Find Out Who's Talking About RS485 Standard And Why You Should Be Concerned > 자유게시판

Fig. 4-3 is a functional schematic illustrating what the SMINI does. The arrows in Fig. 4-3 indicate the direction of signal flow, and double lines with a slash and number indicate multiple parallel wires - four, seven or eight in our case. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. This serial port was no longer a "RS-232 compatible" serial port since the connector and signal levels no longer matched the RS-232 standard. Sometimes it is necessary to transmit over longer distances, or to run networks with spurs off the main cable run. On the serial side of the SMINI, one of two special I/O circuits couples the PIC16F877 to the serial I/O lines coming to SMINI from the main computer, the PC. Its main hardware functions are address decoding, baud rate generation, parallel-to-serial and serial-to-parallel conversion, input/output port selection, input port buffering and output port latching.

Most functions are handled directly by the PIC16F877 including all the parallel-to-serial and serial-to-parallel conversions and determining the operational timings for both the serial and parallel lines. Circuit cards and programmed PIC16F877 chips are readily available from JLC Enterprises; all you have to do is follow my instructions. Besides being more than twenty times faster and much more powerful than the MC68701 used in the original design USIC, the programmed PIC16F877 is about half the cost. The cost is $34. RS-485 does not specify any protocol, addressing scheme, or connector. If handshaking is required, it can be attempted using using X-On / X-Off handshaking protocol, but it is unlikely to work. Software within the 877 can configure each line of each port as either input or output. For example, if the 877 wants to read data from Input Port A on Card 2 it activates the port enable line connected to the enable input on Input Buffer U3.

RD0-RD7 used bi-directionally as inputs to read the 8-bit parallel data coming in from the 3 SMINI input ports and as outputs to send the 8-bit parallel data going to the 6 SMINI output ports. RB0 - RB5 used as outputs to drive the 6 latch enable lines for separately activating each of the 6 SMINI output ports. RB6 - RB7 used as outputs to drive 2 of the 3 input buffer enable lines for separately activating 2 of the 3 SMINI input ports. The functional blocks, noted as U9 through U13, perform as buffer/line-drivers for the 6 output ports. For SMINI outputs, say Output Port C on Card 0, the PIC16F877 places the desired output data on the 8-wire I/O data bus and then activates the port enable line connected to the enable input on Output Buffer U11. Correspondingly, U11 takes the data found on the I/O data bus and transfers (and latches) it to the correct 8 output pins connected to the railroad. RC6 used as output for sending serial data back to the PC. RC7 used as input for reading in serial data coming from the PC. When an RS-485 driver gets its data directly from a UART (with no added inversion), you would expect the "A" and "B" wires to match the voltages in the RS-485 standard for the voltages on the wires, but they will not (unless the driver inverts its input).

4 (DI - the driver's input pin) is high, the output voltage on pin 6 (The A output) will be positive with respect to pin 7 (the B output). This works well for most applications, and is how the original input cards (the CIN24 cards using the 8255) function. While the RS-485 specification does not specify cabling, these recommendations should be used for RS-485 systems as well. ↑ "RS-422 And RS-485 Applications Ebook" (PDF). RS422. RS422 is also specified for multi-drop (party-line) applications where only one driver is connected to, and transmits on, a "bus" of up to 10 receivers. The provided diagram illustrates the equivalent output schematic of the RS485 driver. These connect U1 to the SMINI's interface-unique circuitry for either RS232, or RS485 I/O. Which circuit is used depends on whether you are using RS232, RS485 standard or RS485 (RS422) I/O. RC5 used as output to drive the transmitter enable line for enabling the RS485 transmissions from SMINI back to the PC. RE0 used as output to drive the 3rd SMINI input port enable. RC4 used as input to read in the SMINI address DIP switch segment A6. To place multiple SMINI cards on the same RS485 4-wire cable, we must be able to set a unique SMINI Address, UA, for each card.