I'm trying to use a pl2303ta usb-serial adapter with a nulsom
ns-rs232 level shifter. The goal is to connect an RPi3 to an old
Graphtec MP3100 pen plotter.

On its own, with tx looped back to rx using a resistor, the pl2303ta
seems to work fine; opening up a screen session at 9600 baud  allows
me to type with no problem, everything I type shows up on the terminal
window.

When I interpose the Nulsom ns-rs232 level shifter and loop pins 2 and 3
back, about five characters can be typed, then characters quit showing up
in the terminal window. Screen does not exit, just quits working. Unplugging
the USB connectors does cause screen to exit, plugging the connector back
in lets me restart screen and type a few more characters.

Dmesg reports

[304143.580388] usb 1-1.5: New USB device found, idVendor=067b, idProduct=2303
[304143.580398] usb 1-1.5: New USB device strings: Mfr=1, Product=2,
SerialNumber=0
[304143.580402] usb 1-1.5: Product: USB-Serial Controller
[304143.580406] usb 1-1.5: Manufacturer: Prolific Technology Inc.
[304143.581172] pl2303 1-1.5:1.0: pl2303 converter detected
[304143.586617] usb 1-1.5: pl2303 converter now attached to ttyUSB0
[304160.462033] pl2303 ttyUSB0: pl2303_get_line_request - failed: -32


Might this be a driver problem? Most references to the pl2303 are to
other, presumably older, chips. Uname -a reports
Linux raspberrypi 4.9.35-v7+ #1014 SMP Fri Jun 30 14:47:43 BST 2017 armv7l
GNU/Linux
so the OS is as up-to-date as I know how to make it.

Thanks for reading, and any ideas.

bob prohaska

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