Micro Peripheral: MENTA
Micro Electronic mNemonic Teaching Aid
Menta comes from Dataman Designs, the people who brought you Softy and Softy 2, and in fact uses the same vacuum formed case as Softy 2. This case, along with its general shape and the membrane keyboard, means that Menta looks very much like the Sinclair ZX80, but in fact it isn't a personal computer at all; neither does it work in BASIC.
Instead, Menta is aimed at education, for teaching machine code programming and the rudiments of computing; and at engineers for micro based system development. The basis of the machine is a Z80 microprocessor running at a little over 2.2MHz (half of TV colour subcarrier frequency, in fact), supported by a 2K monitor program in EPROM and 1K of RAM. The RAM is split into four 'pages' of 256 bytes each, and each page's contents may be displayed on a TV as 16 rows of 16 bytes, each byte being represented by two hexadecimal digits. The usual UHF output lead is provided to feed your television, and a seven-segment LED display indicates which page is currently being shown on the screen.
Apart from a handful of logic ICs, the other major component is an 8255 I/O port, with a total of 24 lines (three groups of eight) which are available on a 26-way connector. Many of these lines are also used to monitor the keyboard, and to drive the seven-segment display, a beeper which makes an annoying squeak whenever a key is pressed, and a cassette interface which uses only one jack socket doubling for input and output; this stores the entire RAM contents using Dataman's Transwift system, which only takes 4-8 seconds to do the job.
Menta's monitor program is what gives the machine its personality, and does all the things you would expect, plus some you would not: especially considering the small size of the ROM. All the 'housekeeping' is taken care of by software, including nearly all the TV display work; a divider chain takes care of the parts which are too fast for the microprocessor.
From the user's point of view, hexadecimal characters may be entered into the memory to form a machine code program which can be seen in its entirety on the screen. The program may then be run all at once, or in a single step mode where the effect of each instruction may be examined; this is very useful for debugging recalcitrant programs, since the only way of stopping a program which is running at full speed is to press 'interrupt' or 'reset' by which time the damage has usually been done.
To aid the programmer, the contents of all the Z80's registers are displayed at the bottom of page 3, and may also be changed if required in the same way as any other area of RAM. A cursor on the screen indicates the byte into which information will be entered next, and this cursor may be moved freely about by four keys (up, down, left and right); the cursor's address is displayed along with the register contents so you always know where you are, on page 3 at least.
One useful feature is the Assembler mode; when this is in use, all the common Z80 instructions may be entered in two or three keystrokes using the mnemonics printed by each key. You can, of course, easily key in any instruction in hex form if the assembler won't handle it. There is even a facility for calculating the displacement for relative jumps; although fiddly, it saves time and removes a common source of error. All things considered, the assembler is a very useful part of the machine.
The manual is an important part of any educational product. I dream in hex code some nights, so it's difficult for me to say how useful the Menta book is to a beginner; certainly everything seems to be there, from the binary number system up to a fictitious microprocessor controlled drinks machine, which sounds like something from 'The Hitchhikers Guide to the Galaxy'! There is also a circuit description, a list of Z80 mnemonics, data sheets on the Z80 and 8255, and some sample programs including a simple music program which turns the bleeper into a one octave monophonic 'organ'. A separate book contains a listing of the monitor program, which is liberally documented and makes fascinating reading for those who can understand it.
Asa reference book for the busy engineer, however, the manual would be vastly improved by the addition of an index; as it is, there aren't even any page numbers and finding something quickly is made difficult if you don't even know if it's in the book at all!
Quibbles apart, though, a well thought out and useful device; the ports could also make the machine into a handy drum controller or music sequencer. You could make a programmable drum machine in conjunction with this month's percussion generator, for instance, and still have outputs left over for a synthesiser!
Menta is available from Dataman Designs, (Contact Details) and costs £115 plus VAT.
Review by Peter Maydew
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