Profibus Plugs and termination resistors

How to properly use Profibus plugs and termination resistors (2016-04-14 15:18:33) reproduced

Tags:profibus plug Termination resistor DP plug

Plug and termination resistors play a very important role in Profibus communication, they are very simple to use and do not have many complicated settings, but because of their simplicity, many engineers ignore the details in their use, causing many communication problems.

1 The structure and simple usage of Profibus plug

Figure 1Profibus Plug Structure

This is a common Profibus plug, if we have a, B two stations to do Profibus communication, how should connect the plug? Since there are only two stations on the bus, it is clear that the terminating resistor must be hit on the on position. So the wiring on the plug is going to go in one out.

Figure 22 Connection to a DP site

The correct way to do this is to connect the two plugs into the line end. Because the terminal resistor and the plug outlet End is 2 selected 1. The terminal resistor is on, the input end is connected with the terminating resistor, the connection to the outlet end is disconnected, the terminating resistor is off, the input end is disconnected from the terminating resistor, and the outlet terminal is connected.

2 Common Profibus bus connections

Figure 3 main station at the bus end point

Figure 3 shows a general Profibus bus connection method, where the main station is located at one end of the bus and the terminating resistor is on.
Then connected to the back of the site, the middle of the site terminal resistance to play off, the last side of the site terminal resistor hit on.

Figure 4 Main station in the middle of the bus

Sometimes the main station can also be installed in the middle of the Profibus bus due to the distribution of the field equipment, as shown in practice 4.

The terminal resistor on the device can not power off, 5 of the Profibus plug in addition to the 220-ohm terminal resistor has two 390-ohm bias resistor, and the bias resistor must be connected to the power supply.

Figure 5 Termination resistor and bias resistor

If the terminal device needs frequent power outage maintenance, or terminal equipment only terminal and No 9-pin D-type socket, you need to use the active Terminal module as a Profibus bus terminal (6ES7 972-0da00-0aa0).

Figure 6 Profibus active Terminal module

If the Profibus cable is not long enough, you need to connect the two cables, you can not simply twist the two copper cores, because this will damage the characteristic impedance of the cable, may lead to communication problems. It is best to use the connector in Figure 7 to connect two cables that need to be connected.

Figure 7 Profibus Connection Connector

3 How to use the terminal resistor of the RS485 repeater
The maximum length of the Profibus communication cable depends on the baud rate of the communication, and if the cable exceeds the maximum length of the communication, a RS485 repeater is required to extend the communication distance.

Table 1 relationship between bus length and transmission rate

Transfer Rate (KBIT/S)	9.6-187.5	500	1500	3000-12000
Bus Length (m)	1000	400	200	100

There are terminal blocks on the repeater and the Profibus cable can be connected directly to the terminal. In addition, the repeater is also equipped with a terminal resistor. The usage is the same as the cable plug.

Figure 8 1 network segment only into the line, 2 network segment has the incoming line outlet

Figure 9 1 network segment only into the line, 2 network segment only into the line

Fig. 10 1 network segment has incoming line outlet, 2 segment has incoming line outlet

4 Olm Plug Termination resistor use method

If the field equipment communication distance, or the field electromagnetic interference is serious, you can use Olm to convert electrical signals into light signals, using optical cable to transmit signals. The Olm has a RS485 electrical interface and requires a Profibus plug to connect the cable. and Olm electrical interface regardless of the connection between the main station or slave station, the same way.

Figure 11 1 Network segment only into the line, 2 network segment only into the line

Figure 12 1 Network segment has an incoming wire outlet, 2 segment only into the line

Fig. 13 1 network segment has incoming line outlet, 2 segment has incoming line outlet

For a olm with only one RS485 interface, it can be considered that there are only 1 network segments, the same method.

5 DP/DP coupler Plug Termination resistor use method
Two DP master stations can use DP/DP coupler to transfer data. There are two RS485 interfaces on the DP/DP coupler, and the connection method is the same as Olm.

Figure 14 1 network segment only into the line, 2 network segment only into the line

Figure 15 1 network segment only into the line, 2 network segment has the incoming line outlet

Fig. 16 1 network segment has incoming line outlet, 2 segment has incoming line outlet

Profibus Plugs and termination resistors