Profibus question

Spartan10 · Dec 11, 2008

One of our service guys in the field hooked-up several pieces of equipment via profibus (Siemens S7-300 master). He's having problems communicating and I found out he did not daisy chain the profibus connections. I told him this was wrong, but want to make sure. Must the connections be daisy chained?

(Unfortunately, I think I already know the answer)

Thanks.

LJBMatt · Dec 11, 2008

How else did he do it? For standard 9 pin D-shell it has to be 2 wires in and 2 wires out to the next station.

Spartan10 · Dec 11, 2008

He has two connector "piggy-backed" at the PLC.

JesperMP · Dec 11, 2008

I have seen that done before, and I think that it ought to work, despite it doesnt look exactly pretty.

I guess he may be a little ignorant about how to make the cable connections.
Try to make it the normal way, and be sure that terminations are ON at the cable ends.

If it still does not work, the most typical beginner error is that the actually used configuration doesnt match exactly the programmed configuration.
If it still does not work, then investigate the HW Diagnostics on the master CPU.

ivo.maenen · Dec 12, 2008

Also do not forget to terminate on both ends of the network. Further more there is an 'in' and 'out' for each connector. Make sure that you use the 'in' at both ends.

Ken M · Dec 12, 2008

I often recommend going to the http://www.profibus.org web-site - home of the Profibus Organisation. In the "Downloads" section you'll find an Installation Guide. Use this as your bible. If you do this bit correctly and the equipment still doesn't work, then challenge the equipment manufacturer. If his equipment doesn't work when installed according to Profibus Org's rules then what's he going to do to fix it. The stuff is either Profibus-compatible or it isn't. (Of course you might need to double or triple check your wiring first to make sure you're compatible

)

S7Guy · Dec 12, 2008

Pigggy backing at the PLC is fine. Let's say you have ten node. There is no problem making two branches with five nodes originating at the PLC. I've done and seen it many times, and avoids the use of a repeater when you want to have two physical chains due to equipment layout.

Take a look at the terminating resistors at the PLC end. If I remember right, one may have to be on, and one off, but I am not exactly sure.

JesperMP · Dec 12, 2008

S7Guy said:
Take a look at the terminating resistors at the PLC end. If I remember right, one may have to be on, and one off, but I am not exactly sure.

There are always two activated termination resistors. These are at the very ends of the physical daisy chain.

S7Guy · Dec 12, 2008

So, if they are piggy-backed, the one on top would be off, and the one connected to the PLC would be on? I never wrote it down when I actually did it, but I seem to think they were of opposite states. I could be wrong though.

JesperMP · Dec 12, 2008

To my logic they must both be off.
The cable doesnt end at the "piggy-backed" connector. The cable goes in at one connector and out by the other connector.

If the baudrate is 187.5k, you can often get away with wrong terminations. At 1.5M and higher terminations absolutely must be correct.

Profibus question

Spartan10

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LJBMatt

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Spartan10

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ivo.maenen

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Ken M

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