plcken

It doesn't sound like a huge system - your IO count is pretty low, and 984 logic tends to get big quick. Without looking at the program it would be hard to guess exactly what you'll run into, but as a start: The ladder is scanned in a serpentine fashion. Normally that's not an issue, but I've seen 'clever' programmers use that do to 'tricky' things, things that break when you move them to an AB (or other) platform. That needs to be in the back of your mind all of the time. The counters and timers are pretty straightforward - top rung times/counts , bottom rung enables (on = time/count, off = reset to zero). The bottom rung of a timer will pass power EVEN IF IT ISN'T CONNECTED TO THE POWER RAIL - I've seen 'clever' people use that for 'tricky' things, so you have to watch that. Positive (and negative) transition contacts don't have a direct equivalent in a RS500 system - you'll have to build your own. The OSR isn't exactly the same thing. The SUB block might be subtracting two numbers, but it's far more likely to be there as a test block - top output is A>B, middle is A=B, and bottom is true on A<B. There's lots more things to know, but it depends on what the existing program has in it. Out of curiosity, where is the plant at? Ken

Select "Scan Mode Edit" under the "Program" menu (or right click Program in the system tree and select the same). There is a check box in there for "Automatic Start in Run" - that's the thing you need to turn on.

Actually, IOScanning IS the correct term for what you want to do. The name can be a bit confusing, since it's not actually IO that you're talking to in this case, but you're on the right track. IOScanning is a method of talking between two ethernet devices - in this case your CPU in the hotstandby system and the other CPU. A CPU can read and/or write data from/to an 4XXXXX (%MW in Unity) area in another ethernet CPU or device. It CAN be used to get IO data from Momentum bases that are equipped with ENT communication heads, which probably is where the IOScanning name comes from, but it can also be used to move data between two PLC's, which is what you want to do. It's only real limitation is that it can only read and write to 4XXXXX (%MW) addresses, so you can't directly read coil status information in a remote PLC - it has to be moved into the proper memory areas. The IOScanning is controlled by a table that you build using the programming software. In Concept and ProWorx it's a configuration extension, but I don't remember where it is in Unity. You fill in the IP Address of the remote PLC, the rate you want to read and or write, and the addresses within the local and remote PLC's that the data moves from or to, along with the number of words. There are some limitations, including (I think) a 500 word total limit on words moved. There are WAY more details than that, but that should point you in the right direction. As a bit of fill in: On a Quantum PLC you could have the following communications: IO - This is IO that is in the local PLC rack or in a remote rack connected via coax cable. It uses the S908 protocol (for the remote stuff) and is scanned between 984 ladder segments (or at the beginning of the PLC scan in Unity). That's probably what the other poster was thinking of. PeerCop - This is a Modbus Plus communications for communicating between PLC's or for talking to Momentum IO bases. It uses the Modbus Plus port on either the CPU or the NOM modules, and must be configured in pairs (the remote PLC must have a matching entry). Momentum IO bases have a default PeerCop setup in them when on a ModbusPlus comm adapter. There are two types of comms under PeerCop - Global and Specific. Global, as you might guess, is one to many, and Specific is one to one. IOScanning - What I described at the beginning. This uses ethernet to talk between PLC's or from a PLC to an ethernet device, like an IO base or communications bridge. HotStandby - the hotstandby pair of PLC's constantly talk over their fiber connection, and you can use the reverese transfer area to move data from the standby PLC to the primary PLC. There are messaging instructions, like the MSTR block in Concept / Proworx, that can talk either via the Modbus+ network or the ethernet network, to move data from PLC to PLC. There are the equivalent under Unity. There is also an XMIT instruction that can use the Modbus port on the CPU as a Modbus master to get data from other Modbus slave devices, using serial Modbus protocol. And, of course, there are other add in modules that support other protocols, like Profibus etc. Hope that helps Ken You could, by the way, make the hot standby processor pair talk via ethernet, but it requires a really good knowledge of how the hotstandby system works. You can setup the hotstandby system so that not all of the memory is copied from the primary to the standby via the fiber system. Then you could configure an entry in the IOScanning table to push data from the PLC to the device at the next IP addresses up (which is where the standby PLC will be) and push into this non transfer area. You could also use messaging blocks to do this, but you have to pay attention to which sections are executed by the standby. Of course, it would be easier to just let the standby system move the data - primary to standby just happens as part of the system, and the hotstandby supports a reverse transfer area that moves data from the standby to the primary.