Addressing; Rs-232 Point-To-Point Configuration - Honeywell CIU 888 Protocol Manual

Part No.: 4417589_Rev13
CIU Emulation
in principle, a CIU Prime or a CIU 888 when it has CIU emulation

enabled
The downlink option is especially useful in situations where there are
CIU 858s in the field that act as switches and are spread across the
site. In this situation, only the 'main' CIU (probably a CIU 880) need to
be replaced by a CIU 888 leaving the CIU 858s intact.

1.3 Addressing

1.3.1 RS-232 point-to-point configuration

When a host connected to the CIU 888 sends a GPU message (i.e.
request) to a gauge, the CIU 888 interprets the message and tries to
determine which field port the gauge is connected to.
In order to find out this, the CIU 888 takes the first digit of the three-digit
gauge address (e.g. 402BB) in the request and compares it with the last
digit of the RTU address (e.g. 54) defined for the host port. If the digits
are not equal, the CIU 888 sends the request (inclusive CIU address) to
all configured field ports that support the GPU protocol (see FIGURE 1-
6).
If, on the other hand, those digits match, the CIU 888 uses the second
and third digit of the gauge address (02 in the example above) to
determine if the gauge is already configured in the site database. If this
is the case, the CIU 888 is able to determine the field port where the
gauge is connected to and forwards the request to the appropriate field
port. A request to a gauge address that cannot be found in the database
is broadcast to all field lines.
Notice that when cache mode is enabled and the received request is
also part of the regular scanned items, the CIU 888 will reply with the
last value stored in the database and will not forward the request to the
field. In this way, faster responses are provided to the host and
unnecessary overloads in the field scans are avoided. If GPU
messages are addressed to gauges underneath a downlink CIU,
another extra digit will be found in the request; this is: first digit for CIU
address, second digit for downlink CIU, third and fourth for gauge
address. Hence, the CIU 888 forwards the GPU message to the
downlink CIU, which in turn forwards the message to the addressed
gauge. The responses are transferred back up in a similar manner.
Protocol Manual - CIU Emulation
CIU 888 1 - 7