Replacing OIT on GE 90/70 network

The current system I am looking at has eight GE 90/70 PLCs networked together via Genius Bus Controllers (addresses 21-28). Additionally, there is an ELB 906 PCIM module (address 30) installed in a PC to provide the operator remote start/stop commands for the devices on the PLCs. As best I can tell the 8 PLCs and ELB share data via G type registers. I will be replacing the role of the PC/ELB module with a SCADA server. All logic with be retained in the PLCs.



So my question is how to I get my SCADA system to write/read G registers? More importantly, how can I do so while keeping the existing MMI operational until all PLCs can be tested on the new system and a complete cutover can be done.



My initial thought is to add a 90/70 PLC with a Genius Bus Controller on the network and give the GBC an address of 31. It is my understanding that I’d be able to talk from the SCADA server directly to the CPU via Modbus. Then I can program the GBC on this rack identical to the programming on the ELB except give it an address of 31. Then when the PLC is turned on it would take over primary control of the bus and the ELB would have backup control. At the end of the day I could just turn the PLC off and the ELB would take over. When testing is completely done then the ELB can be taken off the network.



Is this correct?

Anything I am missing?

Is there a better solution than adding a 90/70 PLC to the network?
Any potential problems with talking Modbus direct with a 90/70 CPU?

"I will be replacing the role of the PC/ELB module with a SCADA server"

Could you clear this up? Are you building a new HMI project for the system, which can log data yet provide an operator interface to retain the remote start/stop functionality you currently have? If so, software platform are you using? GE's own Cimplicity HMI Plant Edition? Something else?

So my question is how to I get my SCADA system to write/read G registers?

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If your SCADA has a GE Fanuc driver (SNP for serial communication, SRTP for ethernet), then it can write directly to %G memory. If you have to use Modbus, then you will need to add ladder logic. Modbus can only write to %R or %Q memory.

You don't have to go to the expense of another 90-70 and Genius Bus controller. You could use a 90-30.

You will probably have to modify the ladder logic in all of the existing PLCs if you want to have the existing PC/ELB and the new PLC on the Genius bus at the same time.

I expect that the way the system is currently configured, each of the PLCs plus the PC write to a different set of %G addresses. All of the PLCs can see all of the %G data. Each PLC's configuration sets up where it puts the data it receives from all of the other PLCs.

Take a simple example. Lets say that Genius ID 21 writes 16 bits of data to %G0001 - %G0016 and broadcasts that data on the Genius bus. The PLC at Genius ID 22 can receive that data and store it anywhere it wants to, but it would make sense to store it at %G0001 - %G0016. That way, you know that %G0001 - %G0016 in both PLCs originates in the PLC at Genius ID 21. In similar fashion, you could have the PLC at Genius ID 22 write to %G0017 - %G0032 and broadcast it on the Genius bus. Then the PLC at Genius ID 21 could receive that block of data and store it in its own %G0017 - %G0032 address space.

While you're trying to prove out you alternate SCADA, you won't be able to have both Genius ID 30 and Genius ID 31 writing to the same addresses in the PLCs at Genius IDs 21 - 28. You might be able to get away with it as long as both ID 30 and ID 31 were never present on the bus at the same time. That's if the software will let you store a configuration with two Genius different IDs writing to the same address.

Steve Bailey said:

If your SCADA has a GE Fanuc driver (SNP for serial communication, SRTP for ethernet), then it can write directly to %G memory. If you have to use Modbus, then you will need to add ladder logic. Modbus can only write to %R or %Q memory.

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No, I don't have a GE Fanuc driver. Writing to the %R and %Q and using ladder logic should satisfy my needs.

Steve Bailey said:

You don't have to go to the expense of another 90-70 and Genius Bus controller. You could use a 90-30.

Click to expand...

Good to know.

Steve Bailey said:

I expect that the way the system is currently configured, each of the PLCs plus the PC write to a different set of %G addresses. All of the PLCs can see all of the %G data. Each PLC's configuration sets up where it puts the data it receives from all of the other PLCs.

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Yes, the scheme is nearly exactly like the example you indicated.

I am assuming the %G will largely be used to access the %M registers from other PLCs. What about when reading the I/O data tables (%Q, %AI, etc) from the other PLCs? Will any special programming be required to access those registers? Or will the data I want need to moved to a %G register as well?

Steve Bailey said:

You will probably have to modify the ladder logic in all of the existing PLCs if you want to have the existing PC/ELB and the new PLC on the Genius bus at the same time....While you're trying to prove out you alternate SCADA, you won't be able to have both Genius ID 30 and Genius ID 31 writing to the same addresses in the PLCs at Genius IDs 21 - 28.

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My limited understanding is the backup addressing is 30 and the primary is 31. But, I am guessing based on your response that this is only relevant if I have two GBCs not a GBC and an ELB then.

Which brings me to another question. Lets say I have a primary CPU with GBC (31) on one rack and a backup CPU with GBC (30) on a second rack, and a remote scanner (29) with all the I/O modules on the rack. Under normal operation does the Primary and Secondary CPU both read I/O and write I/O to its respective data tables from the GBC, but only the Primary writes the registers from its datatable to the remote scanner. What I am basically trying to figure out is what information is availabe at the Secondary PLC data tables when it is in backup mode (no read or writes, or just the writes and no reads, or read and writes but they're not written to the remote scanner)

It sounds like you're only using the Genius network to exchange data among the PLCs and the PC/ELB. In that case, you really don't need to be concerned with primary/backup bus controllers.

The intent of primary/backup controllers is in a redundant controller setup. If you have real-world outputs wired to Genius blocks on a Genius bus, those outputs are turned on and off as commanded by the bus controller. If you have two bus controllers, the outputs "listen" to the commands from the bus controller at the highest Genius SBA (serial Bus Address). So if you have a bus controller at SBA 30 and another at SBA 31, the outputs will be controlled by SBA 31. If the bus controller at SBA 31 goes offline, control of the outputs will revert to the bus controller at SBA 30. The ELB module in the PC qualifies as a bus controller.

You don't have to worry about any of that when you're exchanging data among multiple PLCs, but it's the answer to the question you posed in your last paragraph.

In the configuration for the Genius bus controller in each 90-70 you have to set up a destination address for the data received from each of the other Genius devices on the bus. You also have to set up a source address for the data broadcast on the bus by that particular bus controller. That does not have to be %G memory. Using %G just may make it easier for someone who didn't write the program to recognize that a bit originated from some other device.

If you want to delve more deeply into the details of Genius global data, send me a personal message through this site and I'll send you my contact information.