Erik:

Disabling the transmit drivers may very well be necessary in a multi dropped RS422 environment. This is a totally valid scheme where, for instance a master may talk to multiple slaves via RS422. The slaves need to 3-state their drivers.

Thus your general comment that RS485 is the only environment where it is necessary to 3-state the drivers is not correct. In addition, if you look at common RS422 driver chips, you will notice that many of them come equipped with 3-state output control pins for just the reason I indicated.

A multi dropped RS422 interface scheme has some advantages in possible protocol implementations over RS485 networks in that the master can send high priority broadcast control commands to all slaves even if one of them is currently sending a reply to a previous command.

Michael Karas