Multiple Modbus Masters using Ethernet Serial Server


Thread Starter


Is it possible to use a single 'Ethernet/serial server' connected to a modbus slave to be polled by several Modbus masters (PC based applications)?

Normally this would not be possible to achieve, but can any of the serial servers 'manage' such an operation, e.g. hold a proxy slave image and present to the Master as a seperate slave? or some such other technique.
Our company manufactures a device - SCADALink IP Gateway (
that allows multiple Modbus Masters to poll a Modbus Slave. This is made done by multiplexing the modbus requests on the slave port from master ports - either ethernet or serial connected. The Modbus Masters are talking directly to the Modbus slave. Note that timeout parameters to be properly set
in the modbus master(s).

Regards Ben,
email - [email protected]

Yes it possible. You can build your own modbus TCP/IP server and hold the data from modbus slaves for multiple Modbus TCP/IP Masters. You can use for example Zilog eZ80 microcontroller to buld the device. Using this chip I've build Modbus TCP/IP server easy and quick.


benoit galarneau

Use a modbus serial to modbus TCP-IP bridge such as Schneider CEV-300. You will be able to poll a modbus serial slave via many Modbus TCPIP masters.
It is possible that you will overload the serial port on your slave devices. One option is to use an IP based control system to poll the serial slaves and store the current status in memory. Then have your masters poll the control system for the current status. This way you are using the network to distribute the data to the other devices, and you are not overloading the slaves with 100s of polling requests.

Automation Linse

Yes, this works amazingly well. The "bridges" function in 1 of 2 ways:

1) "Pass-Through" is how a Digi One IAP or Modicon CEV function. It is a small queueing system without concern for actual data - if 3 masters send requests, all 3 are queued and when the serial port is idle, one of the queued requests is forwarded to the slave. The other 2 requests sit in the queue and wait. The 'Pro' of this approach is no data configuration; the 'Con' is all Masters see slower responses. If your normal OPC/Master timeout was 1 second and you have 3 Masters running, then you'll need to bump up *ALL* the timeouts to say 2.5 or 3 seconds.

2) "Dual-Port Memory" is how most of the products supporting options for diverse protocol and media conversion work (ie: Modbus to DeviceNet, etc). You literally configure the product on the serial side to pull in selected data using one protocol and store in a virtual slave memory. Then masters of another protocol query this virtual slave memory instead of the actual slave. The 'Pro' of this approach is Masters can see much faster responses - especially if all are polling the same data; the 'Con' is usually a higher cost and more configuration is required.

For simple Modbus bridging, either design works.

best regards
- LynnL, (Experts in Ethernet-to-Serial)