PLC | RS485 to use HMI communication is the best choice

In the last article written, because the relationship between the PLC scan time,
Makes the communication time <scan time,
A lot of time wasted on the PLC scan time.

The HMI is to use the firmware to handle the communication, to avoid the problem out of the PLC scan time.

If you use the HMI to do RS485, another relative problem will occur.
1 HMI communication with the PLC's different way of writing.
2.PLC, HMI & control of both sides of the program, follow-up would be more trouble Debug.
3 HMI will usually far away from the other components, interference issues to consider.

PLC|RS485使用人機來通訊，最好

PLC|RS485使用人機來通訊是最好選擇

在上一篇文章寫到，因為PLC的掃描時間的關系，

使得通訊時間<掃描時間，

很多時間浪費在PLC的掃描時間。

而人機是使用韌體來處理通訊方式，可以避免掉PLC掃描時間的問題。

如果使用人機來做RS485，又有相對的問題會發生。

1.人機的通訊方式跟PLC寫法不同。

2.PLC、HMI兩邊都有控制權&程式，後續DEBUG會比較麻煩。

3.一般人機會離其他元件較遠，干擾問題要考慮。

PLC|RS485 PLC to write the key

RS485 communication speed up to 115200bps (bits per second)
115200bps/1000 = 115.2bp (ms)

Data string = begin code + data (7-8 bit) + Check code + end code = 11bit


115.2bp (ms) / 11bit = 10.5 data streams / ms


The existing PLC scan time of about 4-20ms, in fact, are much larger than thecommunication speed of RS485.

So the PLC simplified way, so at least the number of scans, you can complete thetransmission & reception,
Is the biggest key.