이름 : Delphia
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예식일 : Thoughts Blowing Method On Rs485 Cable
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When PE is cleared (equal to zero), the most-significant bit of each transmitted character will be a data bit. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). Termination resistors also reduce electrical noise sensitivity due to the lower impedance. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. We will pull the enable pin high to put the MAX485 module in transmitter mode. 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. The only way this would not work is if the secondary device only takes "write only to device" transmissions with no possibility of any acknowledgement or return result. 2 wires is all you need to make it work. But it does not work ALL the time. It gets away most of the time with this configuration, with the shield draining noise and providing the ground reference at the same time. And even worse, if you have a non opto-isolated device installed on that network, any noise could "drain" to ground through the device itself instead of flowing through the ground terminal at the end of the line, damaging the device in the process!

To make sure that your USB to RS-485 cable & the device RS-485 port are working, have you tried using them with a Windows PC and the manufacturer's s/w? These characteristics make RS-485 useful in industrial control systems and similar applications. 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. RS-485 only specifies the electrical characteristics of the generator and the receiver: the physical layer. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. RS-485 signals are used in a wide range of computer and automation systems. 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. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data.

Since RS-485 is a multi-point specification, however, this is not necessary or desirable in many cases. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. An example of how a byte(0x3E) is transferred over the two lines of RS485 Communication. Even though the MOSI pin is not connected to anything, the master initiates a transmission using a "dummy" byte. As the master transmits a byte to an active slave (that is, a slave with its /SS input active low), the master receives a byte from the slave. The status of a device as master or slave determines how the various pins must be configured. As the master transmits its data, 8 bits of data are simultaneously received. A mode fault occurs when the SPI senses that a multimaster conflict (MC68HC11F1 Technical Data Manual, p.8-7) exists on the network as explained above in connection with the /SS input. By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus.

If the clock idles in the high state, the leading edge of the clock is a falling edge. They take over at high frequencies while the regulator is really meant for DC and "DC-like" low-frequencies. Standards have been developed to insure compatibility between units provided by different manufacturers, and to allow for reasonable success in transferring data over specified distances and/or data rates. Also, when some devices on the RS485 are NOT opto-isolated, the third wire becomes essential to avoid the units being damaged. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. For the QScreen, /SS is not used for SPI communication because it is used to control the direction of the RS485 transceiver; you can use any digital I/O line as a /SS signal. The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or bus, not a star, ring, or multiply connected network. Utilizing a series of dedicated interface devices, it allows PCs and industrial controllers to communicate. This allows RS-485 to implement linear bus topologies using only two wires. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422.

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