The HP 7475A wakes up with hardware handshaking enabled: DTR starts high and goes low when the internal 1 KB buffer has less than 80 bytes remaining. The plotter also supports XON/XOFF handshaking, a sad software thing you’d use only if you had no other choice.
The color codes over on the left of the top diagram match a prebuilt cable I hoped to repurpose, but it had only five conductors, none of which were DSR or CTS. Pfui!
So I used a hank of gorgeous flexy 9-conductor cable (which came with premolded DE-9 ends of the wrong gender, now amputated into pigtails and back in the GCS), which supported the connections redrawn on the bottom in proper numeric order, used the obvious color sequence (Bn R O Y G Bl V W K), then soldered suitable connectors on each end:
After mentioning that I wished I still had my HP 7475A plotter, Dithermaster sent me one from his heap. As he explained, a mouse family had used it as a combination hotel-granary-latrine:
HP 7475A – chassis latrine
For whatever it’s worth, if you must get a bazillion seeds out of a plotter, ship it halfway across the continent: UPS performs a lengthy three-axis vibration test that shakes all the loose bits through the vents.
You’ll probably want the original HP 7475A documentation from the (unofficial) HP Computer Museum before digging in. Not mentioned anywhere: the two washers at the rear edge of the case are not identical. The one holding the power supply in place is slightly longer than the one at the serial connector. Mine are now color-coded to their locations.
A critter whizzed on U13, the serial adapter chip, just beyond the big black filter capacitor:
HP 7475A – PCB latrine area
I rinsed everything (except, no fool I, the membrane keypad at the front of the PCB) with warm water, flushed the latrine areas with dilute baking soda (alkaline, to neutralize the urea), rinsed with hot water, blew-dry with compressed air, then let the pieces sit for a few days.
After reassembly, the plotter didn’t start up. It’s a third of a century old, what did you expect?
Measuring the electrolytic capacitors showed they were all in surprisingly good condition, with only C27 and C34 (on this Option 001 = RS-232 board) having moderately high ESR. They’re the pale blue axial caps just right of the heatsink, both 22 μF 25 V:
C27: Processor Reset timing (U14 – p. 6-27/6-28)
C34: +5 V filter cap (U21 – power supply p. 6-31)
The corresponding caps on the Option 002 = HP-IB board are C20 and C25. FWIW, if you have an HP-IB plotter, you should probably just hack an Arduino into the motor control connections and run it with Grbl; you’d get a bare-bones plotter eating G-Code, not HP-GL, but that’s not entirely a Bad Thing. Adapting the tool change code to handle the pen carousel is left as an exercise for the desperate.
I replaced the offending caps with 33 μF 50 V radial caps from the heap:
HP 7475A – re-capped PCB
And then it performed its Demonstration Plot (load paper, hold down P1 + P2 buttons, turn on power) perfectly. The fossilized pens left no trace behind; we all expected that.
The serial port connection on the back required, from bottom to top:
All of which came from the Big Box o’ Serial Adapters and produced this rather unsteady ziggurat:
HP 7475A – serial port adapters – typical
Seeing as how I’ve been adapting serial connections since before the HP 74754A was a thing, the Adapter Box has All! The! Adapter! Genders! plusDer Blinkenlights! They don’t come in nearly as handy nowadays, though, which is a Good Thing.
Some optimization pared down the ziggurat and added a short extension cable:
HP 7475A – serial port adapters – hardcore
Eventually, I’ll build a custom cable, but it’s good enough for now.
The switches select 9600 b/s serial data in 8N1 format. Yes, the plotter tops out at 9600 b/s, but remember we’re dealing with a pen plotter that executes terse ASCII commands. It offers both XON/XOFF and DTR/DSR hardware handshaking to prevent overruning the internal 1 kB buffer, plus a myriad other software-selectable options relevant to long-forgotten datacomm systems.
Lest I forget, dots now mark the switch settings for 9600 8N1, A (letter) paper, US (inch) units, direct serial connection:
HP 7475A – DIP switch settings
And then it Just Worked: type IN;SP1; into minicom and the plotter grabs Pen 1. The rest is a simple matter of software.
The garage door opener just ate another rough-duty bulb, so let’s see how a $7 LED bulb fares:
Walmart 60 W LED Bulb – garage door opener
It has no external heatsink fins and the color temperature looks just like the old-school incandescent bulb it’s replacing, so they’re getting a clue about what’s acceptable to ordinary folks.
That’s equivalent to a 60 W incandescent bulb, too, at least according to the package:
Walmart 60 W LED Bulb – package data
I love the “Return the package and reciept for replacement or money back” part…
Now, with the V4 hot end and fans installed, I popped a 24 V supply brick off the heap and connected another set of Powerpoles:
M2 – Powerpole connector block
The 24 V supply now powers everything on the RAMBo board, with the platform heater running from the 40 V supply through the DC-DC solid state relay.
Unfortunately, wiring the LED panels to the RAMBo MOSFET driving the fans didn’t quite work. Turns out that the extruder PWM pulses produce corresponding LED blinks; the V4 hot end draws 1.5 A and that’s enough to flicker the lights. So they’re back on the wall wart and glow steadily again.
For whatever it’s worth, the panels don’t have limiting resistors, just eight 150 mA LED emitters in series…
The Smell of Molten Projects in the Morning 