Back From The Brink

G. E. Chkirantz takes a new look at the Microtan 65 - a DIY micro that narrowly escaped extinction

In the beginning (circa 1981) the Microtan 65 computer system was brought to fruition by Tangerine Computers as a single-board, 6502-based kit. Its main attraction was the facility to build it up to a complete computer system.

About 10,000 systems were sold, despite little exposure, when Tangerine decided to disinherit its child. However, Tangerine dealers Microtanic Computer Systems acquired all rights to the system and is now relaunching the machine.

Presentation

The Microtan 65 is principally popular in kit form, allowing you to expand in the manner that best suits your requirements and your bank balance.

Any of the boards can be supplied ready-built and some, like the disk controller, can only be bought this way because of their complexity.

The whole system is housed in a smart 19" frame with a very neat switched power supply. A high-quality ASCII keyboard with numeric keypad is also available.

Documentation

There are separate manuals for all the individual components in the system. All are of good quality. The documentation for the main board is typical: it includes adequate constructional notes and does not attempt to hide information.

Because the board may be used in standalone form for machine coding, details of the 6502 opcodes are thoughtfully provided, as well as a few games listings.

Most notably, a complete annotated listing of the monitor ROM is also provided with details of how to use the more useful subroutines, including the registers and locations that are used or that may become corrupted.

Documentation for the Microtan's Basic is average but does have very useful extra sections detailing the use of logical operators, space and time optimisation of programs, the USR(X) functions and a list of formulae to derive mathematical functions not intrinsic to Basic.

Keyboard

Two options are available for use with the system: a keypad and a full ASCII keyboard. Both plug into the same socket on the Microtan 65 board and can be used in the most minimal to the most complex systems as the system firmware configures itself automatically to the device in use.

The keyboard is a 73-key, full travel, typewriter pitch type with separate caps and shift lock. The latter features a warning LED and acts exactly as does its counterpart on a typewriter while the caps lock forces letters only into upper case.

This is well thought out and provides options which should suit most uses. Auto-repeat is unfortunately omitted, although there is a repeat key.

The keyboard may be housed in a heavy black metal case which is certainly strong if rather stark and angular-looking.

Display

A variety of options exist for video output of which the most primitive is a 32 column by 16 row, upper case only display of outstanding stability. This circuitry is contained on the Microtan 65 board. Further choices on the main board include lower case and a limited graphics facility giving a resolution of 64 x 64.

In an expanded system further options include a 64 x 25 colour board with teletext-style characters, lower case, block graphics and inverse or flashing output. This board has both monochrome and RGB outputs but there is an optional PAL encoded and UHF modulator to produce composite video.

For high resolution graphics, a monochrome board originally made by Tangerine is on offer and a colour board is also available. Both offer a resolution of 256 x 256. Neither was supplied on the review machine but extra software is available to drive them.

Microtanic says an 80-column board is under consideration and a final bonus to graphics programmers is the ability to mix text and colour high resolution graphics on screen.

The System

The Microtan 65 is basically a 12-slot motherboard with a 64-way Eurocard connector carrying the bus information. Three of the slots are dedicated to particular boards, namely the Microtan 65 board, the Tanex 7K RAM card and the TANDOS disk controller board.

These slots are all clearly marked and have the connectors offset to avoid errors. A smaller two-slot expansion board is also available to house the Microtan and Tanex to form a discretely powerful package.

The complete system bus has extensive paging facilities controlled by software switches which are sensibly located in the system ROM area and allow up to 256K of memory to be accessed at the flick of a switch.

The Microtan 65 board itself houses the 6502, 1K of RAM, 1K of ROM, the keyboard interface and the video circuits. The Tanex board is its closest companion, providing full address decoding and data bus buffering that could not be fitted on the Microtan.

The memory map of the system is allocated by this board as 47K of RAM, 1K of I/O, and reserved areas for languages in ROM and system firmware.

