System designers face several challenges when adding general purpose I/O to system buses such as PMC and VME. The most obvious and common barrier is the limited diversity of COTS I/O modules for these bus types. This often translates into a cost issue when custom products are required. Another common barrier is the lack of open slots for the I/O devices.
Fortunately, designers can select the best SBC for their application and then adopt external I/O devices to expand beyond traditional bus-based design choices. The best solution is to use Ethernet and USB I/O devices. The vast majority of CPU modules provide Ethernet and USB interfaces, and I/O manufacturers offer a wide variety of these products to support various instrumentation and control requirements. By using Ethernet and USB I/O devices, engineers can design their core system architecture without concern for I/O expansion and can realize the cost effectiveness of these widely available I/O devices.
Distribute I/O anywhere using Ethernet
Interfacing I/O using Ethernet is becoming a popular trend for many reasons. Most SBCs provide at least one Ethernet port and are commonly connected to a network; therefore, interfacing them to Ethernet I/O devices is a natural choice. The networked I/O modules can be placed closer to the signals being monitored, which greatly simplifies field wiring. Another benefit of many Ethernet I/O products is the ability to access them from any authorized host computer on the network.
Ethernet serial servers make adding RS-232, RS-422, and RS-485 communication ports simple using virtually any Ethernet port. Adding more ports is as easy as connecting another serial server. Application software can use a COM port redirector software that allows serial ports to appear as virtual COM ports to the host machine. Using this approach, standard serial operating calls are transparently redirected to the serial server. In turn, this guarantees compatibility with legacy serial devices and enables backward compatibility with existing software. For more precise control, application software can communicate directly to the serial ports using raw socket mode. One example of these serial servers is Sealevel Systems‚Äô SeaLINK family, ranging from 1 to 16 ports.
Digital and analog I/O signals are also easily interfaced using off-the-shelf, Ethernet-connected devices. Some Ethernet I/O products take advantage of the Modbus TCP protocol to simplify software development and provide interoperability with other Modbus I/O devices. Modbus TCP transports Modbus data packets using the standard TCP/IP protocol, and many third party test and measurement software applications provide support. Acromag markets a family of Modbus TCP-compatible analog I/O devices such as the BusWorks 900EN Series with modules offering analog input and output interfaces to voltage and current signals. The modules are compact and mount on DIN rail for easy installation in an instrumentation cabinet.
I/O connectivity via USB
Developed for use with desktop peripherals, USB is now commonly used for connecting I/O in industrial and OEM applications. USB is fast and, since the introduction of USB 2.0, can achieve a raw data rate of 480 Mbps. Many SBCs have external USB ports with standard USB Type A connectors and often include convenient internal header connectors with two or more USB ports. This makes USB devices easy to implement. However, the point-to-point nature of USB can limit the implementation options for I/O-intensive applications.
One solution for addressing this USB port limitation is to interface multiple modules from a single USB port with USB data acquisition products. Each I/O module can connect to the host with a locking, high-retention USB Type B connector that eliminates accidental disconnection. Modules can include an RS-485 Modbus RTU downstream port for connecting additional expansion modules. This allows up to 246 Modbus RTU I/O expansion modules to be daisy-chained together to achieve the required I/O count using a single USB host connection. Each module is assigned a unique Modbus address that can be set by hardware switch or via software. The host computer uses the standard Modbus protocol to communicate with these modules. Offerings such as Sealevel Systems‚Äô SeaI/O USB data acquisition products (Figure 1) can facilitate this process.
External I/O options expand traditional bus-based design choices
Designing an effective I/O solution for a VME or PMC system can present unexpected obstacles. The traditional approach of adding separate I/O bus boards for serial, digital, and analog I/O expansion can leave engineers frustrated with the lack of selection and high cost of these bus-based products. Another common concern when adding expansion boards is the availability of backplane slots.
System designers now have options that extend beyond the typical backplane add-in board. Using COTS Ethernet and USB I/O solutions allows valuable rack space to be reserved for ever-increasing disk capacity, communications, DSP, and other bandwidth-critical expansion requirements. Ethernet and USB I/O solutions are fast, simplify field wiring, and streamline repairs and upgrades. The broad availability of serial, digital, and analog I/O products from a variety of manufacturers usually proves more flexible and cost effective than PMC and VMEbus expansion boards, which makes USB and Ethernet I/O devices the optimal solution.
Sealevel Systems, Inc.