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Old June 11th, 2019, 10:07 PM   #1
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zacslade is offline
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Location: Phoenix, AZ
Posts: 76
SLC 5/05 MSGing itself

Hi all,

I have a SLC 5/05 which stores encoder feedback for three off-brand drives in registers F27:0, 1, and 2. Those registers are populated via a MSG instruction, with the following setup:

-- This Controller --
Comm Command: 500CPU Read
Data Table Addr: F27:0
Size in Elements: 3
Channel: 1

-- Target Device --
Message Timeout: 23
Data Table Address: F6:0
Local / Remote: Local
MultiHop: Yes

(Control bits are all 0 except for EN, which is 1)

The multihop lines are:
From Device, From Port, To Address Type, To Address
This SLC 5/05, Channel 1, Ethernet/IP Device (str),
Logix Backplane, N/A, Backplane Slot (Dec), 0

So, I interpret all that to mean that this controller is using a message instruction to read data from itself, moving floats from F6 to F27. I go take a look at F6 to see what's there -- there is no F6. My registers from 5 to 7 are:

C5 (Counters)
R6 (Control)
N7 (Integers)

I had to look up what a Control Register does. I found the description in the manual linked below, and I can't imagine how this works if you have errors on more than one ASCII instruction simultaneously, but that still doesn't solve the mystery of how I'm getting these encoder values.


What's going on here? Where should I look next? Anybody seen this kind of setup before?
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Old June 11th, 2019, 11:43 PM   #2
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sparkie is offline
Join Date: Nov 2014
Location: KS
Posts: 1,042
That is a funky setup. I can't contribute much, but I was curious if there could have potentially been another controller receiving this information, and the devices were moved to this PLC. I would not do it, but I could see this being a fast and dirty way to change as little logic as possible.
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Old June 12th, 2019, 08:41 AM   #3

widelto is offline
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Location: Barranquilla, Colombia
Posts: 2,744
What is your PLC IP address.
My two cents.
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Old June 12th, 2019, 08:53 AM   #4
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Firejo is offline
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Location: Redmond, WA
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You can get a SLC to message itself but not with the setup you are showing. When you add a MultiHop you are telling the MSG instruction to use CIP rather than PCCC (at least that's what happens when I try to duplicate what you've got setup). When I enable the MSG it errors saying the connection was broken which is what I would expect if it were trying to establish a CIP connection to itself. When I change MultiHop to "No" and enter the address of the SLC into the address spot it works and I can move data (although that is a very strange way to do it). Also, it fails, again with a broken connection message, if I enter an address ending in "0".
Is the IP address of your SLC ""? It looks to me like it is communicating (or was communicating) with a logix PLC somewhere on the network although I don't understand the IP address ending in "0". That is pretty much never done and in fact some equipment won't allow it (I don't know about AB).
Go Hawks!!!
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Old June 12th, 2019, 08:55 AM   #5
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Firejo is offline
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The other thing (I forgot to mention) is that you can't address a register that isn't there. If you don't show an "F6:0" register it can't read or write to it.
Go Hawks!!!
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Old June 12th, 2019, 10:52 AM   #6
Ken Roach
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It doesn't make any sense to send a message from the controller to itself, and certainly not with a Source data table that doesn't exist.

This must have been a MSG intended to read from those drives (are they connected over Ethernet ?) and then abandoned during development, with an invalid multi-hop path.
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