Communications

Hi all,
I'm a newbie on this forum but have been using this site for quite a while and was thinking that it's time to use the forum for what it's designed for. Getting help from other Automation engineers.

Anyhow I got another question that I hope someone can help me make the correct decision on.

I am designing a seperate project (not the same one from my last post) and have three SLC503's (1747-L531) and a SCADA (Intouch). I have a 485 network between the 3 PLC's and they are using link couplers to communicate to each other (1747-AIC).

The problem I have is in deciding on how will I get the SCADA to communicate to all 3 PLC's? I can use a 232 cable to the first PLC and use messaging between the three PLC so that the SCADA only communicates with the first PLC or I can use what I have been told of a KE box? This KE box (not sure what this is) I am told will get the SCADA onto the 485 network so it can then see all 3 PLC's and I can do away with my messaging.

Anyone got some advice on the easiest and most reliable way to do this.

I have attached a picture of what I see the network design is at the moment.

My budget is a little restricted but I am sure if the KE box thingy is not too expensive then I should be able to use it.

Thanks.

Personaly from experince I would swap out the 5/03 and replace them with 5/05's for the ethernet and faster processor speed. DH485 probally will not cut it here in this application.
If your buildings do not have a network already installed and they are physically close then go for a wireless netwok.

If the KE box is in the budget use it. Communicating directly to each PLC is always better. If the first PLC with all the messaging logic gets hosed your communication network goes down. (not good).

I would probably use a 1747-UIC (USB to DH485 adapter).
These are relativly cheap and should allow communications with the processors over the dh485 network. The only thing is I'm not sure if Intouch supports it. I know that it will allow any rockwell software that uses RSLinx to communicate. That's something you will have to look into. I'm sure Intouch could be of some assistance with this matter as well. If they don't support the UIC, then you may need to add a PKTX card to the PC. However, PKTX cards are pretty expensive, so this probably isn't the most desirable solution.

I'm not sure about the KE box. It sounds like it is a SLC mounted version of a PIC module (DH485-RS232 converter). This would probably have the same limitations as the UIC listed above.

Someone mentioned communications issues.
This can be the case with too much traffic on the network, but in your case, (3 PLCS + 1 HMI) you're almost certainly ok (unless you're sharing a ton of data between nodes)

jimbo3123 said:

Someone mentioned communications issues.
This can be the case with too much traffic on the network, but in your case, (3 PLCS + 1 HMI) you're almost certainly ok (unless you're sharing a ton of data between nodes)

Click to expand...

Don't forget that DH-485 is SLOW! If you are sharing much data with your SCADA node, I would look at other options (like changing to 5/05's and running the SCADA on Ethernet). If not, you will need to be very careful with polling frequencies / quantities of data / arrangement of data in contiguous blocks in PLC memory / etc.

Good luck,

Marc

Thanks MrDave, Jimtech67 and Jimbo2123, I will certianely look into each one. I don't think that I can at this stage swap out the 5/03 but I will be checking out the Intouch support for the UIC.
I have used the PKTX cards before for DH+ but they were about €1000 here. A bit steep really.

Thanks again for your help.

Thanks Marc,
The process is very slow anyhow, it's a water treatment plant so if I have a response of a few seconds I don't really mind. I am being very careful with my coding and comms to the SCADA and am keeping the data to as little as possible and all contiguous.

Although DH-485 is slow (Ken Roach correctly calls it a Data Sidewalk) it has worked well for me on a lot of wastewater applications.

I can offer two low cost options. First, you can use one of the SLCs as a master, and using message blocks collect all of the appropritate data in it. Then the PC can tie directly to the master PLC 232 port using DF1. That is the approach I use, since the PLC comm handling won't have the blue screen of death.

A second approach is to use the PC with DH-485 and give it its own AIC+ module. Then daisy chain all of the RS-485 wiring from the terminal strips. I haven't done this with a PC, but I have done it with Operator Interfaces and it works well. The PC would tie to the DB9 RS232 port.

1747-UIC

I'm just checking out with Wonderware and Rockwell to see if the 1747-UIC can be integrated into a Wonderware I/O Server. This would solve all my problems.
Has anyone done this before?

Help and Advice

David,

A bit of help and advice for you, please take this constructively as that is the way it is given.

First off, Tom Jenkin's suggestion of another AIC at a cost of €130ish + VAT is a solution that has worked well for me in the past and should suit your needs.

Secondly, if your end-user (customer) has any idea of controls, I will be suprised if he accepts your design to let one PLC be the master for all the others..what would happen if the master fails.....your SCADA package is worthless! You need to be able to communicate to each PLC individually even if it is DH485.

Last but by no means least, I wouldn't be posting all my details so clearly on this forum. Ireland is a very small country these days and the controls community is tiny i.e everybody knows everybody. A lot of Irish people check this forum out every day and they will form opinions (good or bad) of other Irish companies. Your company is a systems integrator working in a very small market here and I would advise you to keep their name out of this forum. As I said at the start, this advice is meant to be constructive and I hope you see it as such.

Best of luck to you with the project,

Regards,

Colm

Colm thanks for your advice. Points taken.
I think it was a moment of madness to divulge so much information.