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When the computer is to read data from the SMINI input ports, it simply tells the PIC16F877, which gathers the data from the 3 input ports, sets it up in the prescribed format, and transmits it bit-by-bit back to the computer. This is significant because a serial interface transmits data sequentially, one bit after another, as opposed to a parallel interface, which for example might transmit one byte (eight bits) at a time. Because there can be up to 128 SMINI cards connected on the same 4-wire cable, the system must be able to discern between which SMINI is involved with communications at any point in time. The signal will eventually stabilize (until the next bit transition) so the slower the bit rate the more time the signal will have to stabilize before it is sampled (usually by a UART). The use of the terms bit and byte will become second nature as you begin to put the interface to use.

Two 5-pin headers are provided for use with RS485. Note: The first SMINI production run cards labeled these "cards within a card" as 1, 2 and 3 versus the 0, 1 and 2 description provided in this manual. To understand how to terminate a network (the wires) you must first understand transmission lines. Additionally, connections between nodes should be contiguous from the computer to the first node, then on to the next and then the next. Connections between the SMINI and the railroad are made using Molex-style right-angle header connectors. When using RS232, the serial connections between the computer and SMINI are made using the 3-pin header along the top right edge of the card. It is a complete self contained node, being sort of a gee-wiz do-all type of card. Because typical computer interfacing distances are very short compared to the 4000-foot limit of RS485, the type of cable used is not overly important. PIC processors are the most popular family of microcontrollers and the 877 is ideally suited for our interfacing needs.

Bus Probes are a family of low-power environmental sensors with an RS485 interface. The second important function of the resistor-divider network is to bias the bus voltages toward VCC/2. Both methods ensure that the bus maintains a logical high state, corresponding to a positive differential voltage. This creates a fail-safe bias voltage that causes the negative terminal to have a lower voltage than the positive terminal and the output of the comparator to be in a known high state when applying a 0-V differential voltage to the A and B pins. PURPOSE AND SCOPE OF COVERAGE - This standard is applicable to the interconnection of data terminal equipment (DTE) and data communication equipment (DCE) employing serial binary data interchange. The signals are named from the standpoint of the DTE. 25 wires. Not all of the EIA-530 signals use RS-422 levels. Half-duplex RS-232 implementations exist but require all of the hardware and wires that a full-duplex network requires. Thus, combining nodes using the Classic USIC with the newer SMINI and SUSIC nodes requires operating at either the 9600 or 19200 baud rates - the rates where the two ranges overlap. It is important to understand that all four serial interface standards allow different baud or data-transmission rates.

The main advantage of a smart serial interface with built-in MPU and memory is great flexibility. The 4-wire design, as implemented by the C/MRI, takes advantage of both worlds, RS422 and RS485, by dedicating one set of the more advanced RS485 transceivers to sending data from the PC and a second set for handling data being received by the PC. And, because we are using the more advanced RS485 ICs, we expand our maximum node capability from 16 to 128. Because of this doubling up of the number of RS485 transceiver ICs and the corresponding use of 4-wires, RS485 standard this arrangement is typically referred to throughout the commercial and industrial world as a "Full Duplex" implementation of RS485. Unit Load: The network’s unit load impacts the number of devices that can be attached to it. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. I've been told that 10BASE-T Ethernet and SCSI cables use a bunch of RS-485 pairs -- is that right ? To be on the conservative side, it is always best to use dual twisted pair shielded transmission cable for RS485.