Micro­power

  

November 1981, Volume 1, Number 3











Page 8 of 33











READING TRS-80 PROGRAM TAPES

by Mike Fox

There is a great deal of software available for micro-computers, but it is generally not possible to exchange programs between systems because the data is stored on tape in different formats. This article describes a method for reading and converting TRS-80 tapes for the Nascom. The project needs both hardware and software, and is for TRS-80 Level 2 Basic (also Video Genie in the U.K., P.M.C-80 in U.S.A, and System 80 in Australia and N.Z.), but it could be modified for other machines.

The TRS-80 writes tapes at 500 Baud. An 80 microsecond clock pulse is sent to the tape every 2 milliseconds. The data bits to be stored are represented by inserting an extra 80 microsecond pulse between two clock pulses for a 1, and leaving the gap empty for a 0. This of course is incompatible with the CUTS standard used in the Nascom II. Therefore a small circuit consisting of one LM3900 (an IC containing four operational amplifiers) and a couple of dozen discrete components is used to input the signal from the cassette via the Nascom PIO. Figure 1 shows the circuit diagram of the interface, while a suggested Vero layout is shown in figure 2. Make sure that pin 11 of the Nascom 2 PIO plug is connected to 0 volts on pin 16.

COMPONENTS REQUIRED

Resistors Capacitors Semiconductors

R1 1 kohm R2 150 kohm R3 330 kohm R4 560 kohm R5 330 kohm R6 1.8 megohm R7 470 kohm R8 680 kohm R9 470 kohm R10 470 kohm R11 1 megohm R12 1 megohm R13 10 kohm R14 10 ohms R15 470 kohm R16 470 kohm

C1 220 pf C2 220 pf C3 50 F C4 100 F C5 0.1 F LM3900 Quad. Op-Amp. D1-D4 Smallsignal silicon diode

SOFTWARE FOR MICROSOFT BASIC

The first part of the program reads the tape and loads it into the correct memory location for Nascom 2 Basic. As the reading is done by software timing, the delay values in the program will vary for machines running at 2 Mhz and 4Mhz. At the start of the tape there is about 4 seconds of nulls (00), followed by a sync character of A5 hex. When this character is detected the program starts to load the data from the tape starting at address £10F6; as it is stored, the data is also displayed on line one of the screen. The first four characters are SSSn, where n is the program identification. These are not used, and the actual Basic program starts at £10FA. The end of the program is indicated by three nulls, which cause a jump to part two of the tape reading routine.

In this second section, the token values used in TRS-80 Basic are converted to


This is an OCR’d version of the scanned page and likely contains recognition errors.











Page 8 of 33