Special Hardware Requirements:
There are two types of encoders, each having different resolutions:
- Encoder with 13 bit resolution: Type SVE10-06213
- Encoder with 25 bit resolution: Type SHE86-10214
Manufacturer: Hohner Electrical Engineering, D-78532 Tuttlingen/Germany
It is theoretically possible to connect 8 encoders to an RS 485 interface.
This sample program describes how the position value of an RS 485 encoder (manufacturer: Hohner Engineering) can be displayed using the S7-200 PPI interface.
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Possible Interfacing Problems:
Using a Hohner encoder along with the S7-200 may lead to the following problems:
Limited current output by the PPI interface of the S7-200
The available current is limited to 120 mA, and the Hohner encoder needs 140 mA for supply. Here, the encoder has to be supplied with current by a separate 24 V power supply unit.
Another solution would be to use an EMC-board. This board is an isolated RS 485 interface to the encoders. The supply effort for the S7-200 interface is then only 20%, or < 30 mA.
Noise on the encoder line
Noise signals on the encoder line can be dampened by using a terminator: 6ES7-972-0B00-0XA0, or a resistor combination as shown in the figure below.
Waiting time between two transmitted words too short
The waiting time between two words is only a 1-bit time unit for Hohner encoders. This may be an insufficient amount of time for the CPU 212 or CPU 214 to reset the UART block. The UART will not be ready to receive the next character for 2-bit time units, and a parity error is likely to occur. This problem may be corrected by increasing the waiting time of the encoder to 2-bit time units.
Download Project Example:
