Could someone give me a good definition of passthrough

hello,
I am playing with a panelview 1000 (allen bradley). processor: Control logix. The panelview is communicating to the processor via an DHRIO module. I am using the Remote I/O option to communicate with my RIO panelview. in the panelview set up there is an option for setting the PASSTHROUGH to be active or not. I did a litle searching for a good explanation but could not really come up with one. Could some one explain to me what this option is? is it only used with DH+? The panelview is the only device hooked to my DHRIO card. thanks. any help is welcomed!

Often a PLC has only one data connection, and if you are using an HMI, then the HMI uses that connection. So if you need to go online with the PLC, then you have to disconnect the HMI.

Passthrough allows you to avoid this. It allows you to plug into the HMI, and talk to both the HMI, AND the PLC via the one serial connection between the computer and HMI.

Not only does this save cable shuffling, but it allows you to debug the code with the HMI connected, which can be handier than flipping bits via the programming application.

Actually, it is a transparency of a device for communication between devices or networks connected on both sides.
This may or may not involve a protocol conversion in the transparent device.
May be used with any protocol, not only DH+.
For example, SLC5/05 CPU features passthru between Ethernet and DF1 ports.

Passthru was generally a term used only on the PLC5/SLC500 family. It refers to the ability to bridge from one port in the system to another, ie between CPU ports, and ports on other modules such as R.I/O and Devicenet. It is quite a nice feature especially for programming maintenance. Not a lot of other PLC's did this kind of thing so readily at the time.

The limitation was that the CPU had to perform all the comm conversions and/or squeeze the data across the IO backplane. In some instances the resulting performance was fairly modest. Also your CPU firmware had to be the right version to support the right feature and you usually had to do a little config in the CPU to make it work.

The CLX family does "bridging and routing" between ANY ports in the system. Multiple hops over multiple networks is easy to achieve. With only the exception of support to legacy networks there is no configuration to setup, and there are few performance limitations. The term "passthru" is not really applied to the CLX family anymore as it has been superceded by the native functionality of system.

Thanks Philip for the plain explanation.
I still don't have a clue what Sergei said.
GAS

ControlLogix is very good at bridging and routing all of its native networks except one: Remote I/O.

If this PanelView were connected using DH+, ControlNet, EtherNet/IP, DeviceNet, or DF1, this would be a no-brainer of just "drilling down" using RSLinx Classic.

But PanelView download via RIO Passthrough was always a difficult combination of controller firmware, middleware (RSLinx) and application software (PanelView Standard file transfer utility) in the PLC-5 and SLC-500 controllers. As far as I know, you can't use RIO to download to a PanelView that is connected to a 1756-DHRIO channel in RIO Scanner mode.

Get a CompactFlash card and a PCMCIA Type II adapter and use those to load your application programs into the terminal. You'll be happier.

Thanks for the great info everyone! That really helped a lot.