AB PLC-5 Scan time alarm

sparkotronic · Sep 15, 2011

Hi All,

I'm having a problem in my workplace.

We have a standalone Ethernet network with several PLC5 processors attached to it.

One of the processors is dropping off the network and I'm not sure if it is network or processor related.

I'm never there when it happens and by the time I get there, the system has returned to normal.

I did put in a watchdog timer which monitors a bit changing condition on the PLC which is dropping out, but I was wondering if there is some sort of routine that I can add to the processor which would tell me if there is a problem?

If it is the processor then could I put something in which monitors the scantime and would flag an alarm if it exceeds a preset value?

Thanks in advance.

rdrast · Sep 15, 2011

The PLC5 already has a watchdog timer built in. If it times out, the processor hard faults. The time setpoint for that is in the processor config, but that only monitors scan time, not communications.

sparkotronic · Sep 15, 2011

rdrast said:
The PLC5 already has a watchdog timer built in. If it times out, the processor hard faults. The time setpoint for that is in the processor config, but that only monitors scan time, not communications.

Hi,

Would I be right in thinking that if the processor locks up then it would hard stop?

If this happened, would it need a manual reset?

Thanks

rdrast · Sep 15, 2011

Yes. It hard faults, and needs to be reset with the keyswitch, or the programming software.

JesperMP · Sep 15, 2011

I think that some managed switches can log the connection activity on its ports.
So you could install the switch near the PLC5 and connect one port to the PLC5, and another port to the ethernet cable going to the network.
That can reveal if the problem is with the PLC5, or on the network side (most likely the cabling, or noise in the path).

Another thing is that if you cannot pinpoint the error, then maybe you can try to improve the cable installation. A real fat clamp over the screen and with good connection to the panel ground should be standard, but is rarely ever done.

sparkotronic · Sep 15, 2011

JesperMP said:
I think that some managed switches can log the connection activity on its ports.
So you could install the switch near the PLC5 and connect one port to the PLC5, and another port to the ethernet cable going to the network.
That can reveal if the problem is with the PLC5, or on the network side (most likely the cabling, or noise in the path).

Another thing is that if you cannot pinpoint the error, then maybe you can try to improve the cable installation. A real fat clamp over the screen and with good connection to the panel ground should be standard, but is rarely ever done.

That's an interesting point, the exsisting installation is wired unshielded Cat5e cable.

Is there any indicators that I can look for that could point to signal noise ect?

At this moment in time I'm running a program called Total Network Manager, this pings the devices at set intervals.
When we lose the PLC, it still shows on the network.

I also have been looking at the web based PLC interface, but it means very little to me.

Any pointers would be great.

Regards, Colin

JesperMP · Sep 15, 2011

Unshielded ethernet cables !
Apart from short patch cables, all our ethernet cables are shielded, and of good quality too.

edit: Dont go for office-grade shielded cables. These often only have thin foil as the screen. The foil may break, and it also have a tendency to loose the connection to the gnd in the connector. A good shielded cable has both a foil and a braid. The braid makes the difference for getting the connection to the connector or the clamp.
If in doubt which cable will do, go for Profinet cable.

JesperMP · Sep 15, 2011

I'dd like to add, that practically every time our customers has bypassed our recommendations, and gone and installed unshielded ethernet cables, it has been a catastrophe.

rdrast · Sep 15, 2011

JesperMP said:
Unshielded ethernet cables !
Apart from short patch cables, all our ethernet cables are shielded, and of good quality too.

We have nothing but unshielded cabling in the plant. Miles and miles of it. Some in conduit, some not, and have never had a communications issue with Ethernet. YMMV.

fuzzy logic · Sep 15, 2011

I could agree with both of you Jesper and rdrast...

Although shielded cable can offer best immunity to noise, often due to bad termination or defective equipotential wiring for the shield to ground, it is *the cause* of communication issues...
So, the very important thing is to have very good quality of cable, best if cat.6 twisted pair..., and you may use unshielded one.
Twisted pair design preserves signal balance through cable to provide noise immunity and return loss.

However, different path for Ethernet cables than power or noisy cable is always recommended.

It is not a rule, of course, just giving my two cents on past experiences with that stuff...

- fuzzy logic

JesperMP · Sep 15, 2011

Does there exist ethernet cables that are NOT twisted pair ???
You do not know that the "TP" in TP and UTP stands for "Twisted Pair".

All Profinet cable I know of have double shielding.
I know that Ethernet/IP allow both shielded and unshielded. Dont know why since the principles shoud be the same for both standards. As I see it, there is no reason to take any chances.

fuzzy logic · Sep 15, 2011

Yes Jesper obviously I know that.

Was just to underline the why the unshielded cable is still good and how can take noise far from it...
Also I was indicating Cat. 6 conductors because are twisted more tightly for better performance.

As you now, shielded cables and terminations are often matter of discussions and everybody has different good/bad experiences due to grounding considerations.

Was just to say that recently, I've had more success in using Unshielded cables (UTP)

- fuzzy logic

fuzzy logic · Sep 15, 2011

fuzzy logic said:
Yes Jesper obviously I know that.

It was just to underline why the unshielded cable is still good and how it can keep noise out ...
Also I was indicating Cat. 6 cable, because conductors are twisted more tightly for better performance.

As you Know, shielded cables and terminations are often matter of discussions and everybody has had different good/bad experiences, due to grounding considerations.

It was just to say that recently, I've had more success in using Unshielded cables (UTP)

Apologize for grammar errors in my previous post... just trying to correct some of them... in red inside the quote...

- fuzzy logic

AB PLC-5 Scan time alarm

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