MSG Read or Write?

[Fircombehall 0]

Hi, i need to MSG some integers to another SLC 5/05 via Ch1 TCP/IP. 
 

The CPU that i am sending to already has other SLC’s messaging it. Maybe 2/3 others. 
 

Let’s say the PLC with the integer i need to send is PLC “A” and the destination PLC is PLC “B”. 
 

1) Do i setup Read MSG from PLC B and extract data from PLC A. This way i can time the MSG function to make sure it doesn’t coincide with other MSG blocks. 
2) Do i setup write MSG from PLC A and write my integer to PLC B. 
3) Do i write to PLC C (one that is already messaging PLC B) and jump onto an existing MSG block between the two meaning i don’t have to add to the traffic queue going to PLC B?

Any advice greatly appreciated. 

[Joe E. 259]

In general, I prefer to use MSG Read instructions. That way the destination registers show up being written in the program cross-reference. I think I remember seeing somewhere that reads take slightly lower resources as well, but I could be mis-remembering that. I wouldn't think that the data traffic would be too much, but it all depends on how much data is flowing back and forth and how often the MSG instructions are executing. I like having things simple and easy to understand, and I think having a read MSG straight from the source PLC is simpler. There's also less to go wrong, like if PLC C is shut down for some reason PLC B would lose the data from PLC A.

About the only time I deviated from using read instructions is when I went between a PLC5 and a CompactLogix. I found the routing and MSG path to be far easier to manage in the Logix 5000 PLC than in the PLC5, especially since it was hopping from DH+ to Ethernet via a ControlLogix backplane. In that case, I made sure to scrupulously document it and put info in the destination register's description as to its source. I know, that violates "simple and easy to understand", but I didn't have a lot of options at the time. We had about zero budget and the PLC 5 didn't have Ethernet. We had the ControlLogix hardware sitting around so it was cheap to install.

[pturmel 157]

For anything that needs to be sent regularly, I agree with Joe that it should be reads initiated at the destination.  If the data transfer represents an unscheduled or semi-randomly occurring event on the source PLC, I would use a write from the source.

[Fircombehall 0]

Thank you both for your replies, much appreciated. Read it is!

[Fircombehall 0]

Oh… one more thing. When setting up a new MSG block would setup a new integer file and dedicate it to the control block?

For example if all my program integers are in N7, then setup a new N8 for the control block?

[pturmel 157]

1 hour ago, Fircombehall said:

setup a new N8 for the control

Not a bad idea.  I'd make it big enough for multiple control blocks--future use.

[Joe E. 259]

4 minutes ago, pturmel said:

Not a bad idea.  I'd make it big enough for multiple control blocks--future use.

I agree. Make it a lot bigger than you need. You can edit code online but you can't add/remove LAD files or add/resize data files without a download.

[BobLfoot 301]

Here is one reason why "old timers" prefer all reads.

MSG READ TO PLC A FROM PLC B N7:0

Step 1 - PLC A sends to PLC B  - "I need your N7:0 data please send it"

Step 2 - PLC B sends to PLC A - "Here is the N7:0 data you asked for and it's checksum"

Step 3 - PLC A validates checksum, if OK Read Done , Else Repeat at Step 1.

 

MSG WRITE TO PLC A FROM PLC B N7:0

Step 1 - PLC B sends to PLC A - "I wish to write my N7:0 data to you, are you ready?"

Step 2 - PLC A sends to PLC B - "I am ready to recive your N7:0 data, please sned it"

Step 3 - PLC B sends to PLC A - "Here it the N7:0 data I said I'd send and it's checksum"

Step 4 - PLC A sends to PLC B - "I have received the data and the checksum and the status is GOOD/BAD"

Step 5 - If PLC B recevies BAD CHecksum Status thenr epeat WRTIE, else END Write.

 

As you can see the READ needs only two set data packets per data message while the write requires 4 data packets per data set message.

 

ANECDOTAL DATA

I had an SLC 5/04 network using DH-485 to exchange data between four SLC 5/04 the DH-485 speed was 19,200 baud

To send an integer from A to B, increment by one in B and send it back to A took anywhere from 5-15 seconds using MSG WRITES.

Covnerted to MSG READS the timing went down to 1-3 seconds.