Counter and timer boards are programmable computer cards that perform digital counting and/or timing functions. Typically, they plug into slots onto motherboards or connect as serial or parallel devices. Counter and timer boards can generate pulse width modulated (PWM) signals for motor speed control, count the number of times a switch toggles, and count pulses from a motor wheel encoder disk. Pulses or toggles increment counters, which can advance forward, backward, or both forward and backward. Counter and timer boards also generate timed interrupts for analog-to-digital data acquisition and provide fine resolution timing. They are used to measure system latency, time software, and monitor product conveyor lines.
Selecting counter and timer boards requires an analysis of several performance specifications. Devices vary in terms of the number of counters, the maximum count frequency, and the on-board clock or oscillator frequency. For example, some counter and timer boards include as many as 12 independent 16-bit counter / timers. Counter resolution is measured in bits and represents the number of events. In terms of output compatibility, some counter and timer boards include digital circuitry that is compatible with transistor-transistor logic (TTL). Others use complementary metal oxide semiconductor (CMOS) circuits. Various types of connectors are available, such as D-types with 9, 25, 37, and 50 pins.
Bus type is the most important system specification for counter and timer boards. Peripheral component interconnect (PCI) is a 64-bit bus standard that runs at clock speeds of 33 or 66 MHz. CompactPCI (cPCI) is based upon PCI but housed in rugged 3U or 6U Eurocard packaging. PXI, an acronym for PCI eXtensions for instrumentation, is a superset of cPCI that includes timing and triggering functions. VersaModule Eurocard (VME), the bus upon which the form factors for cPCI buses are based, is the basis of VME eXtensions for Instrumentation, a standard for many electronic platforms that defines electrical and mechanical backplane characteristics. Other common bus types include industry standard architecture (ISA), extended industry standard architecture (EISA), and IEEE 1394.
Counter and timing boards can include multiple digital or analog I/O channels and selectable input frequencies. Some boards include compatible software for controlling or monitoring data acquisition or signal conditioning from a supervisory or host computer. Other devices include one-shot outputs that produce only one pulse when triggered. Numerous regulatory agencies and standards organizations confer approval upon counter and timing boards. Examples include the Federal Communications Commission (FCC), Underwriters Laboratories (UL), and the CE Mark.