GFK-0825F Chapter 4 Operation 4-9
4
Data Transfer Between the BIU and the Host
Each bus scan, a Bus Inter fa ce Un it exchanges the followin g data with the host:
It sends an input message with up to 128 bytes of discrete and/or analog inputs.
It receives an output mes sa ge with up to 128 bytes of discret e an d/or analog outputs.
The exact length of these messages is determined by the network I/O map configured for the Bus
Interface Unit.

Data in the BIU's Network (Bus) Map

As mentioned previously, the BIU has 256 bytes of memory available for each of the four types of
data (discrete and analog inputs and outputs: I, Q, AI, and AQ). For each data type, up to half (128)
of the 256 bytes can be used for the data that will be exchanged on the Genius bus. During BIU
configuration, starting addresses and lengths are set up for each of the BIU's four types of I/O
memory. These addresses an d lengths define the BIU's net work I/O map. The data from each ta ble
(I, AI, Q, and AQ) must be contiguous.
BIU Memory Type
Designation Amount Available in BIU M aximum Length for
Network Map
I 256 bytes up to 1024 contiguous bits
Q 256 bytes up to 1024 contiguous bits
AI 256 bytes up to 64 contiguous words
AQ 256 bytes up to 64 contiguous words
Note that although it is possible to configure up to 128 bytes of memory in the network map for
each data type, the maximum amount that can be included in the map is a total of 128 bytes of
inputs (I plus AI) and 128 bytes of outputs (Q plus AQ).
Data to be exchanged on th e bus mu s t be con fi gured to use references wit h in the network map
configured for the BIU. Data of individual I/O modules may be configured anywhere within
available memory. It is possible to have inputs or outputs within the I/O station that are not
exchanged on the Genius bus-that is, data that is completely local to the I/O station. For example,
the I/O station might include a Micro Field Processor performing local data processing.

Communications on the Genius Bus

After a Bus Interface Unit completes a successful login with the host, it begins sending input data
on the bus and accepting out p u t data from the bus. The BIU communica tes on the bus repetitivel y
and asynchronously. When th e BIU receives the Genius bus comm unications token, it sen ds the
most recent data from the con figured portion of its I and AI m emories on the bus. As mentioned,
these memories are repeatedly updated whenever the Bus Interface Unit scans the input modules in
the station.
The Bus Interface Unit recei ves new outputs from the h ost when the host's bus controll er ha s the
communications token . It places these outputs int o th e configured portion of its Q an d AQ ou t put
tables. These outputs ar e then passed to the devices in th e st ation on the BIU's next I/O scan.