Modbus rtu and tcp
- Thread starter M HARISH
- Start date
sigmadelta
Member
Use a gateway which has a Modbus RTU Slave and Modbus/TCP Master (client). For example:
https://www.anybus.com/products/gat...ateway---modbus-tcp-client---modbus-rtu-slave
https://www.anybus.com/products/gat...ateway---modbus-tcp-client---modbus-rtu-slave
TheWaterboy
Lifetime Supporting Member + Moderator
Spectrum Controls makes a gateway that is reasonably priced and easy to use.
I think that will not work, on a Modbus RTU network can only exist a single master, there cannot be other masters, even connected through a Modbus TCP to Modbus RTU gateway
Are you assuming that the masters on both sides can send requests to the slaves on RTU side?
Are you assuming that the masters on both sides can send requests to the slaves on RTU side?
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KirkC
Member
No doubt it will work with one or more master(client) on the TCP side and without a master in the RTU side, but not if there is another master on the RTU side, the requests from this master will collide with the requests coming from the gateway.
KirkC
Member
Even the original BM85 bridge multiplexer from Modicon could let up to 3 Modbus masters talk to 1 slave.
danw
Lifetime Supporting Member
That's a Modbus Plus device. Modbus Plus (a licensed protocol) uses a token method for regulating the traffic on the bus. Regular Modbus (the open protoocol) has no such provision, which is why 'only one master allowed at a time'.
Even if the TCP clients were put on the RS-485 network with Modbus TCP/485 converters, the 485 bus would be swamped with packet collisions because there's no means of defining who (which client) speaks when to allow for one-at-a-time communication.
The outfit Miille pops up on control.com when this issue arises, because they have an RS-485 bus moderator product.
http://www.miille.com/
ICP DAS has a product, the TSH, that will moderate between 2 RTU masters but this app has a total of three master/clients.
KirkC
Member
danw,
The BM85 had 4 Modbus serial ports (232 or 485). You could connect a different Modbus master to 3 of the 4 serial ports and connect a Modbus slave to the 4th. The bridge multiplexer would allow all 3 master to talk to the 1 serial slave. (Forget about the Modbus-Plus port). I am absolutely certain about this.
The BM85 had 4 Modbus serial ports (232 or 485). You could connect a different Modbus master to 3 of the 4 serial ports and connect a Modbus slave to the 4th. The bridge multiplexer would allow all 3 master to talk to the 1 serial slave. (Forget about the Modbus-Plus port). I am absolutely certain about this.
Even single PLC can have several different (serial) modbus ports and share same data of PLC. Newest writed data wins and is echoed to other ports.
What makes you think that this is not possible, when there is special gateway which have several RS-ports and one TCP?
But OP is question is: one master device, 4 serial slaves and 2 TCP slave. No even need to think that there is collisions as there is only one master.
One or two Serial to TCP gateways is needed.
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A TCP client in a Modbus TCP network is a Modbus master, not a slave.
My bad. TCP client and server goes allways wrong.
Still, if there is serial to modbus gateway, then there is 3 TCP clients (masters) and it is possible on TCP side.
If 2 TCP masters are Scads or HMIS, they (probably) write only from change.
Problem could be if TCP master is PLC, which write continuously (which probably isn't case as there is 2 different TCP masters).
Even if TCP master is PLC, PLC code can modified to write only from change.
Still, if there is serial to modbus gateway, then there is 3 TCP clients (masters) and it is possible on TCP side.
If 2 TCP masters are Scads or HMIS, they (probably) write only from change.
Problem could be if TCP master is PLC, which write continuously (which probably isn't case as there is 2 different TCP masters).
Even if TCP master is PLC, PLC code can modified to write only from change.
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danw
Lifetime Supporting Member
1 2003 thread on control.com addresses the issues and solutions:
https://control.com/forums/threads/modbus-many-to-many-connection.15318/
https://control.com/forums/threads/modbus-many-to-many-connection.15318/
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