A Universal LPT Interface for VHF Log

  by N3FTI

Both the BCD chart on this page and the hardware are by Steve Kerns, N3FTI.  Steve designed an interface board to provide for logging-program-controlled bandswitching of transverters, etc. from Dave Mascaro W3KM's VHFLog program The PackRats subsequently put together a kit including the board and all parts, and it sold in fairly large numbers.

The board will cover 10 bands.  It uses the BCD-coded output from the computer's LPT port to control the band switching.  I modified mine slightly to cover 11 bands, so that I can go from 50 MHz thru 24 GHz.  I put it all in a rack mount.  Here is a picture of my unit.

The board works not only with VHFLog, but also with RoverLog, N1MM Logger, and should soon work with WriteLog.

The schematic is here.

The BOM is here.

Steve's article on the board is here.

Table 1 provides the logic of the outputs provided by the computer's LPT port, along with a pin out for the LPT port.

Table 1

Band (MHz)
A
B
C
D
50
0
0
0
0
144
1
0
0
0
222
0
1
0
0
432
1
1
0
0
903
0
0
1
0
1296
1
0
1
0
2304
0
1
1
0
3456
1
1
1
0
5760
0
0
0
1
10,368
1
0
0
1

A
LPT Pin 2
B
LPT Pin 7
C
LPT Pin 8
D
LPT Pin 9
Grnd
LPT Pin 15

The text that follows is by Steve Kerns, N3FTI.

Although there may be more, I know of only one commercially made band switch, the Top Ten Box. Although the Top Ten Box is a very well designed interface, it was designed with the HF station in mind. Among its shortfalls are its six-band limitation (I need to switch 10 bands!), the lack of “dry” relay closures, and the absence of buffered BCD outputs. My existing system required both NO, NC and isolated BCD outputs to control my existing equipment. After speaking with other amateurs; I realized that a more flexible interface would be a welcome addition.

This paper describes a universal interface that provides decoding of the BCD outputs to decimal DPDT c-form relays, along with buffered BCD outputs that are also DPDT c-form.

The circuit is very simple! ISO1 – ISO4 provide a level shift from the ~5v LPT output to the +12 (or 13.8V) used by the interface board. ISO1 – ISO4 could be four discrete devices, but a single PS2505-4 (which has four opto-isolators in a single 16 pin DIP package) does nicely! Transistors Q1- Q4 along with relays K1 - K4 provide the buffered BCD circuits to drive my microwave IF switch, while IC 1 provides BCD to decimal decoding. Transistors Q5-Q13 operate in emitter follower configuration, receiving decimal drive from IC 1 and provide voltage to drive K5-K13.

My pet peeve is any published amateur project that is not assembled on a printed circuit board! Although this circuit could be built on a perf-board, it is quicker to assemble it on a printed circuit board! The board for this project was auto-routed, tested for compliance and compiled with the help of the software program QCAD. QCAD is a commercial software package that auto routes multi-layer printed circuit boards from a CAD drawn schematic. Once the board is routed it can be printed on a standard printer or files can be generated for your favorite printed circuit manufacturer.

For the initial design, I had three printed circuit boards made from a prototype board house. I assembled two while I sent the third to Ed, WA3DCR. During the last Packrats meeting; Ed had mentioned to me that he would like to integrate the design into his station. Along with the board I sent schematics, parts layout and a complete parts list. In no time Ed had the board built and installed into his existing transverter switch box. No problems were encountered and Ed now enjoys control of his station via the keyboard.

The interface became so popular that the Mt Airy VHF Radio Club authorized the production of a kitted version for its members and friends. The kits included a professionally manufactured printed circuit board, all the parts required to populate the board, and detailed assembly instructions. The kits are no longer available, but Steve, N3FTI, has a limited number of boards available at a very reasonable price.

TECHNICAL UPDATE : The diodes that parallel the relays on the board should have the anode lead toward the box, not the cathode toward the box as stated in the construction instructions.


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