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The SMINI provides everything required to set up a very capable node on a single card, at a very reasonable cost. This chapter is devoted to showing how to set up your C/MRI as a serial interface, RS485 standard including step-by-step instruction for building the Super Mini-Node Interface Card (SMINI). Fig. 4-6 shows a sub-schematic detailing the latch and output transistor portion of the SMINI card, configured for both the standard current-sinking and optional current-sourcing. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. 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. 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 typical multi-node setup is illustrated in Fig. 4-1b. Fundamentally, every multi-node application requires that all nodes, plus the connection leading toward the computer, need to be "daisy-chain" connected using RS422/485, a 4-wire cable. Fig. 4-1a illustrates the simplest arrangement for systems comprising just a single node.

Fig. 1-1, in Chapter 1, shows a photograph of the SMINI card. 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. It requires a single 3-wire RS232 cable to connect between the computer’s RS232 serial port and the RS232 header on the SMINI or the SUSIC. 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. The software simply checks to see that the input has remained stable for two or more samplings before it accepts any change in input.

Each switch is separately controllable by software giving you total freedom to turn each circuit on and off as desired. The circuit ground should be connected to chassis ground via a 100 ohm resistor. Two wires deliver 5Vdc power and ground. SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. The most common USB connector, referred to as Type A, has 4-conductors. This is the connector found on most modern peripheral devices and installed on most, if not all modern PCs. With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. Line A voltage, implying A, the green wire, is indeed connected to the driver inverting signal, as seen in a whitepaper.

Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. Failure to stay within this range will result in, at best, signal corruption, and, at worst, damage to connected devices. It is very handy for anything that has an Ethernet port for the ModbusTCP, including devices like Raspberry Pi based controllers industrial controllers. Including the diode provides an ultra fast reset to minimize the wait time required when cycling power. An RS232 to RS485 Converter Card, such as the RS485 card supplied by JLC Enterprises and covered in detail near the end of this chapter, provides the required conversion to make the connection to the computer’s RS232 serial port. Additionally, in every case, the RS422 or the RS485 full duplex implementation is used to make the "daisy-chain" connection between multiple nodes. Additionally, if any kind of RS485 interfacing is available, then it can be used for ModbusRTU as well.