1. In most RS-485 networks, the terminal node causes more problems than it can solve. In order to check which node has stopped working, it is necessary to cut off the power of each node and disconnect it from the network. The resistance value between the receiving terminals A and B or and - is measured using an ohmmeter. The fault node's reading is typically less than 200 ohms, and the non-faulty node's reading will be much larger than 4,000 ohms.
2. Which line is A and which line is B is not very clear. Different manufacturers use different labeling rules, even if the B line should always be the higher voltage in the idle state. Therefore, the A line is equivalent to - and the B line is equivalent. It can be detected with a voltmeter while the network is idle. If the B line is not higher than the A line voltage, then there will be a connection problem.
3. RS-485 uses a pair of unbalanced differential signals, which means that every device in the network must be connected to ground through a signal loop to minimize noise on the data lines. The data transmission medium consists of a pair of twisted pairs, and a shielding layer should be added in a noisy environment.
4. When there is no device to transmit and all devices are in the listening state, a three-state state will appear in the RS-485 network. This will cause all drivers to enter a high-impedance state, returning the floating state back to all RS-485 receivers. In order to overcome this unstable state, the node designer typically installs pull-down and pull-up resistors on the A and B lines at the receiving end to simulate the idle state. In order to check this bias, the voltage from line B to line A should be measured while the network is powered and idle. In order to ensure that it is far from the indeterminate state as shown in the figure, it is required to have at least a voltage of 300 mV. If the terminating resistor is not installed, the biasing requirements are very loose.
5. Once the RS-485 network is up and running, record every detail of its configuration. Includes terminal information, offsets, line types, and spare parts information. If you can afford it, you should purchase some spare parts and store them in the cabinet.
6. Each reliable medium to long distance networking technology has some form of built-in isolation, in addition to RS-485. It relies on the system designer to ensure that the network does not include any ground loops. Isolating each node will increase the reliability of the network by an order of magnitude.
7. Although isolation is the first line of defense against power surges, adding multi-level surge suppressors can attenuate larger surges and ensure they are within the limits of network isolation. It is best to install a surge suppressor at a location where the network has a high-performance ground point. connects it to the earth at the same point, just like other network equipment or the electrical system of a factory .
8. A twisted pair grounded RS-485 network can transmit data both upstream and downstream. Since no two senders can successfully communicate at the same time, the network appears to be idle in a time slice after the last bit of data is transmitted, but in fact the node has not brought its driver into a three-state state. If another device attempts to communicate during this time period, an unpredictable conflict will occur.
To detect this conflict, use a digital oscilloscope to capture a few bytes of 1's and 0's. Determines the time it takes for a node to enter a three-state state at the end of the transfer. Make sure that the RS-485 software does not attempt to respond to requests that are shorter than one byte (slightly greater than 1 ms at 76.8 kb/s).
2. Which line is A and which line is B is not very clear. Different manufacturers use different labeling rules, even if the B line should always be the higher voltage in the idle state. Therefore, the A line is equivalent to - and the B line is equivalent. It can be detected with a voltmeter while the network is idle. If the B line is not higher than the A line voltage, then there will be a connection problem.
3. RS-485 uses a pair of unbalanced differential signals, which means that every device in the network must be connected to ground through a signal loop to minimize noise on the data lines. The data transmission medium consists of a pair of twisted pairs, and a shielding layer should be added in a noisy environment.
4. When there is no device to transmit and all devices are in the listening state, a three-state state will appear in the RS-485 network. This will cause all drivers to enter a high-impedance state, returning the floating state back to all RS-485 receivers. In order to overcome this unstable state, the node designer typically installs pull-down and pull-up resistors on the A and B lines at the receiving end to simulate the idle state. In order to check this bias, the voltage from line B to line A should be measured while the network is powered and idle. In order to ensure that it is far from the indeterminate state as shown in the figure, it is required to have at least a voltage of 300 mV. If the terminating resistor is not installed, the biasing requirements are very loose.
5. Once the RS-485 network is up and running, record every detail of its configuration. Includes terminal information, offsets, line types, and spare parts information. If you can afford it, you should purchase some spare parts and store them in the cabinet.
6. Each reliable medium to long distance networking technology has some form of built-in isolation, in addition to RS-485. It relies on the system designer to ensure that the network does not include any ground loops. Isolating each node will increase the reliability of the network by an order of magnitude.
7. Although isolation is the first line of defense against power surges, adding multi-level surge suppressors can attenuate larger surges and ensure they are within the limits of network isolation. It is best to install a surge suppressor at a location where the network has a high-performance ground point. connects it to the earth at the same point, just like other network equipment or the electrical system of a factory .
8. A twisted pair grounded RS-485 network can transmit data both upstream and downstream. Since no two senders can successfully communicate at the same time, the network appears to be idle in a time slice after the last bit of data is transmitted, but in fact the node has not brought its driver into a three-state state. If another device attempts to communicate during this time period, an unpredictable conflict will occur.
To detect this conflict, use a digital oscilloscope to capture a few bytes of 1's and 0's. Determines the time it takes for a node to enter a three-state state at the end of the transfer. Make sure that the RS-485 software does not attempt to respond to requests that are shorter than one byte (slightly greater than 1 ms at 76.8 kb/s).
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