Measurement Computing USB-1208LS - Manual (Page 9)

9
Chapter 3
Functional Details
Analog input acquisition modes
The USB-1208LS can acquire analog input data in three different modes – software paced, hardware paced, and
burst scan.
Software paced mode
You acquire one analog sample at a time in software paced mode. You initiate the A/D conversion by calling a
software command. The analog value is converted to digital and returned to the computer. You can repeat this
procedure until you have the total number of samples that you want from one channel.
Software pacing is limited by the 20 mS round-trip requirement of a USB interrupt-type endpoint operation.
The maximum throughput sample rate in software paced mode is 50 S/s.
Hardware paced mode
You acquire data from up to eight channels in continuous scan mode. The analog data is continuously acquired,
converted to digital values, and written to an on-board FIFO buffer until you stop the scan. The FIFO buffer is
serviced in blocks as the data is transferred from the USB-1208LS to the memory buffer on your computer.
The maximum continuous scan rate of 1.2 kS/s is an aggregate rate. The total acquisition rate for all channels
cannot exceed 1.2 kS/s. You can acquire data from one channel at 1.2 kS/s, two channels at 600 S/s and four
channels at 300 S/s. You can start a continuous scan with either a software command or with an external
hardware trigger event.
Burst scan mode
In burst scan mode, you acquire data using the full capacity of the USB-1208LS 4 k sample FIFO. You can
initiate a single acquisition sequence of up to 4096 samples channels by either a software command or an
external hardware trigger. Captured data is read from the FIFO and transferred to a user buffer in the host PC.
Burst scans are limited to the depth of the on-board memory, as the data is acquired at a rate faster than it can be
transferred to the computer. The maximum sampling rate is an aggregate rate. The maximum rates that you can
acquire data using burst scan mode is 8 kS/s divided by the number of channels in the scan.
External components
The USB-1208LS external components are shown in Figure 2.
1 Screw terminal pins 21 to 40 3 Screw terminal pins 1 to 20
2 LED 4 USB connector
Figure 2. USB-1208LS external components