There are 3 main physical layers for Modbus.
RS232 : One master and one slave. Typically a cable with 3 conductors with max length of approx a couple of hundred feet. Usually easy. Sometimes some jumpers are required at one end to defeat handshaking.
RS485: One master and up to 128 slaves but take care to read more if you plan on more than 32. There are two wiring systems – so called 2-wore and so called 4-wire. They can be incompatible but usually 4-wire devices can be made to work on 2-wire systems. Each device must have a unique address and all devices must be set to the same baud rate, data bits, stop bits and parity. Usually easy to implement. The RS485 physical layer allows up to 128 devices to be installed on a single network with a max physical length of 4000ft and speeds up to 115k baud. Using repeaters allows the length to be increased. Compare to Ethernet where the spec allows a max of 100 meters (330ft) on a single unrepeated segment.
TCP/IP: All devices are essentially peers. A single device can be a master and a server. Routers can be used to connect sub-nets together. Broadcasts are almost ever used so are not an issue.
RS232 : One master and one slave. Typically a cable with 3 conductors with max length of approx a couple of hundred feet. Usually easy. Sometimes some jumpers are required at one end to defeat handshaking.
RS485: One master and up to 128 slaves but take care to read more if you plan on more than 32. There are two wiring systems – so called 2-wore and so called 4-wire. They can be incompatible but usually 4-wire devices can be made to work on 2-wire systems. Each device must have a unique address and all devices must be set to the same baud rate, data bits, stop bits and parity. Usually easy to implement. The RS485 physical layer allows up to 128 devices to be installed on a single network with a max physical length of 4000ft and speeds up to 115k baud. Using repeaters allows the length to be increased. Compare to Ethernet where the spec allows a max of 100 meters (330ft) on a single unrepeated segment.
TCP/IP: All devices are essentially peers. A single device can be a master and a server. Routers can be used to connect sub-nets together. Broadcasts are almost ever used so are not an issue.
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