Connect Graf-Systeco’s display D1000 over CAN bus
In a very simple example we will show you how the display type D1000 from Graf Syteco can be connected to the mini PLCs STG-550 and an STG-650. In this example, the status LED of the STG-550 will be turned on over a button on the display. Furthermore, the current analog value (potentiometer) of the STG-550 will be displayed as an analog instrument. However, before the value is transferred to the display, it passes the STG-650 which then transfers the value to the display. Of course, the STG-550 could communicate directly with the display, but that would be too easy. :-)
The STG-550 is reading the voltage drop over the potentiometer and sends the value to the STG-650 over the CAN-ID 0x001. The STG-650 sets a limit to 0-180 and sends the limited value over the CAN-ID 0x00C to the display D1000.
The display D1000 sends the keypress to the STG-650 over the CAN-ID 0x002. The STG-650 shows the receipt of the message over its own status LED and then sends the message to the STG-550 over the CAN-ID 0x00B. The STG-550 then shows the status of the button over its own status LED.
To send messages with the STG-550 over CAN bus, the CAN bus has to be initialized first. For that matter, the baud rate has to be set (see CAN Controller Init/Info).
In our example, the STG-550 does not communicate directly with the display, but with the connected STG-650. However, the STG-550 could also directly communicate with the display.
The STG-550 receives the signal LED-On over the first Byte of the CAN-ID 0x00B. In our example, the signal is sent from the STG-650. But the display could also directly send the signal. Now, the STG-550 converts the Byte signal over a convert block into a Boolean signal (LED-On > 0=HIGH, LED-On=0=LOW). Now, this Boolean signal can directly be connected to the LED block, which triggers the status LED of the STG-550.
A potentiometer is connected to the STG-550. The position of the potentiometer is received as voltage value over the voltage divider of the potentiometer. In our example, 0V and 24V are fed to the potentiometer. That is how, depending on the position of the potentiometer, a voltage drop of 0V up to 24V can be measured over the voltage divider. The measured value will then be converted into a floating-point number (FLOAT) 0.0-24.0 with the function block Analog Input IN1 and will be written on a signal line. As the position of the potentiometer is visualized with an analog instrument (0 to 180°), the voltage value has to be multiplied by 7.5 to reach a range of 0 to 180°. As only integral degree values should be sent, the floating-point number has to be converted into a Byte signal. That means, that the necessary CAN message is much smaller. The CAN block CAN send, sends the signal to the STG-650 over the CAN-ID 0x001. However, the display could also directly receive the message and display it.
In our example, the STG-650 actually does not make much sense. The messages are received over CAN bus from the STG-550 and sent to the display over another CAN-ID. Furthermore, the STG-650 receives the message to turn on the display LED and sends this message to the STG-550 with another ID. We just wanted to show you what is possible with the CAN bus system and more than one PLC.