자유게시판

HP is a company that makes and market particular hardware / software for speciallist application. It finds particular application in the Telecom and Industrial Diesel Generator where remote monitoring is employed. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s. The probe can be cut to any length from 150mm to 300mm. An RS485 interface with a definable address allows it to be used directly on a modbus network. RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. The standard is jointly published by the Telecommunications Industry Association and Electronic Industries Alliance (TIA/EIA).

The foreword to the standard references The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs. RS-485 signals are used in a wide range of computer and automation systems. The A signals of all devices should be connected with each other and the same goes for the B signals. For RS232a crossover cable is used where pin 1 is connected with pin 6 at the other connector, pin 2 with pin 5, etc. This interconnects the transmit and receive signals of both PLCs, and also the handshaking signals DTR and DSR. If handshaking is not necessary, an ordinary 4 wire telephone cable could be used, provided that the pins are cross-connected as shown in the picture below. Two wire telephone cables will not function because in that case the ground signal levels on both sides will be floating. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states.

The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conformant receivers detect a differential input down to 200 mV. These characteristics include: definition of a unit load, voltage ranges, open-circuit voltages, thresholds, and transient tolerance. These characteristics make RS-485 useful in industrial control systems and similar applications. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. Similar to Original Bobuino; ATMega1284, RS232 port, FTDI FT232RL USB, SD Card Socket, Screw terminals for 4 serial ports, power, 2 input buttons, 3 switch closures, additional SPI-Dual UART with RS485 interface drivers, 3 additional Status LEDs, Arduino shield compatible. Depends what you mean by a serial cable - Fibre, Ethernet, RS232, RS485, RS422, USB, HDMI, SMPTE292 are all examples of serial communications media. Relay Board with '1284, RS232, USB, directly driven relays.

Mega shield, relays driver by D22-D29. Uno shield, shift register controlled by any 3 pins. It also defines three generator interface points (signal lines); A, rs485 cable B and C. The data is transmitted on A and B. C is a ground reference. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection. Add some jumper wires for chip-to-chip SPI, I2C, or UART-UART comm's. RS-485, like RS-422, can be made full-duplex by using four wires.