A.1 RS-485 Networks
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-5560 Series Controller is 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 standard, 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-5560 Series Controller. 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 nor- mally 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.
A.2 Basic Network Layout
Multi-drop RS-485 implies that there are two main wires in a segment. The con- nected systems tap from these two lines with so called drop cables. Thus all connec- tions are parallel and connecting or discon- necting of a node doesn’t affect the network as a whole. Since ADAM-5560 Series Controller use the RS-485 standard, they can connect and communicate with the host PC. The basic layouts that can be used for an RS-485 network are:
Daisychain
The last module of a segment is a repeater. It is directly connected to the main-wires thereby ending the first segment and starting the next segment. Up to 32 address- able systems can be daisychained . This limitation is a physical one. When using more systems per segment the IC driver current rapidly decreases, causing commu- nication errors. In total, the network can hold up to 64 addressable systems. The lim- ita- tion on this number is the two-character hexadecimal address code that can address 64 combinations. The ADAM converter, ADAM repeaters and the host com- puter are non addressable units and therefore are not included in these numbers.
