Profibus terminating resistors

ajw3

Member
A
Join Date
May 2010
Location
Pittsburgh
Posts
4
Hi, I'm new to this site & new to profibus. I've dealt with various other networks but not this one. What I don't understand is this. I have a S7-400 in the middle of a network. The connectors are labeled "in" & "out". On the "out" side of the connector from the CPU I have 8 remote racks. At the end of that I have my terminating resistor. But what about the other side. I have two nodes before the CPU that feed to the "in" side of the CPU. How do I terminate these. If I turn on the termininator I'm switching "off" that input.

Please help
 
The end node of the 2 should have the resistor turned on - and the processor resistor should be off. So you will only have 2 turned on (both ends of network)


On-Off-Off(Proc)-Off-Off-...........ON
 
This doesn't make sense. If the two nodes before the CPU are A & B. And I have a jumper from A to B then the input of the CPU. Then from the CPU out to C & D etc... If I turn on the terminator on A does that not shut off it's output to B & the CPU?
 
They are end of line resistors, it prevents the comms trying to look past your end of line nodes. So where ever you have an end of line node you have a terminating resistor.
 
simple rule of thumb, one cable in plug - switch on. two cables in plug - switch off. I have taught our electricians to check for this before calling me out at 2am and so far it has worked very well
 
I'm using bradharrison connectors. And they have arrows on them designating "in" and "out. The also have a switchable built in terminating resistor. My concern are these in and out arrows. Can you give me any info on this?
 
It´s pretty simple. IN-OUT has no functional importance. You can connect to any of these connections. But... when you flip the switch to ON i.e. connecting the end resistor you also disconnect the connection between in and out. (So if you flip the swith to ON somewhere in the middle on the bus the portion after this will be disconnected.) But here is the cath: resistor is placed on the "IN"-side of the connector. So if you connect the last point of the profibus to OUT instead of IN and flip the swith, the resistor will never be connected to the net, since it is on the wrong side of the switch.
 
electrical_geek said:
It´s pretty simple. IN-OUT has no functional importance. You can connect to any of these connections. But... when you flip the switch to ON i.e. connecting the end resistor you also disconnect the connection between in and out. (So if you flip the swith to ON somewhere in the middle on the bus the portion after this will be disconnected.) But here is the cath: resistor is placed on the "IN"-side of the connector. So if you connect the last point of the profibus to OUT instead of IN and flip the swith, the resistor will never be connected to the net, since it is on the wrong side of the switch.
Click to expand...

Therefore IN-OUT has functional importance with single cable entry at the ends of a profibus network 🤾
 
OK! :)
I tried to make it short. You can make all other connections along the bus at either side. The only points where IN and Out are really important, are at the endpoints of the bus.
 
The terminating restistor is physically connected to the 'IN' side, therefore if you only have 1 cable it must go into the 'IN' therefore both ends go into the 'IN'.

The Brad Harrison plugs have a diagram on them, look at that.
 
electrical_geek said:
OK! :)
I tried to make it short. You can make all other connections along the bus at either side. The only points where IN and Out are really important, are at the endpoints of the bus.
Click to expand...

Not true, for testing purposes you want them all the same direction..

'IN' .... 'IN''OUT' .... 'IN''OUT' .... 'IN''OUT' .... 'IN''OUT' .... 'IN''OUT' .... 'IN'

That way when fault finding you can switch the resistor on at any location to break the network up (good newtork in the above example above would always start from the left).

The only thing you need is to ensure a Master exists in the network if you are testing without a BT200 type tester and you are looking at the light status on the plugs.
 
Thanks, this definitely helps clear some things up. I've worked with devicenet, controlnet, & ethernet. But profibus is new to me. I'll try this in the morning when I go into the plant.
 
PeterW said:
Not true, for testing purposes you want them all the same direction..

'IN' .... 'IN''OUT' .... 'IN''OUT' .... 'IN''OUT' .... 'IN''OUT' .... 'IN''OUT' .... 'IN'

That way when fault finding you can switch the resistor on at any location to break the network up (good newtork in the above example above would always start from the left).

The only thing you need is to ensure a Master exists in the network if you are testing without a BT200 type tester and you are looking at the light status on the plugs.
Click to expand...
If you have the CPU (or DP-card) in the middle, should you have it like this?

RESISTOR....'IN' .... 'OUT''IN' .... 'IN'-CPU-'OUT' .... 'IN''OUT' .... 'IN'....'RESISTOR'

And with the speed of 1,5MBit the max lengh from RESISTOR-RESISTOR is 200m? Not 200m from the CPU/DP-card to each direction?
 

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