danw
Lifetime Supporting Member
1. Modbus TCP vs Modbus UDP
There is recent mention of Modbus UDP (Modbus over ethernet, but UDP, not TCP) on this thread.
http://www.plctalk.net/qanda/showthread.php?p=772283#post772283
I found this description on an opensource page:
http://jamod.sourceforge.net/kbase/modbus_udp.html
I have not run into Modbus UDP, but and I'm interested in whether others have and what their experience is.
Does a Modbus TCP server or client care whether the Modbus packet arrives UDP?
Will its response as a TCP packet be acceptable?
In other words, do both ends have to be UDP in order for server/client to play nicely together?
2. Modbus TCP vs RTU (over RS-485)
I prefer TCP because Ethernet (10base T [twisted pair], not 10base2/5 [coaxial]) is inherently isolated and does not require termination resistors or biasing. It does have a shorter distance limitation than RS-485, but with the available plant Ethernet infrastructures, that is less and less of a problem over time. Essentially, plug in Ethernet and if the IP address, subnet mask and gateway are suitable (all configuration, no wiring), the devices talk.
RS-485 is frequently non-isolated, subject to common mode issues, frequently lacks signal ground connections, requires termination, and if biasing is required it is an engineering job to achieve it. It can be a bear just to get 485 devices to talk to one another, even if the cabling distance specs are longer.
There is recent mention of Modbus UDP (Modbus over ethernet, but UDP, not TCP) on this thread.
http://www.plctalk.net/qanda/showthread.php?p=772283#post772283
I found this description on an opensource page:
http://jamod.sourceforge.net/kbase/modbus_udp.html
I have not run into Modbus UDP, but and I'm interested in whether others have and what their experience is.
Does a Modbus TCP server or client care whether the Modbus packet arrives UDP?
Will its response as a TCP packet be acceptable?
In other words, do both ends have to be UDP in order for server/client to play nicely together?
2. Modbus TCP vs RTU (over RS-485)
I prefer TCP because Ethernet (10base T [twisted pair], not 10base2/5 [coaxial]) is inherently isolated and does not require termination resistors or biasing. It does have a shorter distance limitation than RS-485, but with the available plant Ethernet infrastructures, that is less and less of a problem over time. Essentially, plug in Ethernet and if the IP address, subnet mask and gateway are suitable (all configuration, no wiring), the devices talk.
RS-485 is frequently non-isolated, subject to common mode issues, frequently lacks signal ground connections, requires termination, and if biasing is required it is an engineering job to achieve it. It can be a bear just to get 485 devices to talk to one another, even if the cabling distance specs are longer.