rs422-rs485-Phan 3 docx

Serial Communication EIA RS422 and RS485 Recommended Standard The purpose of this course note is to attempt to describe the main elements in Serial Communication. This application note attempts to cover enough technical details of RS422 and RS485. EIA - Electroonic Industries Association Introduction • The purpose of this note is to describe the main elements of an RS-422 and RS-485 system. • This note attempts to cover enough technical details so that the system designer will have considered all the important aspects in the data system design. Balanced Line Drivers • In a balanced differential system the voltage produced by the driver appears across a pair of signal lines that transmit only one signal. (2-6V) • C: Signal Ground. It isn’t used by a balanced line receiver in determining the logic state of the data line. • ENABLE: (Tristate condition of the driver) • RS-485 diver must have ENABLE control signal. • RS-422 driver may have this signal, but it is not always required. Balanced Line Receiver • A balanced differential line receiver sense the voltage state of the transmission line across two signal input lines. • Signal ground C is necessary in making the proper interface connection. • The 200mV to 6V range is required to allow for attenuation on the transmission line. • Common mode voltage is define as the mean voltage of the A and B terminals with respect to signal ground. Schmitt Trigger using Op-Amp vout = A(v2 – v1) The comparator will switch when V+=0. Then The effect of using a Schmitt trigger (B) instead of a comparator (A). Typical hysteresis curve (Non-inverting) Non-inverting Schmitt trigger Inverting Schmitt trigger The difference between the trip points is the hysteresis H and is given as sss V R R V R R V R R H 2 1 2 1 2 1 2)( =−−+= Let's say you detect a low-to-high transition at 2.5 V. A 100 mV hysteresis would mean that the low-to-high transition is detected at 2.55 V and the high-to-low transition is detected at 2.45 V, a 100 mV difference. Vcc = Voltage Common Collector Bipolar Vee = Voltage Common Emitter Bipolar Vss = Voltage Common Source FET Vdd = Voltage Common Drain FET EIA Standard RS-422 Data Transmission • Each generator can drive up to 10 receivers. The 2 signaling states of the line are define as follows: • When the “A” terminal of the driver is negative with respect to the “B” terminal, the line is in a binary 1 (MARK or OFF) state. • When the “A” terminal of the driver is positive with respect to the “B” terminal, the line is in a binary 0 (SPACE or ON) state. • Like RS232, the data bits are transmitted in reverse order. The terminator is placed at the end of a transmission line or daisy chain bus, designed to match impedance and hence minimize signal reflections. • Figure shows the condition of the voltage of the balanced line for an RS-232 to RS-422 converter when the line is in the “idle” condition or OFF state. • The “A” terminal is equivalent to the “-” designation. • The “B” terminal is equivalent to the “+” designation. • The same relationship also applies for RS-485 systems. • RS-422 can withstand a common mode voltage (Vcm) of ±7 volts EIA Standard RS-422 Data Transmission DVM: Digital voltmeter • As many as 32 driver/receiver pairs can share a multi drop network • Many characteristics of the drivers and the receiver are the same as RS- 422. • The range of the common mode voltage Vcm that the driver and receiver can tolerate is expanded to +12 to -7 volts. • The driver can be disconnected or tristated from the line. • The signal ground line is also recommended in an RS-485 system EIA Standard RS-485 Data Transmission [...]... communications over long bus lines Half duplex CONTROLLING THE DRIVER ENABLE CONVERT RS 232 TO RS485 Half duplex MAX 232 • A MAX 233 converts the RS- 232 signals to TTL levels, and the TTL signals connect to an SN75176B that provides the RS-485 interface • The MAX220–MAX249 family of line drivers/receivers is intended for all EIA/TIA- 232 E and V.28/V.24 communications interfaces, particularly applications where ±12V... specification for serial asynchronous or synchronous communications MAX 233 CONVERT RS 232 TO RS485 Half duplex The MAX481, MAX4 83, MAX485, MAX487–MAX491, and MAX1487 are low-power transceivers for RS-485 and RS-422 communication LM78LXX Series Output voltages of 5.0V, 6.2V, 8.2V, 9.0V, 12V, 15V Popular chip MAX 232 is used to convert RS 232 signals from/to TTL level MAX485 converts TTL level signals to RS485... current flows that would otherwise result from short high-voltage transients, such as those caused by electrostatic discharge (ESD) This is a small RS 232 to RS485 converter project it use for convert RS 232 signal level to RS485 level multidrop It can use with 32 slave to communicate with PC and embedded systems board for long distance less than 1.2Km( 4000 feet) Specification - 9-12Vdc power supply - Small... wire connection - Direct connect with PC on DB9 connecter - Use RTS signal to control direction - 32 Slaves (up to 256 slaves with some transceiver i.e MAX3088) J1 and J2 use for jump R terminate(120) at the end of communication line or last slaver The 75176 use for transceiver RTS active (logic 0 or -3 to -15V) when we need to send data Data Format and Protocols • Information content passing through... converts TTL level signals to RS485 standard RTS line is used to change transmission mode of MAX485 RTS signal is first converted to TTL level by MAX 232 then connected to DE and RE/ pins 2 LEDs are used to indicate current state of the interface CONVERT RS 232 TO RS485 Half duplex Diode 1N4001 provides protection against reverse polarity connection of the supply Adding a resistor to a line may limit damaging... > – and are both configurable via software (device configuration parameters) • Most common generic Handshake are available/selectable with RS 232 interface: – Hardware (RTS-CTS) – Software XON/XOFF  . the “B” terminal, the line is in a binary 0 (SPACE or ON) state. • Like RS 232 , the data bits are transmitted in reverse order. The terminator is placed. reflections. • Figure shows the condition of the voltage of the balanced line for an RS- 232 to RS-422 converter when the line is in the “idle” condition or OFF state. • The