Vetri,
To some extent I am guessing as I am most familiar with the MAX487 chip
but: it looks to me as if the 488 chip is designed to work only on
terminated (~110ohm) lines, while the 489 will work on unterminated ones.
It may therefore also include controlled slewrate driver that reduces the
higher frequency components on the line and assists unterminated operation
(as does the 487). If the line is terminated then power requirements
increase due to losses in the terminating resistors.
So the 489 disable function is probably  a power saving mode.

In either case, it should be fine to connect a number of devices in
parallel, as this is a basic requirement  of RS485 communication.

Richard P



Hi,
I am working on a design which uses four PIC16F877 at different
locations and communicate to a PC using RS485.
I am planning to use MAX chips on each microcontroller board and have
them place the data on the common data line to PC only when they are
addressed by the PC. On the PC side we plan to use MAX232 and a
MAX488E/MAX489E chip.

When I read through the data sheet for MAX488E/489E chips, I could not
understand the distinction between MAX488E and MAX489E chips.
I see from the data sheet that for all these chips their outputs of
drivers are short circuit protected and have thermal shutdown. However,
for MAX488E their driver can not be put in to High Z line (no
enable/disable pins). If this is the case, is it correct to connect the
driver outputs of several MAX488E to the same pair of lines?

Why do MAX489E chips have a separate driver enable/disable line even
though their driver outputs are also similar (short circuit protected
and thermal shutdown) to those of MAX488E chips?

Thanks in advance.
Vetri



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