4 ms·

The parallel printing protocol is synchronous in both directions. Both the Computer and the Printer will wait on each other. Edit: s/port/printing/

pifm_guy 4y ago
But not everything uses that. Some software simply connects a pin of the port to a motor and another to a sensor, then uses software in the computer to turn the motor off as soon as it sees the sensor be activated. Devices that do that often have very tight timing requirements - since original 1990's era computers could respond to a change in a pin change in as little as a few clock cycles.
- zozbot234 4y ago
  This. The parallel port is pretty much the original GPIO.
- deleted 4y ago
  [deleted]
akiselev 4y ago
It’s synchronous but it was offloaded on to the southbridge in Intel/AMD CPUs so you could fill a ring buffer and it’d DMA everything to the peripheral without interrupting the CPU. Drivers could just leisurely fill up those ring buffers and never have to worry about kernel scheduling inhibiting hard realtime. As an input the interrupt could also drop into a driver much faster than userland software. CNCs with MachCNC software and lots of old industrial equipment operated this way since it saved money on custom PCB designs. BART [1] still buys older motherboards with parallel ports wired to the south bridge because USB adaptors can’t guarantee timing and swapping to a PCIe card would require rewriting some old stuff. However it has to be done at the driver level, it wasn’t Postscript or GCode or something like that, so I doubt this piece of gear will be anywhere as useful as parallel ports used to be. [1] https://news.ycombinator.com/item?id=32884814 https://news.ycombinator.com/item?id=32884814
memcg 4y ago
I remember trying to interface a PCs Limited (original Dell) parallel port to a Slo Syn paper tape reader on Excellon PCB drills and routers around 1995.

pifm_guy 4y ago

But not everything uses that. Some software simply connects a pin of the port to a motor and another to a sensor, then uses software in the computer to turn the motor off as soon as it sees the sensor be activated. Devices that do that often have very tight timing requirements - since original 1990's era computers could respond to a change in a pin change in as little as a few clock cycles.

zozbot234 4y ago
This. The parallel port is pretty much the original GPIO.
deleted 4y ago
[deleted]

akiselev 4y ago

It’s synchronous but it was offloaded on to the southbridge in Intel/AMD CPUs so you could fill a ring buffer and it’d DMA everything to the peripheral without interrupting the CPU. Drivers could just leisurely fill up those ring buffers and never have to worry about kernel scheduling inhibiting hard realtime. As an input the interrupt could also drop into a driver much faster than userland software. CNCs with MachCNC software and lots of old industrial equipment operated this way since it saved money on custom PCB designs. BART [1] still buys older motherboards with parallel ports wired to the south bridge because USB adaptors can’t guarantee timing and swapping to a PCIe card would require rewriting some old stuff. However it has to be done at the driver level, it wasn’t Postscript or GCode or something like that, so I doubt this piece of gear will be anywhere as useful as parallel ports used to be. [1] https://news.ycombinator.com/item?id=32884814 https://news.ycombinator.com/item?id=32884814

memcg 4y ago

I remember trying to interface a PCs Limited (original Dell) parallel port to a Slo Syn paper tape reader on Excellon PCB drills and routers around 1995.