How one man solved his RFI problem


LNW-80

Richard L. Brocaw
1850 Pearl Loop
Bosque Farms, NM 87068

I purchased my TRS-80 in 1987. Radio Shack's original design problems were compounded by my rural location. The RFI interference precluded its use in the ham shack.

It seemed obvious I needed a hardware upgrade. The replacement had to be soft- ware-compatible with the TRS-80. I shopped

around and decided on the LNW. I ordered both circuit boards.

The processor board

The circuit boards are silk screened and solder masked with plated-through holes. The processor board has a gold plated edge connector.

Two views of the interior of the LNW-80 built by the author. The expansion board has been lifted out and laid back to reveal the processor board. The power supply is on the right side, and the fan RFI filter and IC regulators with their heat sinks can be seen. Note the use of heavy wire to connect power to the expansion board. The expansion board shows the two piggyback boards: Percom Doubler and the programmable baud rate generator, The default RS-232 parameters are selected by switches seen at the lower right of the keyboard. The wires running off the picture on the left are remote vertical and horizontal position con- trols. The wire on the top right of the keyboard goes to the LED power light. The small to- roids clustered around the power supply are used in place of jumpers to help reduce noise.

The Z80 processor chip runs at 4MHz. Most of the circuit board design is a leap ahead of the TRS-80. The power supply has an off- board large capacitor. The design uses several IC regulators with overvoltage crow- bars and fuses for protection. This circuitry is clustered in one section of the board to keep heat from sensitive circuits. Heat sinks are manatory on the regulators.

80Microcomputing, May 1982 206


"The LNW case is an
attractive low profile cabinet
made of heavy gauge steel..."

There are 90 ceramic and 18 tantalum capacitors on the board with only 0.1 volt of noise on the plus five line. The memory design has Schmitt trigger devices on all lines: the address lines have series termination. The result is a quiet memory. 200 nS memories are required with the 4MHz clock.

The character generator ROM can be a custom programmed 2716 or the TRS-80 character generator. The new Radic Shack ROM with lowercase descenders also works. For the language ROMs the two or three chip Radio Shack set or LNW six chip set will do. For the six chip set the ROMs are 2716s. The configuration is selected by a DIP switch of jumpers.

The processor speed is slowed to 1.77 MHz when the ROMs are accessed by in- serting wait states in the Z80. When run- ning the Basic interpreter, the speed in- crease over the TRS-80 is around 40 per- cent (depending on the application). When running machine language ROM indepen- dent software the LNW is more than twice the speed of the TRS-80.

The LNW-80 allows use of a Radio Shack keyboard or the LNW keyboard. There are two connectors on the circuit board. U30 and U34 must be removed to use the Radio Shack keyboard.

Graphics

The foremost feature of the LNW is the on- board inclusion of high density black and white and color graphics consisting of six additional 4116s with support circuitry. It is controlled by an OUT command to port 254.

The graphics pixels use the same ad- dress space as the Basic ROMs, activated by an OUT to port 254 with bit 3 set. The command deactivates Basic making a ma- chine language driver necessary for graphics. This is fairly well documented in the manual. High and low resolution graphics and characters can be displayed at the same time.

The screen is divided into an inner and ex- tended region. The inner region is 384 by 192 pixels. The extended region is another 96 pixels horizontally. Addressing for the two regions differs slightly.

For color graph ics either a color monitor or a tv with on-board RFI modulator is used. Outputs for an RGB monitor can be wired in. There are two color modes. The first gives you 128 by 192 pixels in eight colors. The high resolution mode gives 384 by 192 pix- els. The bandwidth for this mode requires

an RGB monitor. The color graphics re- quires a custom programmed 82S23 ROM, available from LNW. The ROM contents are detailed in the manual.

Drawbacks

LNW lists a 24 by 80 character video screen in its advertising. This is a little de- ceptive. The LNW-80 has the capability of a 24 by 80 screen by utilizing the high density graphics, but to get it you have to write a driver program.

LNW's six chip ROM set costs $120. Con- sidering that 2716s go for about $6 each . I decided to program my own. But which ad- dress blocks go in which ROM socket? I called LNW and was told the information was proprietary! Some schematic study and experimenting revealed the truth as can be seen in Table 1.

The Expansion Board

The expansion board is of the same high quality as the printed circuit board, except the gold plating costs extra. (The plating is worth it.) It has the same good memory de- sign and Schmitt trigger drivers. There are 43 ceramic and 12 tantalum bypassing capacitors.

The board also has a few drawbacks. The printer port works okay if the printer is only a few feet from the port. You never want to hook a long line to the output of a D-type flip-flop. My printer is 20 feet away and 74LS175 will not drive it. I had to add buffering.

They cut corners on the RS-232 port. Wire jumpers select the default RS-232 parameters. Why not use a DIP switch? The baud rate is also a wire jumper, and is not software-select- able. You can change soldered jumpers about three times before the pads fall off or the plating pulls out. LNW should have added the one extra chip to make baud rates se- lectable. Or they could have laid out the DIP switch on the circut board.

The RS-232 connector pads are spaced for a printed circuit mount right angle 25-pin con-


IC Number  ROM Name      Address Boundaries
           (LNW Manual)  Decimal   Hex
U75        A1            2-4K       0800-0FFF
U76        B1            6-8K       1800-1FFF
U77        C1            10-12K     2800-2FFF
U78        A             0-2K        0000-07FF
U79        B              4-6K       1000-17FF
U80        C             8-10K      2000-27FF

               Table 1.

nector. Do not solder one or, the board. When the board is in place inside the LNW cabinet there is no room to hook up to it with a plug. You also cannot use a straight connector. Besides. the connector pads are 16 inches from the nearest opening in the case. Use 25-wire extension cord.

The system expansion connector for out- side devices is at right angles to the back of the case. You have to take the cover off to use it.

The Keyboard

Here is where the good designers at LNW went out to lunch en masse. The manual says the keyboard "is designed specifically for the LNW-80." The one I got was just a plain unencoded keyboard. A plastic bag with engraved keytops was included with instructions to replace certain keytops with the new ones. One of the new keytops was engraved sideways!

The key switches are the same wire finger switches that gave Radio Shack grief for so long. With this system, if you do not have a keyboard debounce fix, you will get keybounce in spades at 4MHz.

The circuit board is one-sided, with a large number of wire jumpers on top of the board. The keyboard has to sit on them and is difficult to seat fear soldering. Buy your keyboard someplace else. Radio Shack Model I keyboards are cheaper and superior. The LNW case is an attractive low profile cabinet made of heavy gauge steel with room inside for both circuit boards, key- board, power transformer, a muffin fan and RFI filter. Unfortunately it is overpriced. Build your own or buy a cabinet from Cramer.

Conclusion

The LNW-80 is, for the most part, a well designed, fast, effective computer. I use my LNW-80 with the Percom Doubler II and LDOS operating system. I have a time-of- day clock, plotter controller, music board and eight bit general purpose port attached to my expansion bus. I designed and install- ed a piggyback board for selectable baud rates on the RS-232. All is working well.

Building an LNW-80 is not a project for a beginner. The manual is informative, but this is not a Heathkit.


Mr. Brocaw was born in Oklahoma in 1944. He holds a BSEE from New Mexico State University and is now a senior systems an- alyst for Public Service Co. of New Mexico.

80Microcomputing, May 1982 207


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