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When a binary 1 (aka OFF or marking) is being transmitted the voltage on TXData must be less than -3V. Using less resistance is not good, because lower values do not adequately limit current surges resulting from the capacitor being discharged when the input line is switched to ground by the external hardware. The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method. 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. With U9 enabled it transfers the data from the Port RD lines, the I/O bus, to the 8-output pins associated with Card 0 Port A that are connected to the railroad. Thus, two ICs are used where U6 is set up to receive data from the PC and U7 is set up to transmit data to the PC. Four input lines, RC0-RC3, connected to DIP Switch SW2, set the SMINI’s baud rate. RA0 - RA5 used as inputs to read in the SMINI Address DIP switch segments A0 - A5.

RC4 used as input to read in the SMINI address DIP switch segment A6. RE0 used as output to drive the 3rd SMINI input port enable. 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. " Well, first off the resistor should never be greater than about 100Ω. Larger values cause the input voltage into U3-U5 representing a logic low to rise to a point where it is too close to the .8Vdc switching threshold of the ICs, thereby actually increasing noise susceptibility. 5Vdc and 0V logic levels used by the PIC16F877's Transmit (TX) and Receive (RX) lines. On the serial side of the SMINI, one of two special I/O circuits couples the PIC16F877 to the serial I/O lines coming to SMINI from the main computer, the PC. 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. Two wires are used to send data from the PC to the SMINI cards and two wires are used to send data from the SMINI cards to the PC.

For this to happen without interference, the outputs from the U7 transmit ICs on all the other SMINI cards must be held in a high impedance state, effectively an open circuit to prevent loading down the bus. Such undesired action - called bus contention - must be avoided. 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. Table 4-2 summarizes the utilization of the 877’s I/O port lines for the SMINI application. 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. For example, if U1 needs to write to I/O Card 0 Port A, it first sets Port RD to be output then it places the data for output on the Port RD lines.

These connect U1 to the SMINI's interface-unique circuitry for either RS232, or RS485 I/O. 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. 12-V bus common-mode range: This indicates the range of voltages that the RS-485 bus can tolerate without affecting communication reliability. The level refers to error percentage of instrument when it come full range. If you are unsure of the level of noise filtering you need, or for that matter if you need it at all, then a good approach is to assemble the SMINI cards without installing R6-R29 and C24-C47. Three wires are used with RS232. U6 and U7 are identical RS485 transceiver ICs capable of transmitting and receiving over the same 2-wire transmission line to support half-duplex operation - two wires transmitting bi-directionally. If you are pushing any limits, the termination resistor value has to be determined empirically for the individual network. A termination resistor can increase the signal's quality (reduce ringing), but it will also reduce the signal's amplitude.

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