I am using an 4 Port RS485 to ethernet converter to communicate with 16 PLCs. This converter has 4 rs485 lans with 4 plcs daisy chained on each lan. I am able to communicate flawlessly to all devices in each lan, but only when I poll each individual lan. When I poll all four lans ocasionally I get missing data. Does anybody know a fix for this. I am using modbus over ethernet protocol (modnet).

 

Orbin,

What model Ethernet serial device are you using? You may want to look at a different vendor. Sealevel has a model with four RS-422/485 ports, part number 4402. They also have eight port models if you want to further segment your network. Good luck!

http://www.sealevel.com/products.asp?bus_id=8

 

I have no idea what you mean by either "Modbus over Ethernet" or "ModNet". Neither of these are formally specified at http://www.modbus-ida.org so could mean many things.

I agree it sounds like you may need to try another brand - or it may just required Master application and/or Enet-to-485 device configuration changes.

Likely when multiple RS-485 lines talk concurrently you are ending up with gaps between fragments of Modbus response and so the Modbus serial Master is aborting the response. You could try configuring the Master to expect gaps up to 250msec, or configuring the Enet-to-485 device to hold received data longer to pack more data into fewer packets.

I know this can easily work with our own 4-port RS-485 server, but can imagine many design details other won't understand & thus would not work in this situation.

best regards
- Lynn A Linse, LynnL, www.digi.com

 

you can also consider the Hexin brand RS-485 ethernet converter.

http://www.hexin-technology.com/RS-..._TCP_IP_Serial_Device_Server-Product-253.html