Connect 2 PLCs over a switch

Hy everybody,
I have two PLCs and one network card, I used to disconnect one PLC and connect the other one when I needed to switch from one to another.
Now I bought a switch, connected the computer and the two PLCs to the switch. It's not working, I don't know from where to start.
Thank you in advance for your answers.

We'll assume Ethernet even though you didn't say that.

Provide the PLC type. if the Ethernet connection for the PLC is a seperate unit please specify that. Give us the type and model of the switch. Then the computer operating system and software. Finally the IPs of the PC and each PLC (all must be different.)

Are you able to see that you are connected to the two PLCs? Have you changed the IP on your computer? Make sure the IP on the PLCs are the same with the exception of the last set, and make sure the computer is the same first three numbers. Run BOOTP to see if there are requests wanting an IP? Do any of these work?

Thank you for your fast answers, I knew I wasn't giving enough data but I didn't know which are relevant:
The OS is Ubuntu. I set up the ip adresses days ago with BOOTP from windows and made sure the IP adresses have the same 3 first numbers, and that my 3 components have the same mask. I don't know from where I can list connected hardware in Ubuntu. My PLC is a vago. My switch is a netgear.
I used to connect to the PLC just by opening FireFox under Ubuntu and typing the ip adress of the PLC, this doesn't work now.

What cables are you using? If to a switch then use straight thru cables.

If you are using Ubuntu 11.04 or greater, there is an icon in the far upper right corner (near your user name) for your network connection. You can right click on it to view your connection information.

As Bernie suggested, please list all of the exact IP addresses of each component (and your PC) as well as the subnet mask you are using.

Ok, I think the problem is from the cross cables, I need to check that. I'll also try to list the detected devices on my network as soon as I'll go to the lab.
Thank you.