COMMUNICATIONS TAKEN OUT OF THE CRADLE
MICROMYTE has produced a communications system to use on a home telephone which will allow you to SAVE and LOAD programs to another ZX-81 or Spectrum user at 600 baud. The system consists of a cradle for the normal standard telephone, which has an amplifier and filter in it. The power pack is plugged into it and then a lead taken from the alternate power socket to the computer.
If you have a Spectrum the alternate power socket is a jack plug; if you have a ZX-81 the alternate socket is a Spectrum-type barrel socket. A lead with both kinds of plugs is provided.
A cassette is supplied with the software which does all the work. You must first LOAD it into the computer. The program first lowers RAMTOP - on the Spectrum by 1,535 bytes - and then loads the machine code program above it. Separate programs are provided for 16K and 48K machines. The program can be called at any time during a running program or from the keyboard by using RAND USER 64000 on a 48K Spectrum.
The bottom line is taken over for messages once the machine code routine is entered and (T) transmit or (R) receive appears on the screen. Pressing (R) will start the LOADing routine immediately. That routine waits for a "header" from the EAR socket before starting.
If T is selected, you are asked whether you want to SAVE a program (1), variables (2) or a the current screen (3). Pressing the appropriate number leads to a message Press Key to transmit. That should not be done until a few things have been checked.
The cassette lead then connects the EAR and Signal IN socket on cradle. The person at the other end has set up the program to receive and done the same thing. Both yours and the other person's handsets have been strapped on to the cradles using the Velcro strap provided. Anyone else in the room has to be warned that quiet is required or the data might be corrupted by noise in the room.
Once those have been checked you can press any key. The speaker beneath the mouthpiece of the telephone - the end with the label tends to emit a certain amount of noise. That is amplified Spectrum noise and should be ignored. It will, however, identify which way up the telephone handset should be. The sound the transmission makes is rather like a Spectrum program - first a short header, then a long series of data, both accompanied by the flashing bands on the border.
Saving a Basic program seems to work well, as does a screen file, but DATA tends to be corrupted and warning messages of BAD DATA and WARNING flashing in red on the bottom line appear. The program allows you to try again by sending the program again without touching the keyboard. Breaking the system on the receiving side may crash the system.
The best way to send variables would seem to be to send the complete program, variables and all. The DATA saving also appears to take much longer than necessary, as it still appears to take a minute or so to send even the shortest single string. There is no way at present to send CODE except as string variable. The variables used by the program are listed in a table at the back of the detailed instructions. The only ones which may be of use to the programmer are two bytes which set the colours for the flashing border and four bytes which set the start and end of the data sent.
They may be POKEd but there is no indication as to how to use them without going through the Program/Data screen routine which presumably will re-set them.
The cradle is not a true modem and so will not allow you to talk to any other user not using the same system. ZX-81 and Spectrum versions are not yet compatible, either. It is, however, a cheap and useful way of sending programs down the line and since the variables are sent automatically, interactive games can be used.
Sending from within a program means that when LOADed successfully the program will auto-run from the next line of the program.
The system can also be put together without connections other than to the EAR socket and with no knowledge of modems.
Telephone time, however, is expensive and the system is still affected by noise from its surroundings and from the telephone line. If you have a constantly noisy line, it is not the system for you.
The system will work with model three Spectrum and Interface One. The code is all that is required and that can easily be SAVEd and LOADed from Microdrive.
Micro-Myte 60 costs £39.60 from Micro-Myte Communications, Polo House, 27 Prince Street, Bristol 1.Tel: 0272-299373.