ABB S800 I/O User Manual page 18

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Each I/O module is installed on a Module Termination Unit (MTU). The first MTU
with its I/O module connects to the FCI or a cluster's ModuleBus Modem and then
each of the remaining MTUs connect to the previous MTU. With redundant FCIs,
the ModuleBus connects to the ModuleBus Interconnection Unit.
An FCI communicates with its I/O modules over the ModuleBus. The ModuleBus
can be divided into 8 clusters, one base cluster and up to 7 I/O cluster. The base
cluster consists of the FCI and I/O modules. Additional I/O clusters (1 to 7) consist
of a ModuleBus Modem and I/O modules. The ModuleBus Modems are connected
via optical cables to an optional ModuleBus Optical Port module on the FCI.
Within a cluster the ModuleBus is made up of increments that are integrated into
each MTU. The FCI and ModuleBus Modems have a ModuleBus outlet connector
to connect to an MTU. An MTU has a bus inlet and a bus outlet connector. By
adding, on the DIN rail, an MTU to a FCI or a ModuleBus Modem, the bus is
automatically expanded up to a maximum of 12 MTUs. Unique position codes are
automatically assigned to each MTU as the bus is expanded. An inserted I/O
module is assigned the unique position identity of its MTU. Through the
incremental bus design the physical size of an S800 I/O installation is directly
proportional to the number of installed MTUs.
The S800 I/O modules can be inserted and removed from MTUs without disturbing
system operation. The physical lock, which locks an I/O module to its MTU, allows
I/O module removal only when the lock is in its unlock position. The locking
mechanism also acts as a logic lock so that an I/O module is only operational when
the lock is in the locked position. If the lock is in its unlocked position, output
channels are de-energized and I/O modules can be inserted/removed without need to
remove system or field power.
8 3BSE 020 923R101