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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. Of its 40 pins, 33 are I/O lines, configured in three 8-bit ports, one 6-bit port and one 3-bit port. Seven input lines, RA0-RA5 and RC4, are connected to DIP Switch SW1 for reading the SMINI’s node address, UA. Four input lines, RC0-RC3, connected to DIP Switch SW2, RS485 standard set the SMINI’s baud rate. The 4-segment DIP switch feeding directly into the PIC16F877 sets the baud rate generator built into the 877 to the desired serial transmission rate. RC0 - RC3 used as inputs to read in the SMINI baud rate DIP switch segments. RA0 - RA5 used as inputs to read in the SMINI Address DIP switch segments A0 - A5. When the PC transmits data, every SMINI card listens for its address. RC4 used as input to read in the SMINI address DIP switch segment A6. This is handled by the 7-segment DIP switch feeding directly into the PIC16F877.

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. RC5 used as output to drive the transmitter enable line for enabling the RS485 transmissions from SMINI back to the PC. 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. Against, RS-232, this standard allows long cabling distances and multidrop communication links where multiple devices are organized on the same bus. RS-485 I/O (N,O) - These are standard RJ-15 4 pin modular connectors for connection to a multidrop network. And that with an interface which does not require intelligent network hardware: the implementation on the software side is not much more difficult than with RS232. 25.64 shippingOnly 5 left in stock (more on the way). This way only one device, the driver IC on the RS485 card, is in command of the two transmit lines. The Laureate interface board P/N L485 with dual RJ11 jacks allows multiple Laureate meters, counters, timers and remote displays to be daisy-chained to the same RS485 line and to be addressed in command mode.

Line RC7, Pin 26, is U1's serial data input Receive (RX) line, while RC6, Pin 25, is the serial data output Transmit (TX) line. 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. 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. The schematic in Fig. 4-4 is simply an expansion of the functional diagram in Fig. 4-3. It looks complicated but breaking it down into its various components and visualizing that many wires run in parallel to identical devices performing the same function, it becomes quite easy to understand.

Adding a resistor to the end of the cable that matches the value of the cable's impedance will absorb the signal traveling down the line and reduce or prevent reflection. 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. 3.The two-wire RS485 can only work in a semi-duplex mode, and the sending and receiving cannot be carried out at the same time. Low power consumption. Real Time Clock with battery backup. MORNSUN Power is a leading vertical industrial power supply manufacturer carrying over 6,000 standard power supply models to satisfy multiple industrial power supply application needs. 12V. A driver in the high impedence (off) state is able to remain in this state over the common mode range, whether power is applied or not. There are sophisticated gate driver chips that get around this, but I'm actually already using a part that can act as a reasonable driver in this situation: the 74HC2G34 buffer. The driver IC on the RS485 card is capable of driving 32 unit loads or 128 of the ¼ load devices.