What is the use of PROFIBUS Terminator? -Solutions - Huaqiang Electronic Network

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Q: What is the purpose of a PROFIBUS Terminator? When should it be used?

Question added: I'm referring to this specific device, not a standard DP connection port. The part number is 6ES7972-0DA00-0AA0.

Based on the documentation and what I observed on-site, there are no DP slaves connected on this PROFIBUS network. It was converted to RS485 for signal activation, and this terminator was used for that purpose.

A: The terminating resistor serves two main purposes:

1. It helps maintain an idle voltage level of approximately 1.1V on the bus.
2. It ensures that the terminal impedance matches the line impedance, preventing signal reflections that could cause data errors.

When a station is powered down, the impact of the termination resistor becomes more significant:

• The absence of the two 390Ω resistors causes the bus idle voltage to drop to around 0.62V, making it more susceptible to electrical noise and interference.
• The mismatch between the line impedance (150Ω) and the terminal impedance (110Ω) can lead to signal reflection. In theory, a reflected wave with about 15% of the incident wave's amplitude and opposite polarity may occur.

The terminating resistor should be placed at the end of the PROFIBUS DP network, similar to a standard resistor. However, in this case, three resistors are connected in series: 390Ω, 220Ω, and 390Ω. The DP bus is connected to the 220Ω side, while the other ends are connected to the power supply. The 220Ω side typically operates at around 1V, with the 5V supply connected to pins 5, 3, 8, and 6.

The position of the terminal resistor switch is set to "ON" for devices located at both ends of the network (those that only connect to the bus), and "OFF" for devices in the middle of the network to avoid signal conflicts.