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If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans. Although basic data rates are high, RS485 standard there is extra overhead and subsequent delays in the manner in which USB handles data to and from the computer. All four standards can be used to connect your computer to an SMINI and to an SUSIC. Using the SUSIC is the ideal approach for locations requiring large amounts of concentrated I/O such as at a lever-type CTC machine. At node locations requiring more than 72 I/O lines, you can group SMINI cards together. If you desire more I/O than provided by a single SMINI, you can simply distribute additional SMINI cards around your layout to build up to whatever level of capability you desire. 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 SMINI and the SUSIC can make use of four interface standards: Universal Serial Bus (USB), RS232, RS422 and a "full duplex" implementation of RS485.

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. 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. 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. 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. If you are starting out with a single node, with plans to expand at a later date to multiple nodes, then it’s a good idea to configure your first node for RS485. With an addressable capacity of 128 SMINI nodes, with 72 I/O lines per node, the maximum capacity of the C/MRI system using the SMINI is 9,216 I/O lines. This distributed approach minimizes all local I/O wiring. However, how to apply each of the different serial interfacing options, including detailed wiring information for each approach, is presented near the end of this chapter. Fig. 1-1, in Chapter 1, shows a photograph of the SMINI card. 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). An RS232 interface is a system based on voltage levels and performs optimally in situations where there are minimal differences in ground potential.

The desktop is connected to earth ground and the RS-485 port is referenced to the earth ground. 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. 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. Although many C/MRI users effectively make use of USB Converter Cables, their application can pose some challenges. Additionally, if your computer is limited to USB ports only, then you need to employ a USB to RS232 Converter Cable or as we will observe later in this chapter, a USB to RS422/485 Converter Cable. For example, Pin 6, Data Set Ready on the DB25, is one that a printer might use to tell the computer it is ready to accept the next character.