Specifications

47
Schneider Electric
20 mA CL – RS232 – RS422 –
RS485 links (continued)
RS485 link
Type of connector:
b A "SubD 9 points" connector is recommended for Schneider products, in which
case connection is as follows:
v pin 9 (Transmitted Data +) = A,
v pin 5 (Transmitted Data - ) = B
v pin 8 (Received Data +) = A’,
v pin 4 (Received Data -) = B’
v pin 1 (Signal Ground) = C/C’.
Environment:
b Ambient and storage temperature: respect characteristics of products and cables.
b Electromagnetic compatibility:
v respect rules for implementation,
v products should comply with "CE" directives.
Electrical characteristics:
b Electrical levels: ± 5 V (in differential).
b Transmitter: minimum output voltage: 1.5 V / 54 Ω.
b Receiver: minimum sensitivity ± 0.2 V.
b Type of medium: shielded twisted pair.
b Characteristic impedance of the medium: Zc = 120 Ω.
b Maximum voltage in common mode: -7 V to 12 V (for Zc = 120 Ω ).
b Maximum current for transmitter output: 250 mA.
b Load and line polarisation: yes.
b Load resistance = 120 Ω.
b Polarisation resistance = 470 Ω .
Note: If a compromise is made on the maximum variation of common mode voltage,
it is possible to:
b Use a cable with a lower characteristic impedance.
b Connect more loads to the bus (example: for 50 connection points, Umc = -1 to 5
V).
The TSB89 bulletin specifies that if the transition time of the fastest transmitter on the
line is greater than twice the propagation time on the same line (in one direction), the
connection is not a transmission (SIC). Load resistance is unnecessary.
An RS485 link has the following general
characteristics:
b Maximum number of loads on the bus: 32
(for Zc = 120 W and Ucm = -7 V to 12 V).
b Standard topology: chaining of
connection points.
b Possible topologies:
v point-to-point,
v Bus type multipoints (2 or 4 wires),
v Multidrop (1 transmitter and several
receivers),
v 1 receiver and several transmitters.
b Topology not recommended: star.
b Minimum distance between 2 points:
27 cm.
b Maximum length of a tap: 1 m (in
accordance with ISO/IEC 8482).
b Transmission speed: 4800/9600/19200
Bds.
b Type of protocol: Modbus/Jbus in our
case.
b Security required: more detailed study of
the network to adjust the various
parameters.
b Throughput required for use: optimisation
of links according to requirements,
10 Mbits/s max.
b Control lines: no.
Guide to Integration
Modbus / Jbus