RedLab 1208FS User's Guide
Functional Details
21
Power terminals
The
PC +5V
connection (pin 30) is on the bottom screw terminal of the RedLab 1208FS. Refer to the
pinout
diagrams
on page 16 for the location of this pin. This terminal draws power from the USB connector. The +5 V
screw terminal is a 5 volt output that is supplied by the host computer.
Caution!
The +5 V terminal is an output. Do not connect to an external power supply or you may damage
the RedLab 1208FS and possibly the computer.
The maximum total output current that can be drawn from all RedLab 1208FS connections (power, analog and
digital outputs) is 420 mA. This maximum applies to most personal computers and self-powered USB hubs.
Bus-powered hubs and notebook computers may limit the maximum available output current to 100 mA.
Just connecting the RedLab 1208FS to your computer draws 80 mA of current from the USB +5 V supply.
Once you start running applications with the RedLab 1208FS, each DIO bit can draw up to 2.5 mA, and each
analog output can draw 15 mA. The maximum amount of +5 V current available for experimental use, over and
above that required by the RedLab 1208FS, is the difference between the
total
current
requirement
of the 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 RedLab
1208FS USB +5 V as follows:
(RedLab 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 RedLab 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 RedLab 1208FS. Refer to the
pinout diagrams
on page 16 for the location of this pin. Calibration of the RedLab 1208FS is software-
controlled via
Insta
Cal.
Ground terminals
The four analog ground (
AGND
) connections provide a common ground for all RedLab 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 16 for the location of the
AGND
and
GND
terminal 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 16 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:
Summary of Contents for redlab 1208fs
Page 1: ......
