Micromod Micro-DCI: 53MC5000 PLC AND PRINTER INTERFACES - Manual (Page 78)

Table 6-6. PCS Modbus Master Fault Isolation Aids
RS-232/485 ITB Activity Indicators
Inactivity from the XMT (CR13) and RCV (CR14) LEDs could indicate line problems, a hung
device, a misconfiguration between the PCS and PLC resulting from manual database alterations
made at either device, or just no DDI channel activity. Active indicators on the RS-232/485 ITB
do not necessarily mean error free operation, e.g., repeated PCS-PLC transactions attempted
with timeout errors.
A hung PLC can be cleared with the Diagnostic command (Function Code = 8) and the Restart
Communications Option Diagnostic Code 00 01.
6.5 PCS MODBUS SLAVE OPERATION
In Modbus slave operation, the PCS does not initiate any read or write commands, but only re-
sponds to commands it receives. PCS Modbus slave operation is defined as operating with zero L-
bytes and C-words to read, and zero L-bytes and C-words to write.
6.5.1 PCS MODBUS SLAVE MEMORY MAP
The PCS Modbus slave memory map is the same for the DDI-A or DDI-B channels. It has an L-
value range of L0000 - L2047 (2048 coils) and a C-value range of C000 - C767 (768 holding regis-
ters). It is unlike PCS master mode operation, which has dedicated L- and C-memory areas in
memory that serve as transfer buffers to other PCS memory areas under F-TRAN control. The
PCS Modbus slave provides access to all of the L- and C-values (coils and holding registers); how-
ever, a single read of slave memory cannot exceed 512 L-values or 64 C-values. Access to all of
the L- and C-values allows them to be altered by any unit functioning as a Modbus master; there-
fore, PCS control operation can be affected. Prudent judgement should be used when altering
PCS memory contents that have an affect on process control. A Modbus master can send data to
a PCS slave so that it can be viewed on the status displays (status display information is provided
in the PCS instruction bulletin referenced in the Preface of this manual). Also, it should be noted
that the L- and C-memory areas of a DDI channel in Modbus master mode are open to changes
from an external Modbus device functioning as a master if the other DDI channel is in Modbus
slave mode (e.g., if DDI-A is functioning as a slave and DDI-B is functioning as a master, then the
master device connected to DDI-A can send data that overwrites the memory areas used by DDI-
B).
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