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The cost is $34. 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. Circuit cards and programmed PIC16F877 chips are readily available from JLC Enterprises; all you have to do is follow my instructions. If you have to know that it will work before you pull the cable, then get the required length of the cable and move the two devices next to each other, connect them and see if they can talk. 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 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. Simply cut the narrower circuit trace located on the bottom side of the card under each of the resistor locations, and install the corresponding R6-R29 resistors and C24-C47 capacitors.

Which circuit is used depends on whether you are using RS232, or RS485 (RS422) I/O. In RS485 standard, data is transmitted via two wires twisted together also referred to as "Twisted Pair Cable". 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. 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. RB0 - RB5 used as outputs to drive the 6 latch enable lines for separately activating each of the 6 SMINI output ports. 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. 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. L1 (green) blinks to show that U1’s internal program is operating correctly and also provides error codes when not operating correctly.

Also note that the symbol does not show the input. There are a number of other things to note with this model. Only one output line is illustrated because all are identical. 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. 5V TTL signal level, with anti-reverse connection and anti-over-voltage circuit on the power supply side. The RS-485 circuit common is connected to earth ground in both PCs. For RS232 the interface uses only the Pin-2 and Pin-3 data lines and the Pin-5 or Pin-7 ground lines, so it is independent of the sometimes unpredictable RS232 handshaking lines. Seven input lines, RA0-RA5 and RC4, are connected to DIP Switch SW1 for reading the SMINI’s node address, UA. 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. If you prefer, all the circuit boards, including the SMINI, can be purchased fully assembled and tested or as complete kits from SLIQ Electronics. The foreword to the standard references The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs.

RS485 is also used in performance venues to control lights and systems such as the one shown below. The 7-segment switch can set 128 unique addresses, allowing up to 128 SMINI cards in an RS485 system. The same design works whether we are using it plugged into the SMINI or the SUSIC and whether we are using RS232, RS422 or RS485 I/O. 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. Looks to be similar to Ethernet. It is like the Universal Serial Bus (USB) or ethernet that we can find in many of our modern computers. A CRC or checksum can be added to the end of the data being transmitted. 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. If you do not have either one of these situations you can simply skip ahead to Input Line Filtering.