Rs485 Cable Guide > 자유게시판

The following table shows the connection diagram for a standard 9-pin serial cable. The secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent. There are surface-mount resistor pads on the Docking Panel to bring out the RS485 signals to the DB9 Serial 1 Connector. Of the processor’s three synchronous SPI (Serial Peripheral Interface) ports, two are available for inter-processor communications on multi-processor systems, and the third is brought out to the Wildcard expansion bus. Since both channels can operate simultaneously and independently, serial debugging can be performed while the application program is communicating via its primary channel. Two devices are at the ends of the cable, while others are connected somewhere in between. The advantage of using Serial1 for RS485 is that the Serial1 RS485 signals are also available on the Docking Panel, while the Serial2 RS485 signals are available only on the PDQ Board’s Serial Communications Header. The PDQ Board’s transmit data signal /TxD1 (pin 2 on the 9-pin serial connector) is connected to the terminal’s receive data signal /RxD (pin 2 on its 9-pin connector). The RS485 data direction of Serial1 and Serial2 are controlled by the PJ0 and PJ1 processor pins, respectively.

Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: /TxD1 (transmit data), /RxD1 (receive data), rs485 cable and DGND (digital ground). It is NOT necessary to introduce long delays in a network to avoid "data collisions." Because delays are NOT required, networks can be constructed, that will utilize the data communications bandwidth with up to 100% through put. From the PDQ Board’s point of view, these three signals (/TxD, /RxD, and ground) are the only connections required to perform serial communications. To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. Moreover, if Serial2 is running full duplex at 4800 baud, any other interrupt service routine that takes longer than 100 µs is likely to cause a problem.

The Serial 1 and Serial2 ports can be configured for either RS-232 or RS-485 communications at standard baud rates up to 115200 bits per second. The Serial1 and Serial2 ports are is supported by the HCS12's dual on-chip hardware UARTs, and do not require interrupts to work properly. After a factory cleanup, Serial1 is the default serial port for program development and downloading. The adapter plugs into the serial port of the PC and is powered from the port for most applications. This page describes the serial ports and how to use them for instrument control and automation applications. Because a single pair of conductors is used for both transmission and reception, RS485 is useful for multi-drop applications in which a master communicates with multiple slave serial devices, or nodes. Each serial port can be configured for the RS232 or RS485 protocol, and runs at standard baud rates up to 115,200 bits per second. Receiver Input Resistance (Ohms), (1 Standard Load for RS485) 3k to 7k 4k min. These 9-pin standard DB-9 serial connectors are located on the back of the Docking Panel. As described below, the RS485Transmit() function controls the RS485 data direction of each serial port.

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. Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the PDQ Board brings out its serial ports to two female 9-pin D-Type connectors on the Docking Panel. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector, although custom placement of zero-ohm surface-mount resistors on the Docking Panel can route the RS485 signals to the DB-9. For this reason, frame-level cyclic redundancy checks are much more widely used for validating data from serial links, network connections and storage media. These protocols are summarized on this page, but for more information regarding their data formats and their use for simplex or multi-drop serial lines, consult Understanding Serial Communications (but keep in mind that that page is directed to the use of the UART Wildcard, so it uses different driver functions).