Meanlng Of Handshake Bit - GE Profibus - DP Bus Instructions Manual

3.2. MEANING OF HANDSHAKE BIT
The Handshake bit avoids that the same service request is 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 here 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 SOOmSec,
the Master shall send a no-request service (all Bits are set to 0), thus allowing the Slave to perform a commu-
nication Reset. This causes the reset of the Slave Handshake Bit.
The Timeout for the service towards the DGFC option shall be longer (1,5 set). The Master should therefore
have two different Timeouts: one for the services towards the drive and one for those towards the DGFC option.
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7 Rxl
BYTE ASSIGNMENT FPR DATA EXCHANGE