Hands (Further) On

By Viktor

Using The Cassette Interface

When data from a microcomputer is stored on tape the eight bits of each byte being stored are written sequentially onto the tape, usually with the addition of a number of bits denoting the start and end of a byte. In many micros this task is performed by software, but in the Nascom a device called a UART (Universal Asynchronous Receiver Transmitter) does all the work, and the processor merely sends each byte to be stored to the UART and then sits an waits while this chip writes the bit pattern through the cassette interface.

To be effective the method of storing the data bits has to distinguish clearly between a 1 and a 0. In the original cassette interface fitted to the Nascom, a 1 is stored as a pulse of ‘noise’ at a frequency of 1.95 kHz, while a 0 is simply represented by the absence of such a signal. The system worked fairly well at the low speed of operation which was originally chosen, but it is very susceptible to spurious signals, caused by poor tapes or by external ‘noise’ carried along the mains leads, which the interface then interprets as a 1.

A design for an improved cassette interface, which was much more reliable and which would operate at much higher data rates, was published in P.C.W. in December 1978, and this interface, always referred to as the ‘Cottis-Blandford’ after the designers, was adopted for the Nascom 2. It stores data in the CUTS format (Cassette Users Tape Standard, also known as Kansas City format), which represents a 1 by a 2400 Hz and a 0 by a 1200 Hz signal. The original Nascom 1 tapes are not compatible with CUTS tapes, but the Cottis-Blandford interface is available in kit form, and it can be fitted to a Nascom 1.

Two recording speeds are available on the standard Nascom 2, 300 Baud and 1200 Baud. The term Baud refers to the data transfer rate in bits/​second, but don’t forget that this includes the start and stop bits, so that at 1200 baud it takes about 9 seconds to transfer 1K of data. As noted in INMC 80, issue 1, it is possible to run Nascom 2 interface at 2400 Baud by linking TP20 to TP4 and TP21 to TP5. The various Baud rates are selected by switches 1 to 6 on LSW/2. The switch positions for the various rates are:

U = up (nearest edge of board)   D = down   X = either

At higher Baud rates you will have to use better quality tapes and a better tape recorder, because the date is being stored at a higher density, but it is well worth persevering, because once your interface is operating reliably at 2400 Baud you will find even 1200 to be slow, and 300 will seem interminable. By modifying the board it is even possible to operate at higher speeds – 4800 or 9600! However, the best baud rate for exchanging tapes with other Nascom users seems to be 1200.