The RS485 standard was first developed by EIA, then revised by tia named TIA/EIA485-a. RS-485 allows multipoint, bidirectional communication over a pair of twisted pairs of wires, with outstanding manic suppression, data transmission rate, cable length and reliability. RS485 (RS232 to RS485) is a low-cost, easy-to-operate communication system, but in the Some of the details of the improper handling will often lead to communication failure or even system paralysis and other failures.
1. RS-485 (RS485 converter) uses a pair of unbalanced differential signals, which means that each device in the network must be connected to ground through a signal loop to eliminate noise on the data line. The data transmission medium consists of a pair of twisted pairs of wires that should be shielded in noisy environments.
2. In the vast majority of RS-485 networks, the terminating node causes more problems than it can solve. In order to check which node has stopped working, it is necessary to disconnect the power to each node and disconnect it from the network. Use an ohmmeter to measure the resistance between a and b or + and – at the receiving end. A faulty node will usually read less than 200 ohms, while a non-faulty node will read much more than 4,000 ohms.
3. It has never been clear which wire is a and which wire is b. Different manufacturers use different labels to specify which wire is a and which wire is b. The labeling requirements are different for different manufacturers. Different manufacturers use different labelling, even though wire b should always be the one with the higher voltage in the idle state. Thus, wire a is the equivalent of – and wire b is the equivalent of +. This can be checked with a voltmeter when the network is idle. If line b does not have a higher voltage than line a, there is a connection problem.
4. When no devices are transmitting and all devices are listening, a tri-state condition occurs in the RS-485 network. This causes all drivers to go into a high-resistance state, causing a hung state to be transmitted back to all RS-485 receivers. A typical method used by node designers to overcome this unstable state is to simulate an idle state by adding pull-down and pull-up resistors to lines a and b on the receive side. To check this bias, the voltage from line b to line a should be measured with the network powered and idle. A voltage of at least 300mv is required to ensure that you stay away from the indeterminate state shown in the figure. If no terminating resistors are installed, the bias requirements are very relaxed.
5. An RS-485 network with a twisted pair plus ground can transmit data both upstream and downstream. Since no two transmitters can successfully communicate at the same time, the network behaves as if it is idle for a time slice after one bit of data has been transmitted, but the node has not actually tri-stated its drivers. If another device tries to communicate during this time period, a conflict will occur with unpredictable results. To detect this conflict, use a digital oscilloscope to capture a few bytes of 1’s and 0’s. Determine the amount of time it takes for a node to enter the tri-state state at the end of a transmission. Ensure that the RS-485 software is not attempting to respond to a request that is shorter than the time for one byte (slightly more than 1ms at 76.8kb/s).
6. Every reliable medium to long distance networking technology has some form of inbuilt isolation, with the exception of RS-485. It is up to 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 orders of magnitude.
7. While isolation is a defence against power surges, the addition of multi-stage surge suppressors can attenuate larger surges and ensure that they are within the tolerable limits of network isolation. Surge suppressors are installed at locations in the network where there are high-performance grounding points. Connect it to earth at the same point as any other network device or factory electrical system.
8. Once the RS-485 network is up and running, every detail of its configuration should be documented. This includes terminal information, bias, wire type, and spare parts information. Spare parts should be purchased and stored in cabinets if affordable.
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