It also provides a considerable extension to the system as the board can hold up to 7K of extra RAM, 14K of EPROM, and two 6522 Versatile Interface Adaptors giving 32 I/O lines. Some of the I/O lines are used to provide the cassette and RS232 interfaces, others provide a Centronics printer port but they may also be used for other purposes.

Expansion

The RANRAM board forms the main memory expansion for the system with 7K of static RAM and 32K of dynamic RAM using 4116 chips. This boosts the system to its full complement of 47K of RAM. The use of dynamic RAM keeps the chip count and power requirement to a minimum but limits the top speed to 1MHz.

An EPROM switching board is a useful addition, especially for those using cassette-based mass storage. the ROM area is decoded into two 4K and two 2K blocks, which means the 10K Basic, 8K Forth, 8K word processor and two pass assembler plus several toolkits are instantly accessible.

An EPROM programmer allows you to program all common 2K and 4K devices. The EPROM is powered only during command operations so removal is safe. Software is provided to check, copy from EPROM to memory, program either the whoe device or single step, and verify.

Basic

A 10K Microsoft Basic is available for the machine with all the usual features. It lacks the advanced commands now becoming increasingly common but does feature full error messages.

A screen-orientated editor is provided which is set up for the original 32-column screen. If the 64 x 32 card is in use a copy of the original screen is placed in the centre of the new one. Dotted lines highlight the line to be edited and a nice feature is that, depending on which terminator is used on the line, either the next or the previous line may be brought into the edit buffer.

For the machine code programmer, the Microtan comes with a built-in monitor. It is very neat and competent and features the usual facilities with some extras. You can single and multiple step through a program, and set up to eight breakpoints.

It can be further augmented by XBUG which includes a disassembler and single line assembler plus extended printer support and some useful memory management routines.

Additional high level languages are available including Pilot, an implementation of FIG-Forth and a full-blown two-pass assembler. All can reside in EPROM, leaving the RAM area free for programs.

Verdict

The Microtan system will appeal to enthusiasts who do not want a little box full of mysteries, but would like to get intimately acquainted with their machine.

The 65 is capable of expansion in almost endless variations.

There are some problems: the 65's processor clock speed of 0.75MHz scarcely does justice to the 6502 which may now be driven up to four times as fast. I also feel that the price is rather high.

When comparing the system with others available you should remember that it is not a rival to the Oric and Sinclair machines. It is aimed more at the BBC and Apple markets with the advantage that it is extremely flexible, leaves you with more RAM than the BBC and you do not have to buy everything at once.

The system is a taut and intelligent design. With the quality of documentation and backup, it is easy to understand users' almost fanatical loyalty to the machine.

Soldering On...

Microtan in the making - David Guest is the only man for the job

There's nothing quite like a distinctive aroma for bringing out the poet in people. With some it's damp leaves or freshly-mown grass, with others home-cooking or a subtle perfume.

With veterans of the computer business it's solder. As Martin Duvall might have said in Apocalypse Now, there's nothing like the smell of solder at breakfast. (In fact he was talking about napalm but solder can have a similar if less dramatic effect on a carelessly positioned African violet.)

Micros that require you to take up a soldering iron are few and far between these days, so Microtanic might find itself fuelling a nostalgia boom with its Microtan kit.

A new generation of enthusiasts could discover the delights of microcomputer construction, and old-timers could treat themselves to a sentimental journey, the more so since the Microtan 65 is none other than the old Tangerine micro under new ownership.

I, from the PCN team of experts, was hand-picked to build this device because I'd never wielded a soldering iron in my life. I should therefore declare a prejudice - I'd never touched a soldering iron because I'd never wanted to. I'd long suspected that ready-made systems were unreservedly a great boon to mankind and that soldering had not merely gone out of fashion, it had been superseded as a tedious and unreliable operation.

