Momentum processors and the Modbus Port 1

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Thread Starter

Mike Bargnesi

I can communicate to PLC Momentum processor 1760 00 with ProWorx Nxt connected to PLC at Plant floor level. When I try to communicate to PLC at my PC I can only read data and can not write to any registers? I have tried using Proworx Nxt at the PC and I can see the ladder logic rungs update but if I try to force a coil on or write
a value to a register I get an error message that says I have to be logged on to the processor? Using my SCADA software package LOOKOUT I can only read my values from the graphic screns if I try to use a switch I created it ignores the command and never writes to the PLC. My converter is a B&B 485TBLED converter that converts the RS-232 signal to a 485. I have a NULL modem adapter connected to the PLC side and I
have the other converter at the PC side with no Null modem adapter.

Please help
 
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Bob Peterson

I can't tell why your lookout stuff isn't working but the error message indicating you are not logged on usually means someone logged onto that CPU with a programming terminal and then did not log off properly. Example: instead of shutting down the programming software in the normal manner, you can get this message if you just disconnect the comm cord from the CPU while it is still attached. A modicon cpu will only allow one programming terminal to "attach" itself to a cpu. I am not sure how this is implemented.

I have found on Compact CPUs that the only reliable way to get rid of this message is to cycle power to the CPU.

Bob Peterson
 
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Todd Beckmann

Mr. Peterson is correct in that someone else logged in to the processor without logging out. The Modbus RS232 cable typically shorts the
DTR and DSR (Data Terminal Ready and Data Set Ready) lines at the connector. Breaking this connection simply by unplugging the cable at the
processor will remove the log in, you should not have to cycle power.

Todd Beckmann
 
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Bradley G. Hite

Hello Mike

...snip
> When I try to communicate to PLC at my PC I can only
> read data and can not write to any registers?
> My converter is a B&B 485TBLED converter that converts the RS-232
> signal to a 485. I have a NULL modem adapter connected to the PLC
> side and I have the other converter at the PC side with no Null
> modem adapter.
...snip

Your problem is in the NULL modem adapter you are using. Modicon Modbus ports use a custom NULL Modem pin-out not a true NULL modem. You need to look in the Momentum Manual for the correct pin-outs for the Modbus port. If you can not find them e-mail me and I will send you the pin-out diagram.

Once you get the right pin-out all of your problems will go away.

Bradley G. Hite
Intertech Inc.
mailto:[email protected]
http://www.myplc.comTeaching Practical Skills for a Technological World
 
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