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GFK-0825F Chapter 4 Operation 4-11
4
Outputs from the Host to the BIU
Each time the host's Bus Controller has the bus communications token, it sends any outputs it has
received from the CPU to the devices on the bus. Outputs for the Bus Interface Unit are sent in one
output data message, with all configured discrete outputs followed by all configured analog
outputs.
Output Data Message
(up to 128 bytes)
discrete outputs
analog outputs
Configured Q Length Configured AQ Length
AQ starting reference dataQ starting reference data
To
Station
The data lengths are equal to the configured lengths of Q and AQ data selected for the Bus
Interface Unit (regardless of the host CPU type or the actual amount of output data needed for the
modules physically present in the station). Either length may be zero.
Output Data Format
The output data format shown above is required by the BIU.
When generating the output data message, a Series 90-70 Bus Controller automatically places the
Q data ahead of the AQ data. Other types of host must send an output message consisting of the
correct number of bytes of discrete output data followed by the correct number of bytes of analog
output data.
As soon as new output data is received, the Bus Interface Unit checks it to be sure the data is error-
free and of the correct length. The length must match the combined lengths of the discrete output
data and analog output data that have been configured for the Bus Interface Unit's network I/O
map. After verifying the accuracy of the data, the Bus Interface Unit puts the data in its Q and Q
tables, and begins passing it to the output modules in the station. Each discrete output module
receives one byte of data for each eight circuits. Each analog output module receives two bytes
(one word) for each analog channel.
Output Defaults
On powerup, all outputs go to their programmed default state, except outputs which had previously
been forced--they go immediately to their forced state or value.
If CPU communications are lost for 3 bus scans, all outputs that are not presently forced will either
default or hold their last state, as configured.
Conventional I/O modules can have a configured output default value. Intelligent modules default
outputs are always zero.