Choosing between master/slave interface for a virtual slave network

Hello, I am looking into implementing fieldbus connectivity into a server software that collects data from remote devices. My plan insofar has been to implement a Modbus TCP stack on the server, represent my data as virtual slave(s) and use an Anybus X-Gateway for physically connecting the server to a fieldbus network.

Now, as to my real question, I do not quite understand if I should choose a gateway with master or slave interface for the Modbus TCP side. According to the HMS instructional videos, a gateway with master interface pointing to your slaves is meant for networks that don’t have an existing Master device such as a PLC . However, I understand that if a slave interface is used, queries from the Master device from the other side of the gateway (let’s say for example Profibus) should be translated and pass through to my virtual slaves all the same.

Is my understanding correct? For my use case, where one of the “networks” is simply a virtual representation of devices, does it matter whether or not I choose a gateway with Master or slave interface? Does a gateway with a slave Modbus TCP interface know how to translate queries from a Profibus or other fieldbus Master and pass them through in Modbus TCP format? I believe a gateway with Master interface is certain to work, but there are some differences in the specs between master/slave interface gateways which might favour a slave interface, if one can be used for my purposes.

Gateways I’ve been looking at for reference:

https://www.anybus.com/products/gateway-index/anybus-xgateway/modbus-tcp-detail/anybus-x-gateway---modbus-tcp-client---profibus-slave

https://www.anybus.com/products/gateway-index/anybus-xgateway/detail/anybus-x-gateway-profibus-slave-modbus-tcp-server

Hello @sjm

You will need a master on the network reading and writing data on the network. Slave cannot talk to each other and only respond to requests by the master. To exchange data the master needs to make a read or write request to a slave. I recommend taking a look at this site, it is a good resource on the modbus protocol. http://www.simplymodbus.ca/FAQ.htm

If you are trying to get data from all of the slave devices then make it available on a fieldbus my guess is that you will want a modbus master gateway making requests for the data from all of the slaves. The other option would be to have a master read the data off of the devices then write it to a modbus slave gateway.

The gateways do not work transparently. The slave interface is providing info from a buffer that is exchanges with the opposite interface. With a master you first configure them to talk to the slave devices, this varies with the protocol used. It also exchanges data to the buffer that is share with the other interface.

Regards,
Deryck