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
| 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 |
Capacitors
| C1 | 220 pf |
| C2 | 220 pf |
| C3 | 50 µF |
| C4 | 100 µF |
| C5 | 0.1 µF |
Semiconductors
| LM3900 | Quad. Op-Amp. |
| D1-D4 | Small-signal 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