GE P485 instruction manual Input/output data during startup, Fieldbus control system

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:

Fieldbus control system P485

Updates the output data area according to the sub-network configuration

(waiting for bit 13 (data valid) in the control register to be set...)

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

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.

NOTE

8–4 P485 MODBUS TO PROFIBUS CONVERTER – USER GUIDE