Omega OM-USB-1208FS manual Calibration terminal, Ground terminals, External trigger terminal

OM-USB-1208FS User's Guide Functional Details

USB (based on the application), and the allowed current draw of the PC platform (500 mA for desktop PCs and self-powered hubs, or 100 mA for bus-powered hubs and notebook computers).

With all outputs at their maximum output current, you can calculate the total current requirement of the OM- USB-1208FS USB +5 V as follows:

(OM-USB-1208FS @ 80 mA) + (16 DIO @ 2.5 mA ea) + (2 AO @ 15 mA ea ) = 150 mA

For an application running on a PC or powered hub, the maximum available excess current is 500 mA−150 mA

=350 mA. This number is the total maximum available current at the PC +5 V screw terminals. Measurement Computing highly recommends that you figure in a safety factor of 20% below this maximum current loading for your applications. A conservative, safe user maximum in this case would be in the 350-380 mA range.

Since laptop computers typically allow up to 100 mA, the OM-USB-1208FS in a fully-loaded configuration may be above that allowed by the computer. In this case, you must determine the per-pin loading in the application to ensure that the maximum loading criteria is met. The per-pin loading is calculated by simply dividing the +5 V by the load impedance of the pin in question.

Calibration terminal

The CAL connection (pin 16) is an output you should use only to calibrate the OM-USB-1208FS. Refer to the pinout diagrams on page 13 for the location of this pin. Calibration of the OM-USB-1208FS is software- controlled via InstaCal.

Ground terminals

The four analog ground (AGND) connections provide a common ground for all OM-USB-1208FS input channels. Four ground (GND) connections provide a common ground for the DIO, TRIG_IN, CTR, SYNC and PC +5V connections. Refer to the pinout diagrams on page 13 for the location of the AGND and GND pins.

External trigger terminal

The TRIG_IN connection (pin 18) can be configured for either rising or falling edge. Refer to the pinout diagrams on page 13 for the location of the TRIG_IN terminal pin.

SYNC terminal

The SYNC connection (pin 19) is a bidirectional I/O signal. You can use it for two purposes:

Configure as an external clock input to externally source the A/D conversions. The SYNC terminal supports TTL-level input signals of up to 50 kHz.

Configure as an output to synchronize with a second USB unit and acquire data from 16 channels.

Refer to the pinout diagrams on page 13 for the location of this pin. For more information on synchronizing multiple units, refer to page 21.

Counter terminal

The CTR connection (pin 20) is input to the 32-bit external event. Refer to the pinout diagrams on page 13 for the location of this pin. The internal counter increments when the TTL levels transition from low to high. The counter can count frequencies of up to 1 MHz.

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