DH+ MSG or Global Status Flags?

OK, I've analyzed our system here, and we are using MSG Writes between our 5 processors. I've been told to use the Global Status Flags File if possible because it's basically a "free ride" around the network. In reading the PLC-5 User Manual 1785-6.5.12 on page 9-5 at the top it says that using a GSFF adds 3ms to your houskeeping time, and at the top of 9-4 it says housekeeping time directly adds to your program scan time. My question is this: Do I add 3ms to the scan time of all the processors to use the GSFF, or go with 2 MSG writes per processor? My instinct tells me the MSG method is more efficient for what little communication there is here between the processors. I'm looking to start a heated debate over this one, just so we all can come out of it a little wiser.

Vetteboy

What? No Opinions? I can't just let this thread die yet.......

How much data do you have to "pass around"? Do all 5 of your PLCs need to receive all of the data? Do all 5 of your PLCs need to send their own portion of the data to the others?

Are we sure that 3ms number from the manual isn't a typo? You can perform some pretty elabotate data manipulation in the PLC5 in 3ms. It seems to me that simply copying 1 word that is already circulating around the DH+ network into a data file shouldn't cause that big of a processing hit. Unfortunately our PLC5 test rack disappeared so I can't try it out.
I would say go with the global status flags.

Keith

Of the 5 processors, 2 exchange a word's worth of data between them, another receives a word's worth from a whole other one, and then a few bits are passed around amongst a few. I really like the GSFF idea, but my boss thinks that the original programmers didn't go that way for a reason, and I just wanted hard evidence to prove this would actually be better.

Sounds like a strong candidate for the GSFF to me ... that's probably the way I would go.

The original programmer may not have been aware of the use of the GSFF.

Of course, using the GSFF, there's no tangible MSG instruction involved that practically self-explains its purpose. With adequate documentation, I don't see this as a turn-off.

My understanding is that the PLC5 does not have Global Status bits that can be used for sending data between processors on the DH+ network. The SLC5 has this function but I have not been able to find the equivalent in the PLC5. If this is not correct, I would appreciate it if somebody could point me to the Status files used for this operation.

Hi Vic-
Take a look at Page 10-4 in the manual Vetteboy links to in his original post. That might help.
I've never personally used a GSFF because I never realized this transfer mode existed. Maybe I SHOULD read those manuals some day. But it looks like you just need to specify a data file number in the channel configuration for the DH+ channel you want to send the GSFF word on. Then reference the element equal to the decimal address of the station you are interested in.
Like I said, I've never used this. But I may now that I know about it.

Keith

Keith

Thanks for the info. I don't know why I hadn't seen that before.

Vic

I guess the question that comes to mind is what brought this on?

Vettaboy

Looked at it more closely and I feel I can explain why the message instruction was used instead of the GSFF.

The GSFF is an excellent way to share one word of data among multiple processors. The problem with this is that it is the word is only 16 bits long. Therefore you are limited to only bits or integers that can be transfered.

The message instruction does not have this problem. Over DH+ you can transfer up to 120 words and over Ethernet you can transfer up to 709 words of data. The best part you are not being limited by the data type.

From experience I have always used a float file for data transfers between plc's. This way I'm not limiting my self to the type of data that can be sent.

Also there may come a time you would like to send more data then what you are prsently sending now. The message instruction is more flexible in this situation.

What brought what on? This discussion? Mostly the lack of information on the internet about GSFF vs. Messaging. Which is better under what conditions etc......