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Jazz2 usb port no longer works after download. Newly configured RS485 doesnt work and can no longer connect to the PLC through the USB.

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     The PLC is a Jazz2 JZ20-R31 with the latest firmware installed. I was attempting to switch from Ethernet communication to RS485. I used the parameters outlined in the U90 help file to code 2 ladder blocks to configure for modbus and also configure the RS485 adapter.  I was unable to get the RS485 adapter working possibly due to incorrect parameters. Screen grabs of the 2 ladder blocks are attached.

The problem is that  I can no longer connect to the Jazz R31 through the built in USB port to correct any errors in programming.  How can I re-establish the USB com port?  Is there a way to reset to factory defults?

Thanks in advance,


Setup RS485 net 1.JPG

Setup RS485 net 2.JPG

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  • MVP 2014

Hi Rob,

You can recover comms only by powering up the PLC into Stop mode. With power off hold down the (i) key while applying power. 

If you do a normal power up, as soon as the program starts it will configure the COM port and lock you out. If you then put it into stop mode from running mode, the COM port remains configured for Modbus. only.

To prevent this from happening there is an alternative code for initialising Modbus. In your net 112 where you store #600 to SI140, change the #600 to #599. This allows the U90 ladder comments to be recognised by the PLC, even though the port is configurred for Modbus. Code #600 configures the port exclusively for Modbus and non-modbus comms are ignored.

This is information is in the Modbus topic in the U90 ladder help. But I first found out about it the hard way, by being locked out of a PLC after downloading the program 🙂

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