Hi all

i have 8 gas detectors in the field  side each with modbus connection facility. 4-20 ma i wired it to the PLC AI card and i need modbus connection from all these gas detectors and connect it to a single port.

modbus protocol used is modbus rs485. the daisy chain connection of all these 8 gas detectors to be done in the field side  or in the terminals of the marsling cabinet?

daisy chain connection in the terminals of marshling cabinet a good practice or not?

Hi manu,

please post the order number of your used devices.

BR,

Noodles

Modbus RTU over RS-485 networks are daisy chained.

Daisy chaining means device to device to device to device, all in parallel.  The connections should local, directly at the field device.   See the graphic in the attachment.

What can create communications problems are stubs, which are cable runs from a tap off the main branch to the device, as shown below.  Stubs create 'reflections' that can created bit collisions on the bus with resulting bad PDU packets and time-outs.   Graphic shown at the link below (forum does not allow multiple attachments)

The field devices can be only wired into the marshalling panel with TWO cables for each device, one cable "going out", one cable "coming back", in order to create a daisy chain.  With double cabling, the master connects to the Device #1 going out, Device #1 coming back cable connects to device #2 going out, Device #2 coming back goes to Device #3 going out, and so on.

Single runs of cable from the field, all connected in parallel in the marshalling panel is network of stubs, which will create comm errors.

OR

The field devices are wired Master to Device #1 to Device #2 to Device #3, etc.