I re-read datasheet for 75C185 and diodes are recomended for
output's protection. On other hand - there is no schema of
internal structure . Such schema is provided in datasheet for
receiver 75189. Input stage is shown as resistor (4k) and
diode (after resistor) with grounded anode. But these ICs are monolithic , so what happens in power-up situations is not
very clear.
I remember one ST75C185B RS232 translator - receivers was working without negative supply (GND used instead) and
was handling non-standard input levels - 0V/+5V instead -5V/+5V.
Drivers was workin fine too, producing same non-standard output.
After time chip dissapeared from market and dasheet dissapeared from www.ST.com too, even in obsolete parts section!!
regards