WorldFIP - Computer Busses - page 318

• RP_RQ (response request transfer). When a station that has made a variable transfer
request receives an ID_RQ frame it responds with an RP_RQ frame coded as follows.
The DATA field contains a list of identifiers, each of 16 bits (it can thus store up to 64
identifiers).
• RP_MSG_xxx (response message transfer). When a station that has made a message
transfer request receives an ID_MSG frame it responds with an RP_MSG_NOACK or an
RP_MSG_ACK frame coded as follows. The Data field contains a 3-byte destination
address, a 3-byte source address and, up to, 256 message bytes. A bit in the control field
indicates if the message transfer is acknowledged or unacknowledged. The destination
and source fields show the addresses of the communicating entities.
• RP_ACK (response acknowledgement transfer). When a destination station receives a
message with a request for acknowledgement it transmits an acknowledgement frame. No
data is transmitted in the DATA field. This frame is very short, as the acknowledgement
information is contained in the control field.
• RP_FIN (end-of message transaction response frame). When a message has been
transmitted the sender, after waiting for an acknowledgement if necessary, transmits an
end of message transaction frame. No data is transmitted in the DATA field. This frame
is very short, as the transaction finished information is contained in the control field.

WorldFIP involves the sending of an ID_DAT, followed by an RP_DAT. The RP_DAT
frame must be send within a given time interval. This is called the turnaround time. Typically
the turnaround time is between 10 bit and 70 bit transmissions. Thus at 1 Mbps the time to
transmit a single bit will be 1 µ s, and the turnaround time will be between 10 µ s and 70 µ s.

20.3.3 Medium allocation mechanism

The WorldFIP network automatically achieves bus arbitration and production/consumption
functions. The bus arbitrator (BA) contains the resources required to scan for variables when
they are required. This involves a scanning table with a list of identifiers to circulate on the
bus. It does this by:

• Broadcasting the name of the variable by sending an ID_DAT frame. This is read by all
the devices on the bus. One of these identifies itself as being the producer of the variable.
One or more other stations can recognise that they are consumers of the variable.
• Next the single producer of the variable transmits the value of the variable within a
response frame (RP_DAT). All the consumers then read this.
• The arbitrator next goes to the next identifier, and follows the same sequence.

20.3.4 Bus arbitrating tables
The bus arbitrating table contains the variable identifier, the periodicity, the scanning time
period, the data type and the conversation time:

Variable Period(ms) Type Time ( µ s)

TEMP_1 5 INT_8 170

TEMP_2 10 INT_16 178

PRES_1 15 OSTR_32 418

PRES_2 20 SFPOINT 194