Материал: m012900e

292 • Common MODBUS functions

Function code FC11 (Get comm event counter)

6.2.10Function code FC11 (Get comm event counter)

This function returns a status word and an event counter from the slave’s communication event counter. By reading the current count before and after a series of messages, a master can determine whether the messages were handled normally by the slave.

Following each successful new processing, the counter counts up. This counting process is not performed in the case of exception replies, poll commands or counter inquiries.

Response

The reply contains a 2-byte status word and a 2-byte event counter. The status word only contains zeroes.

The event counter shows that 3 (0x0003) events were counted.

Modular I/O System

ETHERNET TCP/IP

Common MODBUS functions • 293

Function code FC23 (Read/Write multiple registers)

6.2.11Function code FC23 (Read/Write multiple registers)

This function reads the register values and writes the values into a number of output words (also ”Output Register“).

Request

The first register is addressed with 0.

The inquiry message determines the registers to be read and set. The data is sent as 2 bytes per register.

Example: The data in register 3 is set to value 0x0123, and values 0x0004 and 0x5678 are read out of the two registers 0 and 1.

Note

Should register areas for read and write overlap, the results are undefined.

Modular I/O System

ETHERNET TCP/IP

294 • Common MODBUS functions Watchdog (Fieldbus failure)

6.2.12Watchdog (Fieldbus failure)

The watchdog serves for monitoring the data transfer between the master controls and the coupler/controller. For this the master cyclically actuates a time function (Time-out) in the coupler/controller.

In the case of fault free communication, the watchdog timer does not reach its end value. After each successful data transfer, the timer is reset.

If this time has elapsed a fieldbus failure has occurred.

In this case, the fieldbus coupler/controller answers all following MODBUS TCP/IP requests with the exception code 0x0004 (Slave Device Failure).

In the coupler/controller special registers are present for the selection and status inquiry of the watchdog by the master (Register addresses 0x1000 to 0x1008).

After switching on the supply voltage the watchdog is not yet activated. First step is to set the time-out value (Register 0x1000). The watchdog can be activated by writing a function code in the mask register (0x1001), which is unequal 0. A second activation possibility is to write a value in the toggle register (0x1003) deviating from 0.

Reading the minimum trigger time (Register 0x1004) reveals whether the watchdog fault reaction was activated. If this time value is 0, a fieldbus failure is assumed. The watchdog can be restarted in accordance with the previously mentioned two possibilities or using the register 0x1007.

If the watchdog is started it can only be stopped by the user for safety reasons via a certain path (register 0x1005 or 0x1008).

Modular I/O System

ETHERNET TCP/IP

Common MODBUS functions • 295

Watchdog (Fieldbus failure)

Watchdog Register:

The watchdog registers can be addressed in the same way as the described in MODBUS function codes (read and write). Specify the respective register address in place of the address of a module channel.

Register address 0x1000

Register address 0x1001

Register address 0x1002

Register address 0x1003

Modular I/O System

ETHERNET TCP/IP

296 • Common MODBUS functions Watchdog (Fieldbus failure)

Register address 0x1004

Register address 0x1005

Register address 0x1006

In all registers the length is 1, i.e. with each access only one word can be written or read.

Modular I/O System

ETHERNET TCP/IP