GE Fanuc 90-30 series CPU310

I'd like to add a second HMI to this system, the original being an IC754VSI12CTD-HH connected to the CPU310 via a DB25 connector at the HMI and a DB15 connector at the com 2 port on the CPU310. The DB15 connector has a 120 Ohm resistor between the Tx+ and Tx- leads for two wire RS485 communications. The DB25 end has the necessary 120 Ohm resistor between the Rx+ and Rx- leads. For proper communications I'll need to remove the resistor in the DB15 connector, add a splitter/pass through with a resistor between the Tx leads and then the main HMI should act as one end of the RS485 drop network, the splitter/pass through as the other end at the CPU310, and I can add the second HMI as a drop of the network?

CPU310??? I've been dealing with the 90-30 since it was introduced and I've never heard of that model.

Regardless, the SNP protocol uses a master/slave model. The HMI is the master. You can have multiple slaves of the network but only one master.

To add another HMI, use the other serial port on the CPU. I'm assuming there is another port since you identified the one currently in use as "COM2". Or else connect the second HMI to the DB15 port on the power supply.

I am wrong about the 90-30 it is an RX3i, sorry

The communication that you are using is serial. This usually means that you have a master (sending the commands) and slave (receiving and answering commands).
What protocol is the current HMI using to the PLC?
You can only have one master to the slaves in the network without additional hardware.

Regards,

The current HMI is set as the master with the CPU310 as the slave

What if I turn one of the HMIs off and then turn the other HMI on, essentially leaving one master functional at a time?

What if I turn one of the HMIs off and then turn the other HMI on, essentially leaving one master functional at a time?

Click to expand...

That will work. The slave won't care that a different HMI is acting as the master.
Still, even if you're trying to leave COM port available for programming, why not have the second HMI use Port 1. You can always disconnect the HMI when you need to go online with the programming software

I was attempting to find a solution for what was already installed hardware wise. The com 1 port is a DB9 and wouldn't I still have the problem of two master's and a single slave, or are you saying that it would be two different networks? Yeah it would be a RS232 on the DB9 and the RS485 on the DB15. I would need to have the original HMI switched to a RS232 connection since it is in the PLC cabinet and the new HMI set up to run on the RS485 network as it is 300ish feet away. Not sure how that would affect the existing program...

Seem's like the simpler solution is the wye and on/off switch for the existing HMI. The New HMI is portable and already has an on/off switch in the portable enclosure. I can write up simple instructions to install an on/off switch in the PLC cabinet door, single pole switch cuts the 24V supply, then when they connect the portable HMI at the control station and turn it on it is the only HMI online. The control station is in the weather so the crew won't leave the portable unit connected and since the on/off for the existing HMI will be mounted beside or below it on the cabinet door it would be a simple matter of hitting the switch if someone forgot to turn it back on when they were done.

The two ports are independent of each other and can be used simultaneously. In hardware configuration you would assign a different SNP ID to each one. The original HMI would communicate with one SNP ID, the new HMI would communicate with the other SNP ID.
As long as the two HMIs don't ever need to be in use simultaneously, your approach is fine.