Changing IP Address on 1756-ENET-A

I’m an IT “fixit” guy working at a manufacturing plant that volunteered to help out with PLC’s. I know about .001% of what there is to know so forgive me if my verbiage with my issue is a little off.

We had to change the IP address on our PLC’s and all of them are good to go but three. The PLC’s are ControLogix 5550s with 1756-ENET-A’s. I at least know enough to know that these are old.

Problem:
I changed the IP in RSLinx like I had done on our other ones but I would get an error stating “An unexpected communications error has occurred. Unable to compete the current download to device”, but the IP would change, and we can use it for our operation until power is cycled it. Once power is cycled to the plc the new IP is dropped and the module shows its set to BootP.

Today I have gone online with it using RSLogix to change the IP. I go to the module, change the IP in the General tab and that looks good and it stays saved in there. I then go into port configuration to change it and when I do I get an error stating, “Invalid Gateway Address. Address conflict between Gateway and IP Address, or module unable to receive data.”

These PLCs had their IP address the same for a couple of decades and never lost their IP. The only thing I changed on the module was the IP Address and Default Gateway, I didn't make a change to anything else.

Any help you great folks can offer up would surely be appreciated as I’ve grown tired of having to add the IP back every time the power has to be cycled on these. Thanks!

rajj03 said:

I’m an IT “fixit” guy working at a manufacturing plant that volunteered to help out with PLC’s. I know about .001% of what there is to know so forgive me if my verbiage with my issue is a little off.

We had to change the IP address on our PLC’s and all of them are good to go but three. The PLC’s are ControLogix 5550s with 1756-ENET-A’s. I at least know enough to know that these are old.

Problem:
I changed the IP in RSLinx like I had done on our other ones but I would get an error stating “An unexpected communications error has occurred. Unable to compete the current download to device”, but the IP would change, and we can use it for our operation until power is cycled it. Once power is cycled to the plc the new IP is dropped and the module shows its set to BootP.

Today I have gone online with it using RSLogix to change the IP. I go to the module, change the IP in the General tab and that looks good and it stays saved in there. I then go into port configuration to change it and when I do I get an error stating, “Invalid Gateway Address. Address conflict between Gateway and IP Address, or module unable to receive data.”

These PLCs had their IP address the same for a couple of decades and never lost their IP. The only thing I changed on the module was the IP Address and Default Gateway, I didn't make a change to anything else.

Any help you great folks can offer up would surely be appreciated as I’ve grown tired of having to add the IP back every time the power has to be cycled on these. Thanks!

Click to expand...

in order for a plc to keep it's IP, you typically have to disable the BootP function, or it will revert back to bootp/dhcp on reboot.

I believe the Bootp-DHCP software is a free download as well. use it to set the IP, and then you can disable it from there. I couldn't explain why it reverted back after years of not doing it unless the change in IP didn't take well being done in RSlinx.

https://compatibility.rockwellautomation.com/Pages/Home.aspx

search by product and look for BootP

IT suck!

Both RSLinx Classic and the Rockwell BOOTP/DHCP Utility have the ability to enable and disable BOOTP/DHCP on Rockwell controllers and Ethernet modules, using messages to the TCP/IP Object (CIP Class 0xF5)

For some reason, it sometimes doesn't work. You'll get a message about the device being unable to make the change, or that the setting "could not be changed", and that's it.

I've been using both utilities since 1998 or so, and I'm a reasonably skilled and experienced network protocol troubleshooter, and I still don't know why. Our friend Operaghost might.

One thing to check, especially if you're using the BOOTP/DHCP Utility, is to make sure the "Network Settings" are configured so that the IP address and Subnet Mask are compatible with the Default Gateway. I sometimes go to set a device up for "10.0.x.y", and forget that my "Network Settings" for the utility are still set for a subnet mask of 255.255.255.0 and a default gateway of 192.168.1.1. That fails the basic subnet sanity check and the utility refuses to try to make that setting.

Because any ControlLogix-era device gets its actual IP address from the CIP object maintained in onboard NVRAM, I don't try to use RSLogix 5000/Studio 5000 to make these changes. The configuration isn't being made to the user application program, so I'd prefer to use use the simplest tools, RSLinx Classic and the BOOTP/DHCP Utility.

L33er said:

IT suck!

Click to expand...

Yep

rajj03 said:

Problem:
I changed the IP in RSLinx like I had done on our other ones but I would get an error stating “An unexpected communications error has occurred. Unable to compete the current download to device”, but the IP would change, and we can use it for our operation until power is cycled...

Click to expand...

This is entirely normal. Your PC was connecting to the device and suddenly that device's IP address changed. You lost communications as a result.

rajj03 said:

Once power is cycled to the plc the new IP is dropped and the module shows its set to BootP.

Click to expand...

As was mentioned, the BootP function must be disabled. This is done from the same page in RSLinx where you set the IP address. It is similar to how you would set a PC to a static IP address instead of using DHCP.

rajj03 said:

Today I have gone online with it using RSLogix to change the IP. I go to the module, change the IP in the General tab and that looks good and it stays saved in there. I then go into port configuration to change it and when I do I get an error stating, “Invalid Gateway Address. Address conflict between Gateway and IP Address, or module unable to receive data.”

Click to expand...

The General tab IP address is where you are telling the CPU what address the ENET will be using. It does not actually set the device's address. And honestly, the CPU doesn't really NEED that information when the module is in the same chassis as the CPU. As you stated, you can use the Port Configuration tab to set the device's IP address, though I prefer to use RSLinx instead.

What are you entering for the Gateway address? If you are not using one, then you want 0.0.0.0. If you are using one, make sure its address is on the same subnet as defined by the IP address and subnet mask.

rajj03 said:

These PLCs had their IP address the same for a couple of decades and never lost their IP. The only thing I changed on the module was the IP Address and Default Gateway, I didn't make a change to anything else.

Click to expand...

The confusing thing here is definitely that this was a system that had been working for years without issue. I'm wondering if that function somehow got re-enabled. The BootP function is enabled out of the box for any new device.

Ken Roach said:

Both RSLinx Classic and the Rockwell BOOTP/DHCP Utility have the ability to enable and disable BOOTP/DHCP on Rockwell controllers and Ethernet modules, using messages to the TCP/IP Object (CIP Class 0xF5)

For some reason, it sometimes doesn't work. You'll get a message about the device being unable to make the change, or that the setting "could not be changed", and that's it.

I've been using both utilities since 1998 or so, and I'm a reasonably skilled and experienced network protocol troubleshooter, and I still don't know why. Our friend Operaghost might.

Click to expand...

The gotcha here in my experience is that the subnet mask on the PC is often different from the mask in the Network Settings for the BootP utility, or on the device itself. But even still, after verifying those settings, I've had devices that would just flat out refuse to let the tool enable/disable them. Then a seemingly identical device would allow it. But I have never had an issue with RSLinx not allowing me to manually make this change.

OG

L33er said:

IT suck!

Click to expand...

That was certainly an insightful and helpful addition to the thread.