To send HPGL files to a HP7475a plotter, one
has to be careful not to overrun its serial
buffer, otherwise it will just stop plotting
(or produce garbage) after a short while.

This stupid script regularly asks the plotter
for its status, being careful not to send
it data when it's not ready.

(yes, it should work with pin 20 hardware-handshaking which I initially tried
 and failed, and also xon/xoff, but for some reason I was too stupid to make
 it work that way, so I choose the way outlined in the manual, flow diagram
 on page 10-19)

https://archive.org/details/HP7475AInterfacingandProgrammingManual/page/n191

==== Serial Cable ====

see https://www.pearl-hifi.com/06_Lit_Archive/15_Mfrs_Publications/20_HP_Agilent/HP_7475A_Plotter/HP_7475A_Op_Interconnect.pdf
(upper half of page A6, only using pins 2,3,5 on PC side)

25pin DSub, feamale     9pin DSub, male
HP7475a                 Computer (PC)
 |                           |
Pin  Function               Pin  Signal
---:-----------------------:----:------
 1 : Cable Shield / Common :body: GND (Case)
 2 : --> data from plotter : 2  : RxD
 3 : <-- data to plotter   : 3  : TxD
 7 : Signal Ground         : 5  : GND

==== USAGE ====

usage: hp7475a_send.py [-h] [-p TTY] [-b N] hpglfile

positional arguments:
  hpglfile

optional arguments:
  -h, --help          show this help message and exit
  -p TTY, --port TTY  Serial port (default: /dev/ttyS0)
  -b N, --baud N      Baudrate (default: 9600)