The confusion may not involve the com signals, directly, but the circuits that communicate through them. Is the 24 volt supply powering anything else? Is it grounded or floating? Does the com cable route along side any other cables? Are there any large noise producers (i.e. variable frequency droves, arching contacts) near by? How is the shield on the cable connected at each end?
Again I ask, is either side of this supply grounded?
Yes. Curious.
Sounds like a pretty clean design.
Does this mean that each pair has its own shield? If so, the optimum use of these shields, I think, is to ground each shield, only at the end of the cable that receives the signal from that pair. A snap-on ferrite bead at each end of the cable will also improve the high frequency noise rejection.
How will you ground this?
Since you had this same problem when the PLC and touch panel were talking over RS232, and RS422 is usually pretty forgiving in a noisy environment, I would be more inclined to suspect that either the PLC or touch panel quits talking intermittently rather than that noise or the cables were causing the probelm.
Mark Walsh
Then how do you know if it floats from ground till it arcs back? Just kidding, but I hate indeterminate voltages in a system. I would tie one end to ground, if no part of the system has not already grounded it.
This means thay you have tied the logic grounds into a loop with the supply common. I can't say how much suseptability this has to induced current from magnetic fields, but it would worry me.
I have a new installation, consisting of an AutomationDirect DL450 and one of their "Ez-Touch" panels. Despite being grounded to hell and gone and hooked up '422 with proper terminations and expensive belden cable, I get intermittent but frequent "No Communication with PLC" problems. Not causing a particular problem, but puts a cosmetic dent in an otherwise perfect machine. The problem occurs whether the machine is running or just sitting there.
Power is +24.2v from a 150W supply, and the comm cable run is about 6 feet. AD support says "noise". Shit. What "noise"? Upsetting an RS422 whilst switching nothing?
Baud is 9600 N-8-1. FWIW It did this hooked up RS-232, which is why I went to '422. The PLC and touch panel are in two seperate cabinets. All cabinets tie back to a groungd busbar connected to the 480v ground. The control power transformer is grounded ("Neutral") side.
Anyone want to take a crack at it?
The 24v comes from 150w switcher that powers the rest of the machine. There are two Hitachi 0.5hp inverters in the main cabinet, and the usual assortment of relays, proxes,and those neat new Banner laser sensors. 'Scope shows clean power at the touch panel.
What baffles me is that ithe problem happens wheter the machine is at "idle" (drives off, emergency-stopped,not switching anything) or running full-tilt. Does the com
It has its own conduit between the cabinets, and in the main control cabinet, route for about a foot with the 24v controls wiring. All the vfd wiring is at the top end of the cabinet, about 3 feet away.
According to AD, the shield should be connected at each end to a dedicated pin which they call "logic ground". I'm using an individually shielded belden paired cable. Today's excercise is put in a dedicated 24v supply.
Also..AD specs just using TD/RD and looping back cts/rts each end. If excersise #1 failes then we hookup rts/cts and see if that helps.
Sorry. No, it floats.
Each pair has its own shield in the cable, and they are connected to the so-called "logic ground" of each device.
Steve, I have seen this problem before somewhere in my distant past. First, forget noise, it sounds like you have beat that horse enough.
Thanks. I've seen that before, on AB panelviews, but come to think of it, a bad tag in the panel might be it. I know I'm talking to the right station, but I could have a nonexistent or protected address in there.
I grounded the neg of the dc like John suggested, but that didn't affect it.
I'm off to look at that tag database....
Thanks again.
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