The DAS is of modular form with units built into single width NIM modules. A typical system is composed from five basic modules:
The DAS is capable of handling experiments with up to 240 different analog or digital outputs. The address bus has 8 bits so after allocating 16 addresses for internal use, up to 240 distinct analog or digital inputs can be addressed.
The bit or baud rate ranges from 150 to 38.4 kbaud. There is also a 32k bps position which meets the NASA serial data standard.
The serial data output consists of a frame of 16 bit words sent seamlessly with a unique synchronization word to identify the start of each data frame.
A. Control Module
The Control module has a slide switch to set the baud rate, also an EPROM in a zero insertion force socket. The data frame set up is burnt into the EPROM and is arranged with analog or digital data in the order required by the experiment. The DAS is powered through the control unit and requires ±15 V. If the protective polarity reversal diodes are shorted out, the DAS will work on ±12 V. The output connector has a serial data output line together with a bit clock line. There is also an optically-coupled, bi-phase serial data line powered from internal, isolated power supply.
B. Analog Multiplexer Module
The Analog Multiplexer/ADC has 16 differential inputs of FSD ±10V with a common mode rejection of 86 dB. The ADC limits hard at ±10V. There is no bleed-over between channels with overloads of ±30V.
C. Analog sub-Multiplexer Module
The 16-input Analog Submultiplexer module is used when there are more than 16 analog inputs. Each submultiplex utilizes/monoploizes one of the Analog Multiplex/ADC module inputs.
D. Digital Word Module
The Dual Digital Word module accepts 2 each 16-bit words (each 16-bit word could be made of sub-digital words, i.e. 2 8-bit words). Additional modules are added to provide the needed number of digital words.
E. Serial to RS232 Module
The Serial to RS232 module takes the seamless serial data and synchronizes to the "sync word". Each 16-bit word is converted to RS232 format and can be sent to any computer with a serial input port and appropriate software. Labview and home-made software have beeen used successfully. A finite time is required for conversion to RS232ule, the maximum DAS baud rate can only be 2.4 kbaud or less. The module output s at 9.6 kbaud.
F. Operational Notes
The only connection between modules is via a 25 conductor flat cable with D-type connectors crimped on as required. The modules need not sit side-by-side or in any particular order. Some may be separated from the others by up to 30 feet of cable. Greater separation might require cable ringing suppression devices at the end of the bus. The bus has many functions such as: 8-bit device address bus, Power to the the modules (±15V, +5V), Bit Clock, "End of Data Frame" pulse, "Power-Up Reset" pulse, "Common Serial Data Line" between modules, "Serial Data Output", "Oddd/Even Frames", "Busy Line", and several ground wires paralleling sensitive data lines.
G. Power Consumption
An attempt was made to keep the power drain reasonably low
for applications that must be powered from batteries.
The measured current drains were as follows:
| Module Control Analog Multiplexer/ADC Analog Submultiplexer Dual Digital Word Serial/RS232 |
mA @ +15V 35 45 35 5 100 |
mA @ -15V 0 35 25 0 0 |
There are documentation notebooks on the DAS in 106 LeConte and in John Gibson's lab (Bldg 50-303).
author John Gibson, converted to HTML and edited by George Smoot
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