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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. It also supports a higher level network connection based on this hardware, in the form of a BitBus netowk node implmentation. But, it should be noted that the network running in the reference was running at 300 baud and had other tweaks such as isolated transceivers and high resistance bias resistors on every node. In high quality cable, the conductor losses and the dielectric losses are on the same order of magnitude. 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. Circuit cards and programmed PIC16F877 chips are readily available from JLC Enterprises; all you have to do is follow my instructions. If you have purchased your SMINI already assembled and tested, you may skip to Computer Connections. Standards have been developed to insure compatibility between units provided by different manufacturers, and to allow for reasonable success in transferring data over specified distances and/or data rates. It can be set to a command mode under control of an external master, or to a continuous streaming mode where data is output at specified intervals at rates up to 60 per second.

The second optional grounding arrangement is to connect the circuit common directly to the frame ground without a resistor. A second installation has two desktop PCs sitting very close to each other. The desktop is connected to earth ground and the RS-485 port is referenced to the earth ground. Unfortunately RS-485 section 4.5.3 states that the total load between "A" and "B" should be no less than 54 Ω. RS-485 functions by defining the transmission of ‘0’s and ‘1’s via the positive or negative states of the transceiver terminals, A and B. To perceive the same logical state, all devices on the communication link must generate and detect signals of the same polarity.A reversed device will read and generate incorrect signals, leading to a communication breakdown. 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.

Programming the PIC16F877 FLASH memory requires a special PIC Micro Programmer package. A big advantage of the 877’s FLASH memory is that the ultraviolet erase cycle is not required as it was for the MC68701, used with the previous USIC. Table 4-2 summarizes the utilization of the 877’s I/O port lines for the SMINI application. A single 8-wire I/O data bus connects the PIC16F877 to each of the 9 I/O buffering functions. 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. There are also 9 separate I/O control wires - typically called port select or enable lines - joining the 877 to each of the 9 I/O buffering functions. One of these port select lines connects directly to the enable (EN) inputs for each of the port I/O buffer functions. The most significant difference is that more than one slave can talk back to the master. But this is only true if there is no ringing or noise on the line that switches the receiver back to a 0 at the end of the stop bit. The first bit a UART transmits is the start bit (0) and the receiving UART should see this transition.

The driver has the capability of driving 10 receivers of 4k impedance, but the actual number that can be driven depend on the actual input impedance of the receivers, bit rate, wire, stub lengths, biasing and termination of the network. Now look at the driver schematic symbol you should notice that the "A" output is the non-inverting output and the "B" output is inverting. Fig. 4-3 is a functional schematic illustrating what the SMINI does. 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. RC5 used as output to drive the transmitter enable line for enabling the RS485 transmissions from SMINI back to the PC. 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 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.

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