USB-Serial communication giving strange output

Question

I'm trying to get data from a mercury analyzer (Seefelder-Messtechnik Hg Analyzer 3000) that gives output to a 9-pin R232 serial port to my OSX 10.10 laptop.

I've followed the steps described here to install the PL-2303 driver: http://pbxbook.com/other/mac-tty.html

The device manual (http://www.seefelder-messtechnik.com/V71-3-02-21e.pdf) lists the communication protocol as "9600 Baud, 8 data bit, 1 stop bit, no log, no parities".

I attempt to read from the device by using the 'screen' command:

screen /dev/tty.usbserial 9600

The result is a string of seemingly non-sensical characters that print to the screen in a regular interval:

�8b4����b��8b48bs��8G�8b�8���8������8����< 8�8��b��KW��\b����8b����b� �b�b����KW�K �8b��\G�� �<���8�8b�"��΁�[؁��؉���bG�3�ˁ�G��\K��[W�pb�8��΁8ʱ�\pa���ʁ�c t��8�h¡�38b�8�q�؁����\�8���bS�8b8�8�q���X��8��<��£8���2�8�����ؖ�ؖ�ؖ�8bS��\�܉�ؖ����[S�8��s���fq�8�����������8fq����������S�܊��b���b�؉����\���S��K���ݎ����S��b��b��S����S�\������KS��S�؊��\S�1S�\b�S�؉�\�ذ����KS�\����S����bS�؉�����1S�؊��[؂����ز������؉\�؂��ز��\����i���$\�$���\��8���$��\�\����܂�زXk�B��7��\k�\X�<��8Xkz��Yj��L�������H�\���]j�،k:��Yj�؈��

I've also tried using 'minicom' rather than screen, and get a different ("?]???ܰ??Yk??2"), but also non-sensical result. I saw that there was another SO query similar to mine that remains unsolved: weird characters displayed during serial communication OSX

Any tips? It looks to me that I'm not interpreting the output correctly, but I don't know what to try next.

Answer 1

The solution was to read from the machine at a higher baud rate (~57600), despite what the manual and online reference said. Reading at 57600 baud made the result plain-text and usable. Thanks for your ideas!

Answer 2

I've followed the steps described here to install the PL-2303 driver

I've also had occasional electrical ground problems with Prolific USB-RS232 adapters. Problem would manifest as garbled data that looked similar to a baud rate issue or what you posted.

You can check if it's a ground issue by measuring for continuity between the ground pin (pin #5) on the DE-9 (aka DB-9) side of the Prolific adapter and the ground pin of the USB side (pin #4, "far left", of the A connector). You'll probably measure infinite resistance with a multimeter. Try the same with a FTDI USB-RS232 adapter, and instead I get a dead short between the ground pins as expected.

Be sure to plug the instrument's and PC's power supplies into the same power strip.
As a last resort try grounding the instrument's chassis/case with the PC using copper wire