User`s guide

Page 78

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

DESIRED STATES

The desired states of the digital outputs are directly reflected in the bits in the table … which directly

reflect the coils (bits) in the holding registers in the actuator. This means that the bits still maintain

their original position in all the registers …

For example,

• Coil [3] … is in register [00] – bit [3] … and in tables [16 & 17] in bit position [3]

• Coil [4] … is in register [00] – bit [4] … and in tables [16 & 17] in bit position [4]

• Coil [5] … is in register [00] – bit [5] … and in tables [16 & 17] in bit position [5]

Therefore, the PLC must set/clear bits [3-5] to control the outputs.

For example,

• Coil [1000] … is in register [1004] – bit [0] … and in tables [16 & 17] in bit position [0]

• Coil [1005] … is in register [1004] – bit [5] … and in tables [16 & 17] in bit position [5]

Therefore, the PLC must set/clear bits [0,1,5] to control these outputs.

A read back of Table [16] will

• If a write command issued: reflect the status of the network write & network monitor commands

accepted by the 1746-C.

• If a read command issued: reflect the last status of the network write & network monitor

commands issued out to the network.

Notes on table [16] use:

1. The PLC can monitor when a write happened by viewing the upper nibble of the word for the

particular valve. When it changes from (1110 … 0xE)

 (1100 … 0xC) the write has

happened and the system is still monitoring the states of the relays.

2. The PLC can activate/deactivate monitoring these relays any time by setting/clearing bit [15]

(

see the “Command / Control” section above).

However, anytime the PLC writes a value to the coils in an actuator, monitoring is automatically

started … and will not terminate until the PLC terminates it.

Therefore, the PLC needs to monitor this and deactivate the operation when not needed

anymore … otherwise, the additional polling will slow (lengthen) network scan times.

3. The 1746-C uses a Modbus Command 05 to write multiple coils when writing values to the

actuators.

IMPORTANT NOTE

Since a single command is sent to the actuator that affects all discrete outputs on a

system at one time, it is important that the PLC be aware of the “state” of all coils on

the particular valve before sending a command because ALL OUTPUTS ON THE

VALVE will be affected EVERYTIME one of these commands is sent. This is why

• Monitoring can be activated without writing values.

• Monitoring remains active after a write until explicitly turned off.

By monitoring table [17], the PLC can ensure correct behavior.