1:N NT-Link communication over RS485: possible?

[CX_Luigi]

I am troubleshooting an installation composed by one CJ1G-CPU44, one NT21 and seven NS5 connected in 1:N NT-Link at high speed. Communication failures are frequent daily, and before they happen the entire network slows down sharply or freezes screen, usually when more operators are simultaneously entering data. When the failures happens, operators cycle the power on the touchscreen and the operation resumes. These failures also happens randomly on any panel, no matter near or far the line, relative to their distance to plc. Also, i've noticed that once one or two panels are kept switched off or idle (no operation performed on them), the communication keeps going without problems. Communication cabling is routed properly, far away from any possible interference source and minimized in overall length (30 to 50 meters maximum between controller and the last panel), each enclosure is well cooled ad insulated from the surrounding environment (a textile mill, hot and humid), and the touchscreen temperature itself is within working limits and not overheating. Communication between panels and plc is done through a single shielded twisted pair cable, and the line converters are not original from Omron, but supplied and marked by their italian branch. They are made in India possibly by Renu Electronics, and are configured to run in RS485. According to the NS manuals i read, for communications in NT-Link RS422 two twisted pair are required instead, while the actual installed configuration is to be used for communicating with temperature controllers, so should I change cable from 2 to 4 twisted pairs and reconfigure network communication from RS485 to RS422 as stated in manuals, or the current configuration is acceptable and I have to look elsewhere for the cause of these problems? Thanks for any insight on the matter

[Jay Anthony]

The manual specifically states that RS422 is the method of connection for 1:N NT Link. I have seen installations similar to yours run on RS485, but never with eight total touchscreens. The converters from Renu should work alright, but you should be using RS422. If you are going to run new two twisted pair cable, consider breaking it down into two networks and use two different serial ports. Before you rewire, check the setup of the port on the CJ PLC. The setting for max number of PT's connected is not a hardware setting. Instead it increases the amount of time necessary to push all eight touchscreens on to the wire. Be sure to set this to 7 to get the widest bandwidth for the port. This also applies when you are using Smart Active Parts that communicate to other networks.

[CX_Luigi]

thanks for your reply Jay, this confirms my suspicion about the design of the network, after reading the manuals. Actually the system is composed by eleven touchscreen dispersed around factory floor, and it was already split in two subnetworks, and I do not have more ports (SCUs in this case) available to move some of the panels to another dedicated line. However the cable laid for communication is indeed a 2 twisted pair cores, so I can quickly move to a 4-wires communication.

[james_applied]

Have the NS screens and PLC's got ethernet ports.? If so have you considered using this instead of the R2422 and leave the NT on its own.

[CX_Luigi]

No ethernet plug, only USB and serial on NS. Also down the line there is an NT21, this rules out anything but serial communication.

[james_applied]

THe NT21 would'nt of stopped you leaving the NT21 on serial and switching the others to ethenet thou lack of ethernet ports does mess up the idea. Why is the NT21 screen there as sounds a little strange to have lots of NS screens and 1 NT?

[CX_Luigi]

I have neither designed nor built the system, so I have to work with what is available, although it makes sense to put all touchscreens in a single connection rather that dividing them according to function (the NT is for machine control, the NS are for data entry and process display). But again that is a design decision from manufacturer to which I have no control whatsoever.