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CIRCUIT DESCRIPTION
coupler diode across this line and the MRET line. If the MEN signal causes the opto-coupler to conduct,
the module is enabled and its I/O channels can be energized.
•
MRET (Master Enable Return) This redundancy signal line is bused across the backplane and
continues through the IOBUS Shunt (I/O Bus Continue) and connects to the IOBUS A/B IN/OUT
connectors and screw terminals. The MRET line is terminated when an IOBUS Shunt is in the "I/O Bus
Terminate" position. The line is tied to common in the originating control module and provides a
"ground-loop free" return for each I/O module.
•
OKIN (and OKOUT) This redundancy signal line is daisy-chained from module connector to module
connector across the backplane as a serial input (OK IN) to a module and serial output (OK OUT) from
the same module. The OK OUT from a module becomes the OK IN to the next module in the chain. It
continues through IOBUS Shunt (I/O Bus Continue) and connects to the IOBUS A/B IN/OUT
connectors and screw terminals. Each module has an opto-isolator transistor that connects the OKIN and
OKOUT lines, if the module is OK, forming one long continuity loop. The signal (1.4 mA constant
current source) originates in the control module and loops back onto the MOK line at the right-hand
termination point of the IOBUS. The OKIN/OUT signal is jumpered to the MOK line by an IOBUS
Shunt in the :I/O Bus Terminate" position or by an installed OK Loop jumper. The control module reads
the state of MOK to determine if any of the modules on IOBUS are not OK.
•
MOK (Master OK or Module OK) This redundancy signal return line gives the result of the OK
continuity string. The line is bused across the backplane and continues through the IOBUS Shunt (I/O
Bus Continue) and connects to the IOBUS A/B IN/OUT connectors. The line is isolated from the
IOBUS A/B IN/OUT connectors and screw terminals and is jumpered to the OKOUT redundancy line
when an IOBUS shunt is in the "I/O Bus Terminate" position. When this line is open, the control module
is signaled that one or more modules are not OK.
4.3 DC Power Bus
The Remote I/O Rack has a tripled DC power bus. DC power is supplied to the rack from one to three
external power supplies. They connect to the rack via backplane connectors P1, P2, and P3 to supply 24 Vdc
to the backplane. The tripled bus is arranged as A BUS, B BUS, and C BUS from which installed I/O
modules receive operating power.
Each DC bus is tapped by an I/O module through separate pins in the module's backplane connector (P1) and
applied to the anodes of A BUS, B BUS, and C BUS diodes located within the module's circuitry. The
cathodes of all three diodes are connected (diode ORed) to create a common power bus for the circuit card.
When redundant power supplies are installed, diode OR-ing assures continued operating power in the event
of power supply failure or removal.
4-2
SD39RIO-1
n
January 2001
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