Serial interface COM1 of the Terminal Bases (PM57x, PM58x, PM59x)

Serial interface COM1

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

• Online access
• A free protocol,
• Modbus RTU, client and server,
• 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:

• 2 resistors of 120 Ohms each at both line ends (to avoid signal reflections)
• Pull-up resistor at RxD/TxD-P and a pull-down resistor at RxD/TxD-N. These 2 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.

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.

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).

RS-485 bus with the master within the bus line

The following photo shows a wiring example “master within the bus line”.

“Master within the bus line”, wired at the COM1 bus connector of the Terminal Base

Note

If the bus is operated with several masters, the pull-up and pull-down resistors may only be activated at one master.

The earthing of the cable shields of the bus lines are described in the CS31 System Bus (PM57x, PM58x and PM59x) CS31 system bus (PM57x, PM58x, PM59x).