| Дата рождения: | нет информации |
| WWW-страница: | http://www.prom-electric.ru |
| Пол: | Мужской |
| Профессия: | and open, read, write to, and close the port as needed. vb Dim comPortIndex as Integer Dim dataToWrite as String = "hello" Dim newReceivedData As String 1 The index of the selected COM port in the name array. comPortIndex = 1 UserPort1.SelectedPort.PortN |
| Место жительства: | Австралия, charge and induces a voltage. The result is that the receiver sees a higher voltage than what was transmitted. 117 Chapter 7 A TERMINATION GREATER THAN CHARACTERISTIC IMPEDANCE VOLTAGE ________ ACROSS TERMINATION IT 3T 5T 7T B TERMINATION LESS THAN |
| Интересы: | some RS-485 circuits, all of the nodes and the cable that connects them share a common ground. In other circuits, the line is isolated from the nodes it connects to. In either case, all of the drivers and receivers should share a ground connection, which can have any of several sources. Most obviously, the RS-485 cable can include a wire that connects to signal ground at each node. Or the nodes’ power supplies can share a common ground either through electrical wiring or via an earth ground. In a very short line, multiple nodes can share a power supply. The specifications for RS-485 interface chips limit the permitted difference in ground potentials. Isolating the line is sometimes easier than ensuring that earth grounds at distant nodes are within the required limits. Common-mode Voltage TIA-485-A doesn’t specify the voltage between each output and signal ground except to say that the common-mode voltage must be 7V or less. The common-mode voltage is the mean, or average, of the voltages on the two differential lines referenced to signal ground. The common-mode voltage measured at the receiver is a function of the driver offset voltage 1/2 the sum of the voltages at the A and B outputs , the difference in ground potentials at the driver and receiver, and any noise that appears equally on both lines. If the interface is perfectly balanced, the inputs are offset equally from 1/2 the supply voltage. Any imbalance raises or lowers the offset. To comply with TIA-485-A, components must work properly with common-mode voltages from -7V to +12V. In addition, each of the receiver’s inputs must be in the range of -7V to +12V referenced to the receiver’s ground. With differential signals as large as ±5V, the ground potentials at the driver and receiver can vary as much as ±7V and still comply with the standard’s common-mode limit. The data sheets for interface chips specify a common-mode limit, which is often larger than the minimum requirement. This brings us to guideline #6 prom-electric.ru |
