Data Acquisition

y increasing the resistance of R x and the use of the opto-isolator, guarantees high isolation,typically 500 V.Detection of AC voltages requires the use of a filter to smooth out the AC pulses from theopto-isolator and provide a continuous signal level to the digital inputs. This slows down theresponse to AC voltages (typically 1 ms), however, if the capacitor is switchable, it can be deselected,thus allowing faster response times (typically 20 µs), for DC input voltages.5.10.3 Driving an LED indicatorWhere it is necessary to provide a visual indication of an action being performed or to informan operator of the status of a system or process, and a low level indicator is acceptable, lightemitting diodes (LEDs) provide an easily implemented solution. As the standard TTL outputsfrom devices such as the 8255 PPI chip may not have sufficient drive to operate an LED,special driver circuitry is required, as shown in Figure 5.32.Figure 5.32Driving an LED5.10.4 Driving relaysUsing a plug-in board for digital control, in particular to operate relays that control AC or DCpower or alarms, gives the PC tremendous power for a variety of control applications. Wheredigital I/O interfaces are required to operate relays, special circuitry is usually requiredbecause the relays typically cannot be driven directly from TTL signal levels. In addition, thedrive current required to operate either electromechanical or solid-state relays is much greaterthan that provided by normal TTL circuits, such as the 8255 PPI chip.To accommodate any special signal level and current drive requirements of relays severaloptions are available:• Specialized rack mounted relay boards that interface directly, via ribbon cable, tothe output connectors of common digital I/O boards. The circuitry necessary forthe higher current drive, required to operate the relays, is provided on the relayboards. Individual relay modules may be fitted to the boards to meet the contactconfiguration and rating required for a specific application.