이름 : Manuela
이메일 : manuelamcgahan@yahoo.com
연락처 :
예식일 : Super Mini-Node Interface Card (SMINI) - Part 1
문의내용:

If you are using RS232 you simply install U8, or for RS485 you install U6 and U7. U8, a dual RS232 transmitter and receiver IC, handles communication in both directions when using RS232. Since a receiver may have a loading of less than one, the actual number of receivers that can be connected depend on the unit load rating of the receivers, as well as the wire, bit rate, stub lengths, biasing and termination of the network. You may not have the polarity you expect. Single board solutions may be hooked up as shown, but chassis to chassis systems require a common ground connection to run between the driver and receiver. 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. Common standards: RS232 RS422 RS449 RS485 20 mA current loop Serial data transmission standards including RS232, RS422, RS423, and RS485 were widely used for many data links, RS485 standard proving effective connectivity for the day. While this is incorrect, many to this day still believe that the RS-232 standard includes things such as baud rate and the bit stream protocol used in the IBM serial port.

This is the 7 or 8-bit protocol commonly associated with serial ports. The 8-lines of U1’s Port RD (Pins 19-22 and 27-30) are the bi-directional I/O data bus connecting U1 to the SMINI’s 3 input ports and 6 output ports. Each of the port latch outputs from U9 through U14 pass through a transistor, Q1 through Q48, before reaching the SMINI card’s 6 output ports. When the latch output is low, Q1 is turned off and the load is de-activated. These transistors, typically 2N4401 NPN transistors, operate in an open collector configuration to provide a sinking capability of .25A for loads up to 40Vdc. Railroad loads such as lamps, LEDs, relays, and switch motors are turned on and off by your PC software turning each output transistor on (transistor conducting with collector connected to ground) or off (transistor open circuited). For the system to work, only the transmitter IC from one SMINI can be allowed to seize, i.e. transmit over, the bus at any one time and all other SMINI cards must have their U7 ICs held in their high impedance (open circuited) states. The cornerstone of this complex system is a dependable physical sensing network!

This provides easier system operation and debugging capability. U6 and U7 are identical RS485 transceiver ICs capable of transmitting and receiving over the same 2-wire transmission line to support half-duplex operation - two wires transmitting bi-directionally. Three wires are used with RS232. The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices. To support a distributed serial system, multiple SMINI receiver and transmitter ICs, U6 and U7, are connected in parallel on the same two sets of two wires leading back to the PC. Two wires are used to send data from the PC to the SMINI cards and two wires are used to send data from the SMINI cards to the PC. Because none of the other SMINI cards were addressed, their corresponding U7’s are held in their high impedance tri-state, effectively open circuited, to prevent conflict with the operation of the addressed SMINI.

For this to happen without interference, the outputs from the U7 transmit ICs on all the other SMINI cards must be held in a high impedance state, effectively an open circuit to prevent loading down the bus. 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. Circuit designers will be quick to observe that RN12 is not really required because the high output of the latch would turn on Q1 independent of having RN12. Each switch is separately controllable by software giving you total freedom to turn each circuit on and off as desired. Software is used to define the latch output, and therefore we have total control of the load’s operation. The reason for this is that different installations have different grounding requirements. You should NEVER have U6 and U7 installed if you have U8 installed and vice versa. RS-422 is similar (except they leave the hyphen out) Both standards have a section defining why you may want fail-safe operation, but neither discuss how to implement it. When a "bad message" shows up on the cable, it is more difficult (but not impossible) to figure out which node(s) transmitted that message when you have a shared-medium with a dozen nodes connected to the same single cable, compared to a point-to-point medium with only 2 nodes connected to any particular cable.