GE Industrial Solutions 6KCV301PDP33 Profibus - DP Bus for AV300i - Manual (Page 16)

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GEI-100422
3.2 MEANING OF HANDSHAKE
The Handshake bit prevents the same service request from being carried out
more than once and its function is the same both in the direction from Master
to Slave and from Slave to Master. The explanation below refers to the direction
Master to Slave, but the same considerations may be applied for the opposite
direction.
The default value of this Bit is 0. Every time that a transition of this Bit occurs,
both from 0 to 1 (positive edge) and from 1 to 0 (negative edge), the Slave
carries out the service requested by the Master through the data frame previously
set. Therefore, this Bit acts as a Trigger, through which the Master indicates
the Slave that the data for the requested service are ready.
The Slave responds to the Master in the same way, by causing a Handshake Bit
transition (both positive or negative).
Consequently, the Master is able to send a service on the Bus only if its
Handshake Bit is equal to the one received by the Slave.
During the initialization and in case the Master does not receive the Slave
response within a Timeout of 500 mSec, the Master shall send a no-request
service (all Bits are set to 0), thus allowing the Slave to perform a
communication Reset. This causes the reset of the Slave Handshake Bit.
The Timeout for the service towards the DGF option shall be longer (1.5 sec).
The Master should therefore have two different Timeouts: one for the services
towards the drive and one for those towards the DGF option.