2 Master PLCs to Share the Same RS232 Radio Port

I need to have 2 SCADA Master PLCs (one old and one new) share the same RS232 Master radio port. Does anyone know of a device that would allow automatic toggling between the two PLCs so that each system can update while the system PLC change out is going on? I have more details if you need to know more.
Thanks!
Chuck

Why not temporarily use two radios, each controlled by their respective processor? Even if it means you slow the polling rate to allow air-time for both radios, it is clean. The problem I see is in having to use the same address for both masters (regardless of configuration).

Does the number of field sites kake it prohibitive to dupicate the MSG instructions in the field, with the dupicates using the new Master address? That way, when the old processor is gone, you can simply delete the old MSG instructions in the field. You could even modify your existing MSG rungs such that when the new MSG's start getting .DN bits (vice .ER bits), the old logic no longer executes.

I may be on the wrong track for what your doing, if so I apologize in advance!

This might be a great question for the guys at B&B Electronics. They make a number of traffic-controller serial switches, and code-operated serial switches.

A lot depends on the protocol and how much interconnection the two master PLCs have.

Are we talking about DF1, or Modbus, or some other protocol ? What model of PLC ?

Hi,

If the 2 SCADA Master PLCs have Modbus TCP, you may consider a gateway, such as:
http://www.anybus.com/support/support.asp?PID=493&ProductType=Anybus Communicator
On the Modbus TCP side, both PLC's have access to the gateway and thus are able to read/write to the RS232 port.

//Me

MasterBlaster,
Thank you for your reply. We had thought about taking the customer's Half Duplex frequency and using it in a Simplex mode with one system using one half of the frequency pair and the new system temporarely using the other freq, all this would require would be an additional master radio and antenna. However in a 40 radio system this would require a visit to every radio to reprogram its operating freq and then once the change over is complete, visit every radio and program the half duplex frequencies back into the radios. It will definately work, it is just very labor intensive! The other downside is that the FCC frowns upon utilizing licensed frequencies in ways other than what they were authorized for in the original license. Thanks again!
Chuck

Ken,

I forgot to check with B&B so I will give them a call for sure! On the other hand, I did call Black Box and they did have a device called a "Buffered Data Broadcast" unit but since it buffers communications between the two devices, both the polls and the responses, I'm worried about time outs. We may end up having to try something like this if other solutions don't pan out. Thanks for your time and suggestion!
Chuck

Thanks Pudden for the link, since I am just an RF guy and we do not compete with our integrators/customers I have no idea if this will work so I will pass it on to the PLC integrator and see what he thinks. Oh, by the way, Ken had asked about the protocol, they are using Modbus RTU.
Chuck

I just wanted to say thanks to everyone for your time and helpfulness! I don't post often because I do not have PLC expertise but being a radio guy I bet I could contribute from time to time. Take care all!
Chuck

Just to add on, I've used one of these in the past for a similar application:

http://www.bb-elec.com/Products/Serial-Connectivity/Serial-Data-Tools-Adapters/Port-Combiners.aspx