I am looking for some help with a system that I need to make some changes to. The system currently includes two PCs both running modbus master software (Modbus RTU). The two modbus masters are trying to read exactly the same data from 15 slave devices. The two PCs are connected to the slave devices through independant serial links (i.e. the slave devices have two com ports), but the slave devices only have one UART, so only one PC can communicate with them at a time. At the moment only one of the masters is running at any one time (one is enabled and the other dsiabled), which works OK, but we have to have complicated logic to make sure that only one of the masters is enabled.

What I would like to do is allow both masters to be enabled all of the time, but have some device that sits in between the masters and the slaves that synchronises the communications so that the slave devices think they are only communicating to one master at any one time. There would need to be some kind of buffering/time slicing happening on the device as we need both masters to be getting updates from the slaves as quickly as possible.
I hope I have described my problem sufficiently. Any help with this would be grately appreciated.

Regards
Dave

 

You may want to consider using something like a Digi port server. The port server sits on an Ethernet LAN and provides additional serial ports to your host. Some of the Digi port servers support dual masters.

Joe
PSI Controls
[email protected]

 

Modbus allows only 1 master!

If more than 1 Computer needs to know the signals you have read from Modbus you have to do one of these.

- store your signals in a database system that is readable from all computers in your network.

- write your own "data provider" that allows remote computers to read the signals.

A "data provider" can be found here:
http://www.pvbrowser.org/pvbrowser/sf/manual/rllib/html/classrlDataProviderClient.html

A Modbus class can be found here:
http://www.pvbrowser.org/pvbrowser/sf/manual/rllib/html/classrlModbus.html

 

You probably need to use a terminal server or have both of the PCs talk ethernet to a modbus/ethernet bridge. The bridge would then allow for multiple masters. Only problem then is you need to implement ethernet on the PC end (modbus TCP) which is not overly difficult - there's plenty of free source code around. Of finally - you a micro PLC or RTU in the middle that reads all the data you need from the modbus link and remaps it within its own memory map and itself provides 2 serial ports for your PCs.

 

Hi

FieldServer Technologies has solved this type of problem before with a FS-B4010-01, the unit has multiple serial ports that can be set up as Master or Slave thus will enable two maters to poll data from the same set of Slave devices. Please have a look at http://www.fieldserver.com for more information.

Thanks,
Richard

 

Dave,

try http://www.calta.com (PLC09 Arbitrator).

Regards
Lorne

 

Consider the XPres-DR-IAP (Part# XSDRIN-01)http://www.lantronix.com/device-networking/external-device-servers/xpress-dr_dr-iap.html.

It's an Ethernet to serial bridge from Lantronix. It does MODBUS/TCP to either MODBUS/RTU or MODBUS/ASCII. The firmware can be configured for either "Master-attached" or "Slave-attached' and can support up to eight simultaneous connections.

Regards,
Jeff

 

The MARC Multii-Port is designed specifically for this application. Check this link http://www.miille.com/din366-10p.pdf for a data sheet. You can contact me directly at [email protected] for additional information.

 

Hi Dave,

I have a perfect solution for you. I have port splitter that splits on port in to 2. You can connect 2 Modbus Master on these 2 ports.

Please visit website http://www.renuelectronics.com and see port multiplexer (Part no. PMX-00).

Best regards,

Santosh