Usb-1208Fs Channel Gain Queue Feature; Synchronizing Multiple Units - Measurement Computing USB-1208FS User Manual

USB-1208FS User's Guide

USB-1208FS channel gain queue feature

The USB-1208FS's channel gain queue feature allows you to set up a scan sequence with a unique per-channel
gain setting and channel sequence.
The channel gain queue feature removes the restriction of using an ascending channel sequence at a fixed gain.
This feature creates a channel list which is written to local memory on the USB-1208FS. The channel list is
made up of a channel number and range setting. An example of a four-element list is shown in the table below.
Element
0
1
2
3
When a scan begins with the gain queue enabled, the USB-1208FS reads the first element, sets the appropriate
channel number and range, and then acquires a sample. The properties of the next element are then retrieved,
and another sample is acquired. This sequence continues until all elements in the gain queue have been selected.
When the end of the channel list is detected, the sequence returns to the first element in the list.
This sequence repeats until the specified number of samples is gathered. You must carefully match the gain to
the expected voltage range on the associated channel — otherwise, an over range condition can occur. Although
this condition does not damage the USB-1208FS, it does produce a useless full-scale reading. It can also
introduce a long recovery time from saturation, which can affect the next measurement in the queue.

Synchronizing multiple units

You can run up to two USB-1208FS units on most computers.
You can connect the SYNC pin of two USB-1208FS units together in a master/slave configuration and acquire
data from the analog inputs of both devices using one clock. When the SYNC pin is configured as an output, the
internal A/D pacer clock is sent to the screw terminal. You can use this signal as a clock input to a second USB
by connecting it to the SYNC pin of the second USB.
When used as a clock input, the SYNC pin operates in one of two modes – Continuous or Gated.
In the default Continuous mode, a USB-1208FS ignores the first clock pulse in order to ensure adequate setup
time. Use this mode if the unit is being paced from a continuous clock source, such as a generator.
In the Gated mode, it is assumed that the clock signal will be held off for an adequate amount of time for setup
to occur. No clock pulses are ignored. Use this mode if the USB-1208FS is set up as a slave and the source of
the external clock is another USB.
The SYNC pin (pin 19) is set for pacer output by default. To set it to input requires a software command (see
Figure 15 on page 24).
To synchronize a master USB-1208FS with a slave USB-1208FS and acquire data, follow the steps below.
1. Connect the SYNC pin of the master USB-1208FS to the SYNC pin of the slave USB-1208FS.
2. Run InstaCal.
3. From the
PC Board List
slave. The Board Configuration
Sample channel gain queue list
Channel
CH0
CH0
CH7
CH2
on the main form, double-click on the USB-1208FS you want to use as a
InstaCal
dialog opens.
23
Range
BIP10V
BIP5V
BIP10V
BIP1V
Functional Details