On Thu, 13 Jun 1996, pic wrote:

> I'm looking at designing a multi drop comm's system for control purposes.
> I have been investigating RS485, and RS422 keeps on coming up along with it,
> so I know there can't be much difference between them, but what is the
> difference ?

RS485 is a true multi-drop setup - I believe the spec is 32
drivers/receivers on a twisted pair bus (about a mile long?)?

RS422 is meant for single send/receive pairs - no multidrop.

To meet RS485 automatically qualifies the chip for use as RS422.  So,
manufactures - being frugal - make one chip in high quantities (and at
cheaper cost) than two different chips.  Personally, I am not aware of a
commercially 422 only chip you can buy (I believe you can buy 422
pre-assembled boards though - even though you can't use them as 485 -
they use the 485 chips).

My source on info comes from the National Semiconductor Communications
Data Book - I like the SN75176 chip in particular - cheap from DigiKey
and well designed.

If you order from DigiKey - you can pay an extra buck and get the data
sheet for the chip.


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