Applicable Model(s)

HMI500 Series

Title

Connect Multiple Silver Series OITs to One PLC

Date

08/19/2008

Rev

02

P/N

0907-1075

Summary

The Silver Series has a built-in feature which allows multiple OITs to connect together in a daisychain fashion. One of the OITs is designated as the ‘master’ while the other OITs are defined as ‘slaves’. The master OIT is directly connected to the PLC communications port. Each slave OIT then communicates through the master OIT to get data from the PLC. No additional communications modules are required which provides an effective and inexpensive way to connect multiple OITs to a single PLC.


Solution

Hardware Connection

It is necessary that the OITs are wired together in the following fashion:

More detailed view of the OIT to OIT connections:

For HMI520, HMI530 and HMI550:

For HMI504:

The OITs are connected in a daisy-chain fashion using RS232 communications. This means that if one of the OITs loses power all other OITs further down the chain will be unable to communicate to the PLC.

NOTE: The maximum distance between OITs is 50 feet.

Software Connection

You must configure the OIT that connects to the PLC as the master. All other OITs should be configured as slave OITs.

To configure the Master OIT

1.

Click on Edit, System Parameters and select the PLC tab.


2.

Select Master from the Multiple HMI box.


3.

Select either 38400 or 115200 for the HMIHMI link speed. Make sure this setting matches the settings for the slave OITs.


4.

Select a unique station number in the HMI station no. box and click OK.

To configure the Slave OIT

1.

Click the Edit, System Parameters and select the PLC tab.


2.

Select Slave from the Multiple HMI box.


3.

Select either 38400 or 115200 for the HMIHMI link speed. Make sure this setting matches
the setting for the master OIT.


4.

Select a unique station number in the HMI station no. box for each slave OIT and click OK.

Sharing Data between the Master OIT, Slave OIT & PLC

Master – PLC
The master OIT will be programmed just as if it were the only OIT connected to the PLC.

Master – Slave
Any local word registers (LW) or local bits (LB) located in the master OIT can be accessed and written to by the slave OITs. Do this by using the Ms_LW and Ms_LB memory registers. For example, if the master OIT is storing data in LW10, the slave OIT can access that data by using Ms_LW10.

Slave – PLC
The slave OIT’s may access data in the PLC in 2 ways:

1.

It may read/write directly to the PLC addresses (as though it were the only OIT connected to the PLC).

-OR-

2.

It may write to the local words/bits in the master OIT (via Ms_LW, & Ms_LB registers) and those values may be transferred to the PLC by setting up a data transfer object in the master OIT program.

NOTE: If the master and the slave OIT’s are all accessing the same areas of memory in the PLC, this method is more efficient because the time required to get information from the PLC is reduced.

Using RS-232/RS-485 Converters

If it is necessary to place 2 slave OIT’s more than 50 feet from the master OIT (or an upstream slave OIT), it will be necessary to use a pair of RS-232/RS-485 converters (one at each OIT).

Please note the following:

  • The Silver Series do not provide any handshaking signals. Therefore it may not be possible
    to use 2-wire RS-485.
  • Use isolated converters so that ground loops will not cause problems.
  • Test the Master/Slave communications using RS-232 before installing the converters.

Troubleshooting

Error MessageProblemSolution
PLC No Response
(on Master OIT)
– Cables not connected correctly.
– Correct cables not used.
– System Parameters not configured correctly.
– Review cable diagrams above. Do not use the AUX port.
– See detailed cable diagram.
– Verify system parameters. Multiple HMI box should say “master”.
PLC NO RESPONSE
(on SLave OIT)
– Cables not connected correctly to Master OIT.
– Master-Slave cables are > 50ft.
– System Parameters not configured correctly.
– Review cable connection to master.
– Move slave closer to master.
– Slave needs own HMI # in
System Parameters. Also
verify that HMI-HMI link
speed matches master’s link
speed.
PLC Response Error…Attach PLC…– PLC program doesn’t recognize bits/words being transferred.
– System parameters have “Multiple HMI’s” disabled on PLC tab.
– Check to make sure that PLC
program reflects all data
being transferred.
– Remove all Data Transfers &
then add back in one at a
time until one causes error to
reappear.
This PLC Only Support Read (12) words of Word
data
OIT transferring more words
than PLC can handle
Lessen the # of words being
transferred in the data
transfer.

More Information

For more information regarding connecting multiple OITs, please refer to Chapter 12, Connection
Multiple OITs to one PLC in the Silver Series Installation and Operation Manual that is supplied
on CD ROM with the EZware-500 configuration software.

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