Category: Photography & Images

A long time ago I got an HP54602B oscilloscope with a serial port data link. HP provided a sample app that snarfed screenshots & data from the scope, but it wasn’t really ready for prime time and, besides, I vastly preferred to use OS/2 (!) and then Linux rather than Windows.

Here’s my Kermit script to fetch screenshots. All the software comes more-or-less standard in Ubuntu Linux and (I presume) in most others. If you’re running Windows, you’re on your own.

Scope Setup

The oscilloscope’s HP Plotter setting spits out bog-standard HPGL commands in flat ASCII. I’ve always meant to investigate what HP Printer does, but …

I wish the scope ran faster than 19200 b/s, but that speed works reliably over generic USB-to-serial converters (and the scope can’t feed data that fast, anyway). The other choice, back in the day, was HPIB / GPIB; I’d have had to buy three or four different adapters to suit all the PC data buses since then: ISA, EISA, VLB, PCI …

Xon/Xoff flow control (a.k.a. handshaking) works better than hardware flow control, simply because the cable’s easier to build.

The Factors setting adds a bunch of text to the end of the data stream that’s not useful, except for the fact that an HPGL LB instruction follows all of the useful data and gives the Kermit script something to look for. Otherwise, the only way to detect the end of the stream is to time out after a looong time.

I haven’t the foggiest idea what Resolution does, but High seems appropriate.

Hardware Notes

The scope requires a Null Modem in front of a standard DB-25 to DB-9 cable. I’ve been meaning to rewire my standard cable to eliminate the Null Modem, but …

Adding an LED breakout / monitoring adapter to the serial port loads the signals too much and can lead to puzzling errors. Maybe it’s just my adapter: YMMV.

I’ve run the cable all the way across my basement lab with no problem. This is, after all, good old RS-232, not some high-falutin’ USB or Firewire interconnect.

Taking the Shot

Get a picture you like, poke the Print Screen button, then quick like a bunny run the script. The scope copies the current screen into an internal buffer, then sends out a torrent of HPGL commands. The script will capture the data and eventually spit out a PNG file.

You may want to Stop the trace, rather than leave it running.

In XY mode, the scope seems to have trouble copying the entire trace. I tap Auto Store twice, then Stop, then Print Screen. It’s fuzzier, but copies the whole thing.

What Happens

The script captures the incoming serial data into a log file, processes that text through the hp2xx program to get an Encapsulated Postscript EPS file, then runs that though convert to get a PNG file. The bank shot off EPS results in better-looking output, for reasons I don’t understand.

The 240-second timeout value for the Input command seems long, but it takes a lot of plotter commands to define a four-trace plot. A too-short timeout chops off the tail end of the HPGL stream and prompts bizarre error messages from hp2xx.

The parameters for hp2xx and convert came from protracted and tedious twiddling. The ‘scope image is 512 dots across and 300-some-ish vertically; the output mimics the not-quite-square graticule aspect ratio on the actual screen. If HP thinks it looks good, then it looks good to me.

The active (bright) traces use Pen 2, which I’ve set to Blue (color 4). The graticule, annotations, and stored traces all use Pen 1, which appears as Black (color 1). Tweak -c 14 as you wish.

The pen widths (set by -p 34) don’t actually seem to do very much, although I vaguely recall that using the default width of 1 makes the output entirely too faint.

The PNG has a transparent background that turns white when you actually use it in a document; I suppose you could overlay it atop a background image if you wanted to get cute.

When the dust settles and the smoke clears, you get PNG images like this. It’s an XY plot, so the blue section appears as a bright trace on the oscilloscope’s screen.

Kermit Script

#!/usr/bin/kermit +
# Fetches screen shot from HP54602B oscilloscope
# Presumes it's set up for plotter output...
# Converts HPGL to PNG image

set modem none
set line /dev/ttyS0
set speed 19200
set flow xon/xoff
set carrier-watch off

# Make sure we have a param
if not defined %1 ask %1 {File name? }

set input echo off
set input buffer-length 150000

# Wait for PRINT button to send the plot
echo Set HP54602B for HP Plotter, FACTORS ON, 19200, XON/XOFF
echo Press PRINT SCREEN button on HP54602B…

log session “%1.hgl”

# Wait for end of data stream
input 240 lb

echo … got final lb command

close session
close

echo Converting HPGL in
echo — %1.hgl
echo to PNG in
echo — %1.png

# without labels = no terminating lb info
#run hp2xx -m png -a 1.762 -h 91 -c 14 “%1.hgl”
#run mogrify -density 300 -resize 200% “%1.png”

# with labels = terminating lb
run hp2xx -q -m eps -r 270 -a 0.447 -d 300 -w 130 -c 14 -p 34 “%1.hgl”
run convert -density 300 -resize 675×452+2+2 “%1.eps” “%1.png”

echo Finished!

exit 0