Five Predictions on Rs485 Cable in 2024

When two or more drivers share a pair of wires, rs485 cable each end of the link has a termination resistor equal to the characteristic impedance of the cable. Isolating the link reduces or even eliminates these problems. This causes problems in multipoint applications when the transceivers connected on the bus are in receive mode simultaneously. See Figure 10 for a graphical representation of the bus states and differential input voltage. Figure 8 shows a configuration of a master/slave RS-485 network with no fail-safe biasing. Table 2 shows a comparison of different termination techniques. RS-485 links are a little more complicated because of the fact that they have two signal wires that share a termination as well as a ground return path. In I&I applications, lightning strikes, power source fluctuations, inductive switching, and electrostatic discharge can cause damage to RS-485 transceivers by generating large transient voltages. Since it uses a differential line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres). A TVS diode can be modeled as a large resistance in parallel with some capacitance while working below its breakdown voltage. When a transient is generated and the surge voltage is larger than the breakdown voltage of the TVS, the resistance of the TVS decreases to keep the clamping voltage constant.

The protection is accomplished by clamping the voltage spike to a limit, by the low impedance avalanche breakdown of a PN junction. The differential input threshold voltage (VTH) of a receiver is the voltage on the receiver input at which a transition (low to high or high to low) of the receiver output is guaranteed. It is possible to use longer cables when low data rates are used. When high data rates are used, the application is limited to a shorter cable. The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. RS232 typically uses a 9-pin or 25-pin connector, with specific pins designated for transmission, reception, and ground. The devices use modbus RTU except for one that uses modbus ASCII. The master and slave could even exchange ascii QED-Forth operating system commands. Master drivers for RS-485 must be placed in the middle of slave devices to take advantage of the standard’s impedance guidelines. It receives bytes sent by a slave device via the "master in/slave out" pin, MISO.

The CPOL and CPHA bits configure the synchronous clock polarity and phase and specify when valid data is present on the MISO and MOSI data lines. It can transfer as well as process the data. When it comes to communication protocols for data transfer and control in various applications, RS485 competes with several other communication types. It is a half duplex protocol, meaning that only one party at a time may transmit data. In RS-422 applications there is only one driver on the bus and if termination is to be used it must be placed at the end of the cable near the last receiver. Figure 10. Differential Input Voltage and the Receiver Output State. Figure 12. Differential Input Voltage and Receiver Output State. New generation RS-485 transceivers have an improved feature that includes true fail-safe receiver inputs. One of the key advantages also is that RS-485 can drive data a long distance. RS-232 exhibits a single-ended type of data transfer. When communicating at high data rates, or over long distances in real world environments, single-ended methods are often inadequate. This data is connected to the UART and can cause erroneous system operation.

In RS-485 applications, there are often long links, which can cause the ground potential at different nodes on the bus to be slightly different. On a long line, the reflections are more likely to continue long enough to cause the receiver to misread logic levels. This is known as a bus idle condition and in this case, the differential voltage on the bus (VOA − VOB) is 0 V. Under this condition, the receiver output (RO) is undefined by the RS-485 standard and, thus, the receiver output can produce random data. The time required for a signal to propagate down the line to a receiver determines if a line is considered a transmission line. See Figure 14 for an example of how to achieve signal and power isolation using the ADM2485. Galvanic isolation is a perfect solution if there is no guarantee that the potential at the earth grounds at different nodes in the system are within the common-mode range of the transceiver. When it is low, the transceiver is in receive mode. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers. SD card with 74HC4050 SPI buffering, MAX3232 for RS232 on Serial 2, battery bacled DS1307 with 32.786 KHz crystal.