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The 7-segment switch can set 128 unique addresses, allowing up to 128 SMINI cards in an RS485 system. For the system to work, only the transmitter IC from one SMINI can be allowed to seize, i.e. transmit over, the bus at any one time and all other SMINI cards must have their U7 ICs held in their high impedance (open circuited) states. This provides easier system operation and debugging capability. Including the diode provides an ultra fast reset to minimize the wait time required when cycling power. L1 (green) blinks to show that U1's internal program is operating correctly and also provides error codes when the SMINI is not operating correctly. 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.

When the C/MRI application software, resident in the PC, requests that a given SMINI node send the railroad data to be received as inputs on its 3 input ports, the U1 gathers the data, converts it to serial format, then activates U7’s DE enable line, which then seizes the serial bus for sending the input data to the PC. The RS485 standard defines a balanced two-wire transmission line, which may be shared as a bus line by up to 32 driver/receiver pairs. Only one SMINI, as determined by the PC, may be permitted to communicate back to the PC at any point in time. However for special cases that may arise, it is easy to vary the output configuration to handle the alternative current-sinking case. By substituting a 2N4403 PNP transistor and changing card jumper locations it is also possible to obtain a current-sourcing output configuration which I will cover in the next section. 5Vdc. Each SMINI output port is separately configurable by selecting the appropriate output transistor and installing the appropriate jumper. 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.

The ComBoard RTS input line controls the driver enable line so the device only talks when it is its turn. This way only one device, the driver IC on the RS485 card, is in command of the two transmit lines. The driver IC on the RS485 card is capable of driving 32 unit loads or 128 of the ¼ load devices. Many (on-line) App-notes use 12K ohms as an input impedance, 60 ohms (as a termination), 3.5 mA as the minimum 'load' current, and 10 loads. The required load is 5volts / 3.5mA, which results in a load impedance of (around) 1428 ohms. The filter’s cut-off frequency (in units of cycles/second, or Hz), is calculated as 1/(6.28 x R x C), where R is expressed in ohms and C is expressed in farads. This can be solved, at least in part by ensuring the hardware units (converters, repeaters, micro-processor controls) are designed to remain in a receive mode until they are ready to transmit data. Large capacitors also have a disadvantage by creating longer-lasting surge-currents when card inputs are switched to ground by the external hardware. Circuit cards and programmed PIC16F877 chips are readily available from JLC Enterprises; all you have to do is follow my instructions.

You should NEVER have U6 and U7 installed if you have U8 installed and vice versa. If you are interested in such fast tracking of inputs, and you installed the filter time constant of 47ms, it would take about 9,400 samples before you detect an input change, a totally unacceptable situation when seeking high speed data acquisition. The 8-lines of U1’s Port RD (Pins 19-22 and 27-30) are the bi-directional I/O data bus connecting U1 to the SMINI’s 3 input ports and 6 output ports. Lines RB0-RB7 and RE0 perform as port enable lines to define which I/O port IC (U3-U5 for input or U9-U14 for output) is activated for each input and output operation over the I/O data bus connecting all the ICs to U1’s Port RD. RE0 used as output to drive the 3rd SMINI input port enable. When U1 needs to send data to an output port the U1 software switches the eight Port RD lines to be outputs. RB0 - RB5 used as outputs to drive the 6 latch enable lines for separately activating each of the 6 SMINI output ports. By selectively activating only one of these 9 lines at any one time, the 16F877 sets up direct communication to and from the desired port via the single function 8-wire bi-directional I/O data bus.