E.1 Rs-485 Network - Advantech Adam 4000 Series User Manual

E.1 RS-485 Network
EIA RS-485 is 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:
 Max 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 modules 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 dis-
connected from the host without affecting the functioning of the remaining nodes. In
industry shielded twisted pair is preferable due to the high noise ratio of the environ-
ment.
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 address). In this case the master is a personal com-
puter that is connected with its serial, RS-232, port to an ADAM RS-232/RS-485 con-
verter. The slaves are the ADAM I/O modules. When modules are not transmitting
data, they are in listen mode. The host computer initiates a command/response
sequence with one of the modules. Commands normally contain the address of the
module the host wants to communicate with. The module with the matching address
carries out the command and sends its response to the host.
E.2 Basic Network Layout
Multi-drop RS-485 implies that there are two main wires in an segment. The con-
nected modules tap from these two lines with so called drop cables. Thus all connec-
tions are parallel and connecting or disconnecting of a node doesn't affect the
network as a whole. Since ADAM modules use the RS-485 standard, and use an
ASCII-based commands set, they can connect and communicate with all ASCIIbased
computers and terminals. 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 modules can be diasychained. This limitation is a physical one. When using
more modules per segment the IC driver current rapidly decreases, causing commu-
nication errors. Totally the network can hold up to 256 addressable modules. The lim-
itation for this number is the two number hexadecimal address codes that know 256
combinations. The ADAM converter, ADAM repeaters and the host computer are non
addressable units and therefore are not included in these numbers.
ADAM-4000 Series User Manual 350