ELOE171ZKE001
4
selected (e.g. 8 data bits without parity). 19.2 kbps rate and 9 bits format is set in the
picture. When in operation, received and transmitted data and data flow direction is also
indicated by LED.
3.1
RS-485 Interface Connection
There is the isolation between the RS-485 interface clamp and all the other converter
circuits. The 3kV voltage can be applied for 1 second without a failure. The RS-485
interface signal is placed on A-B clamps. The clamps VCC (+5V) and SG (signal
ground) are used to connect terminators, see the figure.
Both the RS-485 link ends should be
terminated with the 100-120 resistors
(so-called passive terminators), which are
placed between A-B wires. These
terminators are used for the converter
impedance matching, they suppress the
undesirable echo and they influence the
transfer immunity against interference.
Inserting the optical converter into the
metallic link terminates this link right on
the A-B clamps of the converter therefore it
is necessary to connect the passive
converter.
There are also the active terminators beside
the passive ones. Only one active
terminator can be installed to one metallic
section of the link and its role is as follows:
The RS-485 signal is symmetric. The differential receiver interprets the difference U
A
-
U
B
. It does not depend on the signal ground potential. The receiver interprets the
obtained signal
U
A
– U
B
> 200 mV as log. 1 or log. 0.
In addition to these levels the third state can occur, it is so-called IDLE state. No
transmitter is activated, each communicating partner is just listening so
U
A
-U
B
< 200
mV. The problem is how to interpret the third state in the two-state logic. The active
terminator gives the signal into the IDLE state line and it is interpreted as IDLE in the
two-state logic.
