____________________________________________________________________________________________________________
V2
AC500 Hardware
1-34 System
data
AC500 / Issued: 05.2007
The serial interface COM1 is connected via a removable 9-pole terminal block. It is configurable for RS-
232 or RS-485 and can be used for
•
an online access (RS-232 programming interface for PC/Control Builder)
•
a free protocol (communication via the COMSND and COMREC function blocks)
•
Modbus RTU, master and slave or
•
a CS31 system bus (RS-485), as master only
If the RS-485 bus is used, each interconnected bus line (each bus segment) must be electrically
terminated. The following is necessary:
•
two resistors of 120 Ohms each at both line ends (to avoid signal reflections)
•
in addition, a pull-up resistor at RxD/TxD-P and a pull-down resistor at RxD/TxD-N. These two
resistors care for a defined high level on the bus, while there is no data exchange.
It is useful, to activate both the pull-up and the pull-down resistors, which only are necessary once on
every bus line, at the bus master. For this reason, these two resistors are already integrated within the
COM1 interface of the AC500 Terminal Bases. They can be activated by connecting the terminals 1-2
and 3-4 of COM1.
Terminator P
RxD/TxD-P
RxD/TxD-N
Terminator N
+5V
0V
470 Ohms pull-up
470 Ohms pull-down
1
2
3
4
Figure: Integrated resistors (pull-up, pull-down) at COM1, can be activated by connections between 1-2
and 3-4
The following drawing shows an RS-485 bus with the bus master at one line end.
1
2
3
4
1
2
3
4
1
2
3
4
Master at the
bus line end,
Slave within
the bus line
Slave at the
bus line end,
pull-up and
pull-down
bus termination
120 Ohms
activated,
bus termination
120 Ohms
120
Ohms
120
Ohms
Figure: RS-485 bus with the master at one line end
If the master is located within the bus line, it does not need a terminating resistor. The pull-up and the
pull-down resistors, however, must be activated (see the following drawing).
