User`s guide

Page 38

Controlinc 1746-C (Version 5.21) Network Master Users Guide (2004-11-18)

5.2.4. Word [3]: Emergency Shut Down (ESD) Command Word & Indication

The ESD command is a command broadcasted to all actuators connected to the network that an

emergency is in effect and the actuator should start performing the ESD actions its pre-configured for.

The 1746-C broadcasts an ESD on both channels with a 200-millisecond delay between them.

If the PLC wishes to engage the ESD for the network, it writes a 255 (0x00FF) to this word location

(Word [3]) using a write to any table. The Network Master then broadcasts the appropriate ESD

command to all valves on the network (on both channels with about a 200ms delay between each).

When the PLC wants to end the ESD, it writes a 170 (0x00AA) into this word into any table. The

Network Master will then broadcast the appropriate “Stand-Down from ESD” command to all valves on

the network on both channels with about a 200ms delay between each.

Later, after the PLC is satisfied that the ESD is over and normal operations have resumed (stand down

from ESD has completed), it is recommended that the PLC clear this stand-down value (and all aspects

of ESD) by resetting it to zero (again, in any table).

Summary: The appropriate values for the ESD command word from the PLC are:

• 255 (0x00FF) = Engage ESD

• 170 (0x00AA) = Stand down from ESD

• 0 = Emergency shut-down (ESD) control is not active

Once any ESD command is written to any table, the value is propagated throughout all tables.

Therefore, the RLL does not have to remember which one it wrote to.

Warning: Since the ESD is actually a function of the actuator control cards; if an actuator is reset then

that actuator will probably not be in ESD. This also means that if the 1746-C loses power or is

reset, it has no prior knowledge of any valves in ESD. It can turn off ESD once initialization of

the software has occurred. It is the responsibility of the RLL to repeat the ESD broadcast

under the possibility of these circumstances.

Warning: If any or some of the actuators are configured to move on ESD, consider the electrical

power required in performing a network ESD. A large electrical power drop can possibly create a

reset in the actuator by affecting the control voltage.

Note 1: Remember, an ESD command from the PLC will inform the actuators to do what ever they

were configured to do during an ESD condition. Therefore, this could easily override any other

control commands already sent to them or about to be sent (ex: commands placed in any write

Tables [3, 5, 8,] might be rejected by the actuator because of the ESD condition).

Note 2: If monitoring the network, the values of 0xAA & 0xFF are not the values transmitted to the

actuators. The program actually uses a Modbus command 05 to set/clear a bit (coil) in the

do when ESD is either started or ended.