The 7-segment switch can set 128 unique addresses, allowing up to 128 SMINI cards in an RS485 system. This is handled by the 7-segment DIP switch feeding directly into the PIC16F877. Four input lines, RC0-RC3, connected to DIP Switch SW2, set the SMINI’s baud rate. The C/MRI is set up for full-duplex operation - two wires for transmitting from the PC and two wires for reception by the PC. 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. The MCU uses these pins to send data to the RS485 bus in a form determined by the user protocol. Experience with test equipment such as oscilloscopes, signal generators, and protocol analysers. MORNSUN transceiver modules include integrated high efficiency CAN, RS485, RS232 isolated transceiver module, protocol conversion module and serial port to ethernet transceiver module. Three wires are used with RS232.
But actually there are only two gaps - two gaps are enough to make the three ports isolated from each other. Each 24-pin grouping is subdivided into three ports labeled A, B and C. The 8-pins within the different ports are defined as A0 through A7, B0 through B7 and C0 through C7. Although all the I/O lines are physically on the same SMINI card it is convenient to talk about them as being separate (I/O) cards within the same (SMINI) card. 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. From the VA characteristic, we see the following: in the worst cases, with stable potentials of -7 V or 12 V in line, the current of the microcircuit is limited to 90 mA. It is a very rugged static protected design with built-in TTL compatibility including high sink/source current capability of 25mA per I/O line. 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.
Likewise, when U1 needs to read from I/O Card 2 Port C, it first sets Port RD to be input then it pulls line RE0, Pin 8, low to activate, or enable U5 and then it reads the input data that U5 places on the Port RD lines. 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. Looking at RC5, Pin 24, the Transmit Enable (TXEN) line, this is a little more complicated. In "computer-ese" we refer to each pin, or line, as a bit and 8 bits become a byte. The use of the terms bit and byte will become second nature as you begin to put the interface to use. Firstly, it requires fewer wires or channels, rs485 interface making it more cost-effective and space-efficient. When this option is installed, it is then possible to install a conductivity sensor, a pressure sensor, or a flow meter pack to communicate further information via RS485.
It also keeps track of which information is data and which is address and determines when to activate each of the port enable lines for communication with the SMINI’s 9 different I/O ports - 3 for inputs and 6 for outputs. One of these port select lines connects directly to the enable (EN) inputs for each of the port I/O buffer functions. 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 SMINI has six 8-bit output ports labeled as Ports A, B and C for "Cards 0 and 1." Similarly, the three 8-bit input ports are labeled as Port A, B and C for "Card 2." Each I/O port includes an important buffer between the railroad’s connection to the SMINI card and the PIC16F877. Of its 40 pins, 33 are I/O lines, configured in three 8-bit ports, one 6-bit port and one 3-bit port. DIP containing a Micro-Processor Unit (MPU), 8K 14-bit words of FLASH Program Memory, 368 bytes of RAM Data Memory, 256 bytes of EEPROM data memory, a Serial Communications Interface (SCI), built-in baud generator, three programmable timers, and an abundance of I/O pins.
Here is more about RS485 standard look into the web-page.