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I need two Modbus master hosts to communicate with one Modbus slave.
Current (one master, one slave):
MBMaster1<-->MBSlave

Desired (two master, one slave):
MBMaster1<-->|--------|
|BlackBox|<-->MBSlave
MBMaster2<-->|--------|

Details:
MBSlave has a RS485 that's converted to RS232
MBMaster1 has a RS232
MBMaster2 has a RS232
MBSlave does not support handshaking/flow-control
MBMaster1 and MBMaster2 are independent, so 'may' send request to MBSlave at random/same time.

Questions:
What do I have to do beforehand for this to work?
What kind of 'black box' solution do I need to buy?

Thank you,
jmczero @ hotmail. com

Our product, the XLTR-200 serial gateway, has 2 RS485 ports and one RS232 port, each of which can be independently configured as Modbus Master or Modbus Slave ports. It is frequently used to allow 2 master devices to simultaneously access one or more slave devices.

http://iccdesigns.com/Merchant2/merchant.mv?Screen=PROD&Store_ Code=ICC&Product_Code=xltr200

Regards,
Darrin Hansen
ICC, Inc.

Jmc,

One system I am aware of that will fit your needs is the PLC09 Modbus Arbitrator from Calta Computer Systems Limited.

http://www.calta.com/

This unit will allow up to three masters to poll a single Modbus slave.

Ian Milne

The Miille Applied Research Multii-Port is specifically designed for connecting multiple Modbus masters to a network one or more slaves. You can see a data sheet here:

http://www.miille.com/din366-10p.pdf.

Jerry Miille

Hello...

Check out ProSoft Technology's ProLinx line of products - they've been doing this for years.

Here's a snip directly for the product page:
"The ProLinx Modbus Serial Port Expander module allows you to attach up to three separate Modbus master devices to a single Modbus slave."

Specifically, see: http://www.psft.com/content/view/full/8891

CH

Thanks for the info. Are there any that can support up to 115.2k baud? (Not absolutely necessary, but good to know.) Thank you.

Yes. All the new Prolinx units support 115.2 baud.

CH