Multiple Modbus Slaves - Best Approach

[sgsims]

I am using Cognex Cameras (Modbus Slave) with a DL260CPU and an H2 Ecom100.  I have one Ecom card per camera.  Now I have to send the Barcode data out to a PC (Modbus Slave).  I was going to install a separate Ecom Card for each camera for the PC comms.

But it is starting to occur to me that maybe that is not the best idea only because I am limited to the # of ECOM cards I can use due to the base oly having 9 slots.

Is there a better strategy when needing to communicate with multiple Modbus slaves.  I guess my main concern is the # of scans it takes to process each read and write command on the Ecom card.

[Greg]

If you are using ECWX/ECRX IBoxes in the DL260, you can address multiple slaves pretty easily. You can, for example, put multiple ones on the same rung and just enable the rung. The ECWX/ECRX IBoxes have a built-in "token passing" and will cycle through them as fast as possible. The backplane is the bottleneck. During the communication cycle, if I remember correctly with a DL260, each scan can pass about 128 bytes... sometimes more, I believe.

Anyway, the ECOM100 can maintain up to 12 simultaneous Modbus TCP connections.

[sgsims]

Thanks Greg,

So my understanding of the ECWX/ECRX IBoxes is that it takes a scan or two to complete each command.  So if I have let's say 2 ECWX & 2 ECRX I-Boxes on the same rung even with the built in token passing it will take something like 4-8 PLC scans to cycle through all 4 commands.

Does that sound right?

[franji1]

Thanks Greg,

So my understanding of the ECWX/ECRX IBoxes is that it takes a scan or two to complete each command.  So if I have let's say 2 ECWX & 2 ECRX I-Boxes on the same rung even with the built in token passing it will take something like 4-8 PLC scans to cycle through all 4 commands.

Does that sound right?

It will take at least 6 scans because the read has a response packet that must be processed on the next scan (at best).  Depending upon your scan time, it could take more than 8 scans depending upon the response time of your slaves.  Comm is asynchronous and is not guaranteed to execute within any number of scans (the PLC cannot control the remote slave device or the network traffic).