Bulletin No. 8

ADS INPUT AND OUTPUT SIGNAL INTERFACE DESCRIPTION

1. Introduction

   This Bulletin gives concise descriptions of the various input and output interfaces available for users of the RAF Airborne Data System (ADS-II). Unless otherwise specified, they apply to all of the NSF/NCAR aircraft fleet.

   If you have questions or need more detailed information about the topics covered here, please contact the RAF Data Manager.

2. Output Data Feeds
   
   • Serial Data
     The ADS-II data acquisition system aboard NSF/NCAR aircraft can distribute, in pseudo-real-time (about a 3-second lag), a single, ASCII data stream to properly-interfaced user equipment. Up to 16 devices (8 in the front cabin and 8 in the rear cabin) can receive this common data stream which consists of time and a subset of all the raw and derived measurements made aboard the aircraft. Measurement selections and their output order are controlled by a special file. Since all attached devices share these data, users should select and agree on which of the measurements will best satisfy their needs.
     • Equipment used:
       • Black Box DB-8 Data Broadcast Unit (1 input channel, 8 output channels) with RJ-11 modular jacks
       • A serial feed from ADS-II
     • Equipment furnished by RAF:
       • RJ-11 four-wire phone cable
       • RJ-11 adapters to DB-9 or DB-25 connectors (M or F): wired as DCE for connection to DTE (terminal equipment)
       • If you have a special need beyond the above descriptions, please contact us.
     • Protocol:
       • baud rate: 9600
       • data bits: 8
       • stop bits: 1
       • parity: none
     • Data output:
       • Rate: One record per second
       • Format: ASCII
       • Measurements: selected from the project's master variable list and ordered via the project file "ascii_parms"
       • Format: 10-character date (yyyy/mm/dd with leading zeros, if numbers are less than 10) followed by a [SPACE] followed by 8-character time (hh:mm:ss with leading zeros, if numbers are less than 10) followed by 14-character measurement fields (leading [SPACE] character, E format with leading sign, 6 significant digits and 2-digit exponent, e.g., +0.123456e+78), the return [CR] character and the End-of-line [LF] character (There will always be the same number of characters in each record.)
       • Theoretical maximum number of variables (excluding date/time): 61
       • Practical maximum number of variables (excluding date/time): 50
     
     
   • IRIG-B
     For those user instruments able to accept it, IRIG-B signals are routed throughout the aircraft to provide timing information. It is referenced to the on-board GPS system (once per second) and synchronized with the ADS-II.
     • Cabling: RG-58/U
     • Connector: BNC
     • Signal description: IRIG-B
       An IRIG-B format description of can be found at the URL: http://www.jcte.jcs.mil/RCC/manuals/irig/2toc.htm.
   
   
3. Data Input

   This section describes the various methods for having measurements recorded by the ADS-II. There may be occasions when a user only wants his/her data displayed on the ADS-II system (WINDS). The present configuration requires that the data must be recorded before they can be displayed.

   • Analog
     The analog output(s) from a user's instrument can be sampled and recorded on the ADS-II. A wide range of signals can be used depending on level and instrument response speed. The analog input circuitry of the ADS-II has a sample rate of 10K sps (samples per second). Instrument output(s) are assumed to have no frequency components higher than 5K Hz. If this is not the case, anti-alias filters should be installed to assure that this condition is met.
     • Sample rates: 5, 25, 250 and 1000 sps only
     • Maximum signal level: ±10 Vdc
     • Available input voltage ranges: 0-1, 0-5, 0-10, 0-20, ±10 Vdc
     • Note: Full-scale signal levels less than 1 Vdc should be amplified to assure adequate resolution and to minimize external noise effects.
     • Analog connection: three-wire with full floating ground (shielded cable preferred)
     • Ground: single-ended or full floating; cable at source can be grounded or not.
     • Connectors: twinax (BNC adapters furnished, if needed)
   
   
   • Serial
     The default serial input has the following characteristics:
     • RS-232, RS-422 interface
     • ASCII
     • [CR][LF] record termination
     • baud rate: Standard rates up to 38,400 baud
     • data bits: 7, 8
     • stop bits: 2, 1
     • parity: none, odd, even
     If your serial data have other characteristics, the ADS-II can handle a wide range of types and rates. Consult RAF for your special needs.
   
   
   • Pulses
     RAF supports pulse counting with a multiple, 16-bit interface card that can be set up to sample (read and reset) a pulsed signal between 1 and 25 times per second. By properly setting the sample rate, pulses can be counted to a maximum of 1.6M pulses per second without overflowing the 16-bit register. If higher rates are needed, the channels can be cascaded to extend the overflow capacity.
     • Pulse width: 100 ns or greater (pulses shorter than 100 ns can be stretched to 100 ns with an input one-shot circuit, if necessary, but it obviously limits the maximum resolvable pulse-rate to something less than 10M pps.)
     • Pulse amplitude: 5-volt, TTL level (higher amplitude signals can be attenuated)
     • Input impedance: 50 ohms (BNC connector and RG-58/U cable preferred)
   
   
   • Ethernet
     Consult RAF if you have any desire to send data to the ADS-II via an Ethernet connection.
   
   
   • Parallel
     RAF supports parallel data input with the following characteristics:  8-bit or 16-bit with strobe.