In article , bp@www.zefox.net says...
>
> 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

First check your power and gnd wires between the two devices it may be
drawing more 5V via the USB-serial than the USB port can handle. Putting
the USB-serial in a funny mode.

Check the voltage levels on the output of the USB-SERIAL, when that is
the only device plugged in, to determine what power level it should be
operating on. Then try powering the Nulsom ns-rs232 level shifter from
the appropriate 3V3 or 5V pin on the Pi.

The PL2303TA can operate with signals at different output levels, the
VCC being 3V3 or 5V determines the signal levels it sends and receives
to your USB-Serial.


When you get the level shifter working, The printer probably uses MORE
signals at the printer end, as many RS232 printing devices did.

Research NULL Modem cable pinouts.

More than likely at the PRINTER end the 9 way or 25 way D type connector
will need links between the pins for

 DTR to DSR
 CTS to RTS

These are flow control signals to stop, pause or resume data flow from
computer. You might be able to get RTS/CTS handling by the USB-Serial
passed through, but from experience these do not always work well on
MANY USB-Serial adapters. See

http://www.nulsom.com/datasheet/NS-RS232_en.pdf

You still might lose characters at 9600 baud as you are ignoring the
Modem flow control signals, see if you can set the printer up in
XON/XOFF flow control mode, which the terminal software and device
driver will be able to handle.

If you still lose characters drop baud rate to 4800 then 2400 until you
get success.

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