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AC7A/B User’s Guide
USING MULTIPLE AC7A/B UNITS ON ONE LINK
Background
Optomux uses a 4-wire RS-422/485 communication system. Technically, this 4-wire RS-422/485 link is 2 links using
the same 2-pair communication cable.
The pair of wires connected from the AC7A/B RS-422/485 transmitter to the Optomux receivers represents the
RS-422 portion. With RS-422, there can only be one transmitter on a link (twisted pair) because the RS-422
transmitter is always enabled. RS-422 requires termination, but not biasing.
The pair of wires connected to the AC7A/B RS-422/485 receiver from the Optomux transmitters represents the
RS-485 portion. With RS-485, there can be multiple transmitters on the same link (twisted pair) because the
RS-485 transmitters are always disabled when not actively transmitting. RS-485 requires both termination and
biasing.
The AC7A/B was designed as an RS-232 to RS-422/485 converter to be used between a computer and an Optomux
I/O system. This assumes there will only be one AC7A/B on the link and that the AC7A/B RS-422/485 transmitter
is only connected to Optomux brain receivers. This means that the AC7A transmitter can be used with the default
configuration of being RS-422. This configuration does not bias the RS-422 link, and the AC7A transmitter stays
enabled at all times, even when not transmitting. Because of this, there can only be one transmitter on this (RS-422)
twisted pair.
Description
Whenever there are multiple AC7A/Bs on a link, special configuration of the AC7A/B’s is required. With multiple
AC7A/Bs in the system, there will be multiple transmitters connected on the RS-422 portion of some or all of the
AC7A/B’s. In order for this to work, each AC7A/B must be configured so that the RS-422 portion is setup in RS-485
mode. In other words, the RS-422 transmitters on the AC7A/B’s must be configured to be disabled when not
actively transmitting in order to prevent interference with the other AC7A/B RS-422 transmitters.
In addition, because the AC7A/B’s transmitters are now connected on an RS-485 link, the termination and biasing
requirements for the RS-422/485 transmitter side of the AC7A/B’s are different than when the AC7A/B’s are used
in their default configuration. The twisted pair cable connected to the transmitter side of the AC7A/B’s must be
biased in one location only and terminated at both ends of the link.