Any such newcomer to the technique requires guidance, and Microtanic's documentation helpfully suggests that you read the manual before attempting anything. The manual covers almost everything from the rudiments of binary arithmetic to the intricacies of the discouragingly named Tanbug operating system. Its punctation is idiosyncratic, with commas flung across the pages, and the writer's sense of preposition betrays an Iberian origin, but by and large it doesn't read badly.

It is in the Aids to Construction that the manual falls down and this, of course, is where you need it most. The components are listed and the PCB is clearly marked, but not all the devices supplied correspond to those listed (is a 21L14 chip functionally the same as 2114?), nor do the marks on the board tell the whole story.

When you've washed your hands, earthed a piece of aluminium foil and prepared your artificial aids, you should be confident enough to begin. Almost immediately you'll find a problem - the first items to solder to the board are the sockets in which the chips will sit, and you have to put them in the right way round.

The manual advises you to make sure that the Pin 1 indentifier is at the appropriate end, but the sockets lack a Pin 1 identifier. This isn't an unsuperable problem - you can just get stuck in, be consistent, and hope you guessed right. But doubt will grown in your mind and by the time you have to check the polarity of the diodes (again without help from the manual) you may have begun to wonder whether you weren't presumptuous in breaching the sacred inner sanctum of technology. It isn't difficult to retrieve an occasional soldering mistake but if you get the whole board wrong you can't unravel it like a badly knitted woolly.

At least the act of soldering holds on mysteries. Try to sit with a draught taking the air away from you or the famous smell will grip your nose and the smoke will bring tears to your eyes. And something mindlessly rhythmical in the background can be a help - tennis on the TV, perhaps, or a Uriah Heep LP. All you need is a steady hand and an appreciation of the joy of dull, repetitive manual labour.

After the sockets you move on to the resistors, colour-coded and as pretty as a child's jewellery. Here you'll need wirecutters, and don't throw the pieces of wire away - you need three of them for links later (the manual says four but if you can find LKNM1 on the board please write to Microtan asking them to print it on all of them.)

After the resistors the capacitors, and after them the diodes of doubtful polarity. By now the end is in sight and these fiddly bits are like a rest cure after the 14, 16, 20 or more pins of each socket. The transistors and the crystal lead you to the final items - the UHF modulator and the edge-connector. This last unit has upwards of 60 pins, and after that you've earned a rest.

All that remains is to fit the chips into the sockets - the right way round, and of course the right sockets. The mystery of the Pin 1 identifier re-surfaces. What can you do except fit the chips all the same way round and hope for the best?

The chips, incidentally, are like a geography of the world's sweat shops. They come from El Salvador, Indonesia, Singapore, the Philippines, Malaysia, almost everything in the free world where freedom means the opportunity to work for American multinationals at pitiful wage rates.

At the end of the manufacturing process, when you sheathe your soldering iron and apply a dab of Vaseline to the last burn, it's time to take stock. In the literal sense this means checking that there are no bits and pieces lying around that should have been welded to vital parts, but the metaphorical sense is more restful - it involves sitting back and drinking in the satisfaction of a job well done.

There is one drawback. Until you acquire a keyboard, a power supply, a monitor, associated leads, and a 13 amp plug, you have no way of knowing exactly how well the job has been done. Don't let that put you off. A job should have a beginning, a tea break, and an end. When you've applied solder to so many points that the damage is irrevocable you can regard the job as finished. Let the quality control experts worry about the quality.

Prices

Microtan boards £59.95
Tanex expansion board £49.95
40K memory board £79.95
Keyboard £79.95
TANDOS £99.00
Colour VDU board £89.95

Specification (Basic System)

Price: £59.95
Processor: 6502, 1MHz
RAM Memory: 2K expandable to 48K
ROM Memory: 2K expandable
Text format: 32 x 16 b/w
Graphics Screen: 64 x 64 b/w (colour and hi-res options)
Keyboard: Optional
Storage: Cassette, 300 baud
OS/Language: Basic
Distributor: Microtanic Computer Systems, Dulwich. Tel: 01-693 1137
Software included: None