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The 10th ARRL Amateur Radio Computer Networking Conference is now history. Glenn Tenney, AA6ER, and his committee did a fine job of hosting the Conference, which was attended by about 135 people.
Undoubtedly, the highlight of the show was CLOVER-II, by Ray Petit, W7GHM. If you don't know that you know him, Ray was the one who was pushing coherent CW technology some years back. His talk can be found in a copy of the proceedings. He also brought a functioning system to demonstrate to the group. Much work has to be done, but Ray is now working with Hal Communications to produce a later model sometime in 1992. A patent application has been filed but Ray and Hal Communications intend to make the system widely available, and license it to other manufacturers.
Another example of some fine research was the work of Whit Kisner, VE4WK, who presented papers on data compression and speech processing.
Phil Karn, KA9Q, talked about spectral efficiency and emphasized the importance of power control, a subject of recent QST and QEX editorials.
The banquet speaker was FCC Chief Engineer Dr Thomas Stanley. He gave an interesting talk about matters on his plate at the FCC, such as digital audio broadcasting, high definition TV and personal communications systems. He was kind enough to entertain dozens of questions and had a number of informal discussions with small groups throughout the weekend.
Photos of the 10th Computer Networking Conference follow.—W4RI
Conference coordinator Glenn Tenney, AA6ER, and Steering Committee member Dewayne Hendricks, WA8DZP, feeling the satisfaction of a job well done.
Ray Petit, W7GHM, captured the attention of the conference with his novel CLOVER-II HF data communications system.
Continued on page 16.
An Optical, Through-the-Air, Digital Communication Modem Part 1 of 2
By Lawrence E. Foltzer 4250 Deer Meadow Lane Occidental, CA 95465
Introduction
In the August 1990 issue of QEX, I discussed some of the basic concepts of LED-based atmospheric link design. That article presented the results of several experiments showing how to predict the range one can expect for various optical link configurations, and demonstrated that transmitters and receivers with relatively small-sized optics can achieve ranges up to 1/2 mile. This article builds on the theme of small aperture optical links with the design of a high-speed optical modem suitable for digital com munications.
One of the experimental links described in this article used 1/2-inch diameter lenses and achieved distances to 100 feet, while the second link used 1.2-inch diameter optics to achieve distances of 1 /8th mile (660 feet). The objective of this article is to show that one need not use expensive and potentially eye damaging lasers, large lenses or mirrors, or elaborate alignment apparatus to achieve useful optical link performance.
- Simplex Data Link Block Diagram
Fig 1
Electronics Hardware Description
Fig 1 shows a functional block diagram of the unidirectional optical link hardware described in this article. The ¡ink can be readily adapted for full duplex operation between computers using readily available communications software packages, or used for the transmission of digitized voice or other binary digital information. By duplicating the designs described in this article, a duplex or bidirectional link can be readily assembled.
The Transmitter's Signal Source
The transmitter's signal source is a single chip HD6303R microcomputer. The Hitachi HD6303 is a CMOS version of the Motorola 6803 microprocessor, and was used to conserve power, thereby permitting battery operation of the transmitter. The transmitter's CPU was operated in the non-multiplexed mode since its only task in life was to provide a quasi-repetitive signal to the transmitter for link performance monitoring. The schematic of the transmitter's CPU is shown in Fig 2.
The CPU was constructed using perforated board and wire wrapping techniques. The CPU was programmed to send a continuously changing message ("Count = wxyz") encapsulated in a packet-like format, as shown in Fig 3. The message was sent at a rate of 6992 bauds, which was derived from the CPU's 3.58-MHz crystal divided by 512. From the hardware point-of-view, rates as high as 62.5-Kbauds are practical (but have not been tested) at the expense of higher operating current. The message was sent 5 times each second, so that I could readily monitor link operation while conducting range experiments. For transmission robustness, the message included a parity byte, so that only those messages that were received with proper parity were displayed at the receiver. In the beginning of each packet was a command to clear the receiver's LCD display. The clear LCD command was also protected by the parity byte. This has the effect of freezing the display with the last correctly received message on it, and allowed me to move to a new position, point the receiver at the distant transmitter for a few seconds using the coaxial optical telescope system, and then looking at the display to see if a new message was captured. This was all done while hand-holding the receiver. The assembler source code for the transmitter's CPU is given in Listing 1.
The NR2 output of the CPU's serial port is then turned into a series of short duration (2 microsecond) pulses marking both the positive and negative transitions of the NRZ bit stream. The pulses are generated by exclusive-ORing the data stream with a delayed replica of itself as shown on the schematic of Fig 4. The short duration pulses are then fed to an emitter-follower type of LED driver, which drives the LED at a peak current of approximately 0.2 amperes. Since the duty factor of the drive current is less than 2%, the average LED current is less than 2 milliamperes, and the entire transmitter including the CPU
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