RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off. For more reliable, error-free transmission over long distances, you should use RS485. To be on the conservative side, it is always best to use dual twisted pair shielded transmission cable for RS485. Alternatively, if your computer has USB only, then you will need to incorporate either a USB to RS232 Converter Cable between the computer and the RS485 card or a USB to RS422/485 Converter Cable connected between the computer and the RS485 header on the SMINI or SUSIC. All four standards can be used to connect your computer to an SMINI and to an SUSIC. Alternatively, if your computer is limited to USB only, then you will need to incorporate a USB to RS232 Converter Cable to make the connection. The RS485 and RS232 serial communication protocols have been in use for over 50 years and are still widely used throughout business and industry. The error codes have been significantly expanded over those provided with the original Classic USIC.

From the perspective of previous C/MRI components the SMINI combines enhanced features of the original design Universal Serial Interface Card, the USIC, with two output cards and an input card, placing everything on a single board not much larger than the original USIC. 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. This chapter is devoted to showing how to set up your C/MRI as a serial interface, including step-by-step instruction for building the Super Mini-Node Interface Card (SMINI). Any number of characters can be sent, and the transmitter will automatically re-trigger with each new character (or in many cases a "bit-oriented" timing scheme is used in conjunction with network biasing for fully automatic operation, including any Baud rate and/or any communications specification, eg.

Any comments and/or corrections would be appreciated. The only requirement for connecting the C/MRI is that your computer has an RS232 serial port and/or a Universal Serial Bus (USB). Alternatively, if your computer has USB ports only, then it’s possible to add a USB to RS232 or USB to RS422/485 Converter Cable to make the computer to C/MRI connection. In reality, if you are using relatively modern equipment (say 1990 or later) and a low baud rate, line lengths of 1,000 feet (300 m) or more are possible. The transmission speed possible with an RS232 interface is 1Mb/s for a distance of up to 15 meters. Devices using the RS232 protocol are limited to a distance of 15 meters between transmitter and receiver while still achieving the maximum data transmission rate. RS485 offers greater speeds of up to 10Mb/s over a distance of 15 meters. It supports maximum data rates at a length of 1200 meters, making the RS485 protocol an excellent choice for communication between physically distant pieces of equipment. The appendix discusses capacitance and resistance of the cable, then gives an example calculation where the capacitance of the cable per foot (30pF/foot) multiplied by the cable's length, plus the capacitance of the receiver (100pF) gives a maximum cable length of 80 feet.

If CAT-5 (100 Ω cable) cable is used, the termination resistors should be 100 Ω. This resistor is called a termination resistor, designated as Rterm in the following diagrams. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). The second optional grounding arrangement is to connect the circuit common directly to the frame ground without a resistor. Onboard TVS (Transient Voltage Suppressor), effectively suppresses surge voltage and transient spike voltage in the circuit and is anti-electrostatic. Since the driver and receiver are differential circuits, the input and output voltages are specified as differential, but these voltages are also referenced to a circuit common. The EIA has officially disbanded and these standards are now maintained by the TIA. Bit and framing protocols are in layer two of the OSI model. Now, let’s take a look at the main differences between the two protocols. As mentioned above in the Voltages section; On the RS-485 network there will be times when the two wires are not driven by a transmitter.

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