DESCRIPTION AND OPERATION
UNIVERSAL SLAVE EXPANDER BUS INTERFACE
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I-E96-313A
®
UNIVERSAL SLAVE EXPANDER BUS INTERFACE
The DSO uses a custom gate array to perform the slave
expander bus interface function. All the control logic and com-
munication protocol are built into an integrated circuit (IC).
This IC provides the following functions:
•
Address comparison and detection.
•
Function code latching and decoding.
•
Read strobe generation.
•
Data line filtering of bus signals.
•
On-board bus drivers.
MODULE DATA
FC 83 in the master module configuration accesses the DSO on
the slave expander bus. It also allows the master module to
automatically read status data from the slave module, and
write output data to it. The slave address in FC 83 must be the
same as the address set on the slave address dipswitch (S1).
Status Data
Status data consists of three separate 8-bit bytes read by the
master module. The first and second bytes contain readback
data that reflects the state of group A and group B outputs.
Each bit of this data corresponds to the digital output state
(ON or OFF).
The third byte is the module identification and module status.
Slave module identification is in the four most significant bits
(MSB). It identifies the slave module to the master module. It
also verifies the slave expander bus communication integrity
and the master module configuration. Slave module status is
the least significant bit (LSB). During initialization, the master
module writes data to the output register and default register,
and forces this bit to a logic 1. A reset or
time-out
(bus fault
error) resets it to a logic 0. Default data is normally written only
during master module start-up. The master module reads the
module status bit to test for a change in the state of the mod-
ule. It uses this bit as a signal to rewrite data to the module to
ensure that proper default values are in the default register.
Output Data
Output data is two 8-bit bytes sent to the output registers.
Each byte corresponds to group A and group B outputs. Each
bit of data represents one output. The bit value reflects the
state of the output. Logic 1 turns a field device ON; logic 0
turns it OFF.
