8–4 P485 MODBUS TO PROFIBUS CONVERTER – USER GUIDE
ADVANCED FUNCTIONS
Input/output data during startup
DESCRIPTION This section is only relevant when the control/handshake registers are enabled.
Bit 13 in the control register is used to ensure data consistency during start-up and at
fieldbus off-line/on-line transitions. The bit should be treated as follows:
Figure 8-2: Input/output data during startup
When the fieldbus changes from off-line to on-line state, the fieldbus control system
should clear (0) the ‘data valid’ bit in the control register. The P485 will then clear the ‘data
valid’ bit in the status register.
During startup, the P485 waits for the fieldbus control system to set the ‘data valid’ bit in
the control register. Before this is done, it will not communicate with the devices on the
Modbus network.
The ‘data valid’ bit in the status register may in some cases be delayed. This latency can be
caused by a missing node or a bad connection to a node with a long timeout value
assigned to it.
Therefore, the fieldbus control system should not wait for this bit to be set before
communicating with the sub-network devices. It should be considered as an aid for the
fieldbus control system to know when all data has been updated.
As with all changes to these registers, the handshaking procedure (refer to Handshaking
procedure on page 8–3) must be followed.
Fieldbus control system P485
Updates the output data
area according to the
sub-network configuration
Sets bit 13 (data valid) in
the control register
Complete a full scan
on the sub-network
Sets bit 13 (data valid) in
the status register
Ready
(waiting for bit 13 (data valid)
in the control register to be set...)
NOTE
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