Omega OM-USB-1208FS User Manual
OM-USB-1208FS User's Guide
Functional Details
17
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).
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:
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.
= 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.
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.
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.
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.
counter can count frequencies of up to 1 MHz.