Circuit cards and programmed PIC16F877 chips are readily available from JLC Enterprises; all you have to do is follow my instructions. 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. If you really just want to get started, skip ahead to SMINI Parts. If you have purchased your SMINI already assembled and tested, you may skip to Computer Connections. 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 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. The majority of C/MRI applications use current-sinking whereby railroad devices are activated, i.e. turned on, by the SMINI completing the ground connection.

These connectors, used on all previous C/MRI board designs, have proven to be extremely rugged, dependable and cost effective. 4.7 µF sets it to 350 ms. Again, the solenoids I have are more than happy with this. The 4-segment DIP switch, SW2, sets the baud rate with all segments off corresponding to 9600 bps. 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. If more than one segment happens to be turned on, the software within the SMINI defaults to using the baud rate corresponding to the highest switch segment with an "on" setting. 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.

Each of the 24 SMINI input lines feature optional input line filtering for maximizing immunity to electrical noise frequently encountered with pulse-type command control systems such as DCC and Railcommand. 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. If you happen to have one of these early SMINI cards simply ignore the 1, 2 and 3 printing on the card and treat them as if they read 0, 1 and 2. See section Counting Cards/Nodes Using Number Zero in Chapter 2 for change in approach. Each of the SMINI output ports is configurable for standard current-sinking or alternate current-sourcing. 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. 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.

The 24-pin inputs are noted as Card 2 along the right edge. 5Vdc and ground, use the two screw terminals at the top right corner of the card. The left header connects with the cable running back toward the computer and the right header connects with the cable running to the next node. Connections between the SMINI and the railroad are made using Molex-style right-angle header connectors. The heart of the SMINI is part U1, a Microchip Technology PIC16F877-20/P Microcontroller with built-in FLASH memory, making it a smart interface. The PIC16F877 performs all the bookkeeping as to which particular bit the computer is sending and whether it is part of an address or data. In "computer-ese" we refer to each pin, or line, RS485 standard as a bit and 8 bits become a byte. Layer two of the OSI model includes the bit and framing protocol, which is outside of these EIA/TIA standards.