Antenna Computer TTL Signals

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Contents

Introduction

Several hard-wired digital lines are used by the antenna computer to send and receive real-time messages to and from data acquisition computers and others. For example, the TRACKING (/IDLING) output line from the antenna computer is fed to the AOS backend computer and SHARC I to coordinate data acquisition with the telescope movement during on-the-fly (OTF) mappings. Or, the TRACK (/DRIFT) input line is used by SuZIE to perform drift scans with two pre-programmed, alternating initial position offsets.

These digital lines have traditionally been implemented by using components with TTL-compatible interfacing parameters, and are referred to as antenna computer TTL signals. The current antenna computer provides eight (8) output lines and eight (8) input lines. Most of the output lines, or status lines, have pre-defined function. Input lines, or control lines, and some output lines change their function depending on the observing mode and the instrument being used. They are explained below and in individual instrument's section.

Unbuffered Inputs & Outputs

All the bare output lines and input lines are accessible via the RX2 section of the field wiring terminal block at the bottom of the right-most rack in the AOS room. They are directly connected to the digital I/O interface module of the antenna computer. The interface module employs an unbuffered version of Xilinx FPGA with ESD protection and provides TTL-compatible interface, capable of sinking maximum 4-mA. The output lines 3, 4, 5 and 6 are already used by the AOS backend computer.

Buffered Outputs

A blue box near the field wiring terminal block provides buffered interface for upto four commonly used output lines. These lines are driven by SN74128 and have 50-ohm series output registers. They are capable of sourcing 42-mA and sinking 48-mA. Channels 1 and 2, ACQUIRED and TRACKING (/IDLING) signals, are used by SHARC I and others.

Optocoupled Iuputs

A silver box with eight (8) front panel BNC connectors above the field wiring terminal block provides true logic, TTL-level compatible, optically isolated interface for all the input lines. HCPL-2731 optocouplers (I_F 0.5-mA) with 1.1-kohm input registers, followed by 74LS540 Schmitt inverters, give the following electrical characteristics:

The optocoupled input lines provide the same function as the unbuffered lines.

Bug reports, suggestions & comments to:
Hiroshige Yoshida (hyoshida@caltech.edu)
Caltech Submillimeter Observatory, 111 Nowelo St., Hilo, HI 96720, U.S.A.