Real World Instrumentation with Python

PC Bus Interface Hardware

A device designed to connect directly to the internal bus of a computer doesn’t usually have a command-response protocol in the same sense that one would find with the serial or GPIB-type device. Instead, the interface protocol is embodied in the form of callable functions provided by driver software. They are also typically somewhat more difficult to use due to a higher level of software interface complexity.

You can buy various interface cards for a PC that are capable of capturing high-frequency signals in real time, whereas there are no such devices for RS-232 or RS-485, and only a handful with high-speed USB interfaces (and the USB devices often employ some type of buffering, or temporary storage, to help move data between the device and the host system).

The most common type of bus-based interface cards utilize the industry-standard Peripheral Component Interconnect (PCI) bus and the newest variant, the PCI Express (PCIe) bus. A typical PCI multifunction data acquisition (DAQ) card is shown in Figure 7-42. There are also add-on cards available for the industrial VME bus and the instrumentation PXI bus, but we won’t get into those in this book as most of what we will want to do can be done with PCI- or PCIe-type cards.

Figure 7-42. PCI interface card (ADDI-DATA APCI-3001)

Pros and Cons of Bus-Based Interfaces

Bus-based interfaces have a couple of inherent ...