Advantech user manual RS-485 Network

RS-485 Network

EIA RS-485 is the industry’s most widely used bidirectional, balanced transmission line standard. It is specifically developed for industrial multi-drop systems that should be able to transmit and receive data at high rates or over long distances.

The specifications of the EIA RS-485 protocol are as follows:

∙Maximum line length per segment: 1200 meters (4000 feet)

∙Throughput of 10 Mbaud and beyond -Differential transmission (balanced lines) with high resistance against noise

∙Maximum 32 nodes per segment

∙Bi-directional master-slave communication over a single set of twisted-pair cables

∙Parallel connected nodes, true multi-drop

ADAM-5000 systems are fully isolated and use just a single set of twisted pair wires to send and receive! Since the nodes are connected in parallel they can be freely disconnected from the host without affecting the functioning of the remaining nodes. An industry stan- dard, shielded twisted pair is preferable due to the high noise ratio of the environment.

When nodes communicate through the network, no sending conflicts can occur since a simple command/response sequence is used. There is always one initiator (with no address) and many slaves (with addresses). In this case, the master is a personal computer that is connected with its serial, RS-232, port to an ADAM RS-232/RS-485 converter. The slaves are the ADAM-5000 systems. When systems are not transmitting data, they are in listen mode. The host computer initiates a command/response sequence with one of the systems. Commands normally contain the address of the module the host wants to communicate with. The system with the matching address carries out the command and sends its response to the host.

C-2 ADAM-5000