Connecting more than one ChemWeb Server
RS485 bus system
The serial RS485 bus is designed as balanced
It is characterized by the following features:
-data transmission in both directions possible
-data transmission via
-bus structure (addressable interface, up to 32 bus addresses)
The bus works with
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
As the ChemWeb Server does not have a separated RS485 interface, a
RS485 - Master | RS485 - Slave | +5V |
RS485 - Slave | ||
(Adr. 00) | (Adr. 01 ... 31) | |
|
| Ru |
A B GND | A B PE | A B PE |
|
| Rt |
Rt |
|
|
|
| Rd |
Each of the two ends of the complete bus cable must be fitted with a bus terminating resistor Rt. In addition, one resistor Rd must be connected against the data reference potential GND/PE and one resistor Ru must be connected against the supply voltage plus Vp 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.
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