Multiple Master RS485/Ethernet Mux

[treefort]

Hi all,

I need a terminal server that allows for an rs485 master and an ethernet master to simultaneously poll an rs485 slave using different poll/response protocols.

The rs485 master talks to the slave via modbus protocol, and the ethernet master talks to the slave via roc protocol.

I have looked at Moxa but they do not have any suitable products. Their only multimaster device with a serial mux is modbus.

I need this asap, if anyone knows of such a beast, please let me know!

Thanks

 

[danw]

Matrikon OPC has a ROC driver for their OPC server. Any OPC server also has a Modbus driver. I don't know whether both drivers can run simultaneously talking to the same slave, but it might be worth a phone call.

Miille Applied Research (miille.com) in Houston, TX, has specialty protocol converter boxes that can coordinate multiple Modbus RTU masters who want to talk to the same slave. Maybe they've got something that talks ROC.

 

[treefort]

Hey Dan,

Thanks for the reply.

The ethernet master is a Telvent SCADA host. It does talk modbus, but we talk to every other ROC RTU using ROC protocol and want to keep everything the same.

The rs485 master is a prosoft card installed in a SLC 5/04.

I found this device but I would feel safer if a bigger more well know company made them.

http://www.scadalink.com/products/ip100.html

 

[danw]

I suspect that the problem will be that the device might not be able to handle quick-in-succession requests for both Modbus data and ROC data.

Most field devices are designed on the assumption that a single comm port will be used.

Modbus RTU slaves (serial RS-485) in particular can point to the Modbus spec that says 'only one master'. The master is supposed to wait for a reply before sending another poll.

So the tasks involved are not just converting RS-485 to ethernet, they are managing the Modbus and ROC master requests to the slave, waiting for a response and then feed the response back to the master.

Unless this slave device is really powerful and really communications savvy. But most of them aren't.

 

[treefort]

Yeah, I hear you.

I called the support techs last week and asked them if they've left a serial master and an ethernet master continuously polling the same serial slave overnight, and that no errors occured, and they said yes without hesitation.

So, we will see. It better work!!!

 

[danw]

If you're looking for a bigger name company for a serial-to-ethernet converter with Modbus packaging, then try either Lantronix or Digi. They're probably the two majors out there.

Be SURE to get a model that offers a specific Modbus 'option' because the standard serial-to-ethernet converters can be very problematic when dealing with Modbus.

 

[treefort]

Digi, Moxa, and from what it looks like, Lantronix do not have suitable solutions. Digi and Moxa only have multi-master muxs that support modbus only. Lantronix only has single port serial to ethernet converters.