So are Termination Resistors Required?

RS-422 specifies the electrical levels of one transmitter to one of more receivers on a single pair of wires. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. If you are trying to get two ancient pieces of equipment (both of which use true RS-232 drivers and receivers) to work with a long cable, the capacitance limit in RS-232 may apply, but RS-232 does not take bit rate into account. Standard Parking Management System composed of the basic bit computer. That is from where the limit of 128 nodes maximum per distributed system arises. Section 3.2 clearly states that "The maximum length of the cable is not defined". 120 Ω cable should provide the best performance, but the 100 Ω CAT-X cable may you have laying around may also work. The maximum capacitor that fits on the card is 470µF, 10Vdc (Digi-Key P6217) which when combined with the 100Ω resistor produces a filter time constant of .047 seconds (calculated as .000470 x 100) or 47ms, and a cut-off frequency of 3.4Hz. This means that the noise would have to hold the input in the incorrect state for approximately 47ms before the software would receive an incorrect input.

This limits the load termination and is an argument against using 100 Ω cable. The receiving device on each SMINI, the U6 receiver IC, has a high input impedance corresponding to a ¼ unit load. 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. Two circuits are for transmitting and receiving the serial data, circuit "BA" (TX data on pin 2) and "BB" (RX data on pin 3). A number of circuits are for handshaking and other functions. If you prefer, all the circuit boards, including the SMINI, can be purchased fully assembled and tested or as complete kits from SLIQ Electronics. If the circuit common is truly isolated from earth ground, then scuffing your feet on carpet (to pick up an ESD charge) and touching the wires can cause the wires to shift potential several thousand volts. RS-232 typically has a transmit wire, receive wire and signal common wire.

The arrows indicate the direction of data transmission for each two wire set. Typically, this occurs between the transmission of messages by different nodes. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). The next figure shows the AC Thevinin equivalent of the bias resistors with the termination resistor. 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. 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. 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. This is handled by the 7-segment DIP switch feeding directly into the PIC16F877.

RC4 used as input to read in the SMINI address DIP switch segment A6. Each of the SMINI output ports is configurable for standard current-sinking or alternate current-sourcing. The functional blocks, noted as U9 through U13, perform as buffer/line-drivers for the 6 output ports. RD0-RD7 used bi-directionally as inputs to read the 8-bit parallel data coming in from the 3 SMINI input ports and as outputs to send the 8-bit parallel data going to the 6 SMINI output ports. 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. 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. These connect U1 to the SMINI's interface-unique circuitry for either RS232, or RS485 I/O. Due to this and it being able to transmit data over long distances, the RS485 is used commonly as a protocol for POS, industrial and telecom.

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