ABB Symphony Harmony Series, Instruction, Page: 141 / 402

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I/O Block
The Hnet control section also provides a means for the CPU to
interrogate the Hnet to determine its condition. If a bus failure
or stall is detected on either of the Hnet channels, the channel
will be shut down locally until the problem is corrected. The
capability to detect a bus failure allows the I/O module to set
the appropriate output channel default states if communica-
tion with the Harmony controller is completely lost.
The signal relays (A and B) allow the I/O module to isolate itself
from the Hnet in the event of a component failure or to perform
diagnostics. This important feature assures that a fault in one
block cannot affect the communications of other blocks. The
Hnet interface circuitry includes the logic that controls the
relays.
Machine Fault Timer
The machine fault timer (MFT) is a security feature built into
the I/O module. The timer is a one-shot timer that must be
periodically reset by the CPU to prevent it from timing out. If an
error condition exists that causes the I/O module to fail or
operate incorrectly, the timer will not be reset and will cause a
time-out condition. A time-out condition triggers a reset signal
to shut down the I/O module.
The I/O module performs a series of online diagnostics to ver-
ify circuit integrity. A detected hardware failure may cause a
time-out condition. If the cause of the problem is not a hard-
ware failure such as a configuration error, the I/O module and
timer can be reset by the manual reset switch accessed
through the block front panel. A hardware failure will most
likely require an I/O module replacement.
Serial Link Interface
Besides the Hnet interface, an I/O module supports an addi-
tional external serial link used for either redundancy or man-
ual/auto station communication. The CIO-100 block uses the
serial channel for station link. All other blocks use the serial
channel for redundancy link.
Figure 3-10 details the serial link interface. The I/O module
uses a dual universal asynchronous receiver/transmitter
(DUART) circuit to direct data transfer over the serial links.
The DUART receives parallel data from the CPU and converts it
to serial data that it sends to one of the serial link interfaces. It