Siemens Server - Manual (Page 12)

Connecting more than one ChemWeb Server
6
RS485 bus system
The serial RS485 bus is designed as balanced two-wire bus line according to EIA RS485 (DIN 66259,
Part 4 or ISO 8482), which is suitable for long transmission routes (up to 4000 ft) at a high
transmission rate (19200 baud).
It is characterized by the following features:
- data transmission in both directions possible
- data transmission via two-wire line (half duplex operation)
- bus structure (addressable interface, up to 32 bus addresses)
The bus works with differential-mode voltage signals. This ensures high interference immunity.
Transmission control is performed according to the master/slave principle. Communication is always
controlled by a master (ChemWeb Server). Only the master can start a communication. Without a
control command from the master to the respective bus address no bus user (slave) will start a
communication.
Installation of RS485 bus
The RS485 bus consists of max. 32 users (slave) and one master. The maximum overall length of the
cable is 4000 feet (1200 meters). To avoid reflections, the bus must be terminated with resistors at the
line ends. The two lines are provided with a quiescent potential (balancing).
The bus cable is always wired as connection from user to user. In principle, the bus lines should be
routed directly to the terminals or the plug of the bus user. Stubs because of local conditions (e.g. bus
users in switch cabinets) should be avoided. Stubs longer than 0.30 m are not permissible.
A shielded, twisted two-wire cable (twisted pair) with a wire cross-section of min. 22 AWG has to be
used as transmission medium.
As the ChemWeb Server does not have a separated RS485 interface, a potential-linked bus setup is
always required, i.e.: the shielding on both sides of the cable segments have to be connected to
ground. At the same time, a ground bus or a line with a minimum cross-section of 6 mm
2 has to be
laid in parallel to the bus cable and has to be connected to the shielding of the bus cable and the PE
terminal of each bus user. The supply of the ChemWeb Server (GND) also has to be connected to this
ground.
Each of the two ends of the complete bus cable must be fitted with a bus terminating resistor Rt. In
addition, one resistor R
d
must be connected against the data reference potential GND/PE and one
resistor R
u
must be connected against the supply voltage plus V
p
at any other point of the RS485 bus.
With these two balancing resistors, a defined quiescent potential is ensured at the line if no user
transmits.
A B
RS485 - Master
(ChemWeb-Server)
A B A B GND PE PE
+5V
Rt
Rd
Ru
Rt
RS485 - Slave
(Adr. 00)
RS485 - Slave
(Adr. 01 ... 31)