The 512's host processor may be a BBC model B, B+ or a Master 128. In all three cases (and it is the only possibility for the first two) the 512 may be fitted into an external housing such as an Acorn Universal Second Processor Unit or some other proprietary co-processor adapter. In this case the second processor housing must also include a power supply unit to cater for the 512's 5 volt needs.

When fitted in this way, the only physical and electrical connection between the 512 and its host is provided by the external tube. This is an Acorn proprietary fast data bus, rated at 2 Mhz (theoretically up to two million bits <should read bytes> per second) and 32 bits (four bytes) wide. The tube is provided specifically for second processor connection, though not only for the 512, as it has been used for at least four other commercially available boards. (The others are 6502, Z80 CP/M, 32016 scientific and Acorn RISC Machine development Systems).

Uniquely, in the Master 128, there is a second option of using the internal tube, when the 512 does not require any additional housing since it is fitted directly to the Master's mother board within its own case. In this type of installation the 512 also takes its power from the Master's power supply unit. Because there is no separate on-off switch by which to enable or disable the 512, software switching is employed, provided by the host's MOS. Selection between the external and internal tube is achieved by the Master's *CONFIGURE command, as is enabling or disabling the tube interface itself.

While there is no essential difference between the host's support of the second processor whichever connection is used, two particular points are worthy of note when the internal tube is employed.

First, the 80186 processor chip in the 512 becomes extremely hot after even a short time. This is completely normal, but after an hour or so it is likely to be too hot to touch. This does very significantly increase the ambient temperature within the Master's case, and any ICs that may be on the limits of their operating temperature tolerance have an increased probability of transient failure. It is vital that the ventilation slots in the case are not obstructed.

Cases are known of users who, after fitting a 512 internally, begin to experience problems after an hour or two of operation, even when the 512 is not in use. This is because the 512 consumes some electrical power whether it is selected by software or not.

There are two solutions to the problem. One is to have suspect ICs replaced, but identification can prove difficult and expensive. The other is to switch the connection of the 512 to the external tube. Similar problems are rarely experienced when a 6502 second processor is fitted internally, as these run very much cooler.

It is worth noting that the WD1772 floppy disc controller fitted to some Masters has proved to be more troublesome than other chips. If problems are experienced which seen to be related to the floppy disc system after fitting a 512 (especially internally) changing to the more reliable, though slightly slower, WD1770 may well provide a solution.

The second point about the internal tube is that fitting one type of second processor to it does not preclude the simultaneous attachment of yet another, connected via the external tube. It is possible to have both a 6502 and a 512 second processor permanently attached, switching between them as required, with the command *CONFIGURE EXTUBE/INTUBE.

The 512 with DOS Plus 2.1 runs virtually all application packages which are legally written to be compatible with MS-DOS version 2.1, PC-DOS 2.1 or CP/M86, which was the forerunner of DOS Plus. Arguably the most notable aspect of the 512 is that much of the software written for PCs, which have very different architecture, does run satisfactorily.

Not surprisingly, some incompatibilities are more easily predicted than others. The prime requisite is that the package does not require more memory than the 512 can provide. This is typically about 358k bytes in an empty 512 running DOS Plus 2.1 and no other program, but it can be further reduced by background programs, the memory disc and other memory-resident utilities.

Some more recent packages are written to expect 640k, which is virtually the standard memory size for PCs these days. This problem is further compounded by the fact that DOS Plus takes about 90k more memory than MS-DOS for its memory resident portion. The result is that, even if software is suitable for a 512k MS-DOS machine, it's still entirely possible that it will not run in the 512 because of lack of memory.

Screen support provided by the host 6502 is naturally limited to the facilities that are available when running in native BBC mode. Despite the fact that the 6845 CRTC (Cathode Ray Tube Controller) employed in the BBC Micro is the same chip as is used by many PCs, the display facilities available are not the same.

The major difference between the screen displays provided by the 512 and other DOS Systems is the restricted colour capability, especially when operating in 80 column text mode. In these modes the BBC Micro uses its native screen mode 3, which is a single colour mode. Although the 512 offers a CGA (Colour Graphics Adapter) compatible screen map to DOS applications, the true PC CGA screen of 80 columns in colour is ultimately restricted by the BBC mode 3 display.

Keyboard support by the 6502 host can offer very particular problems in DOS, especially if a Model B or B+ is used. The prime difficulty is that the keyboards employed by PCs simply have more keys than those of BBC Micros. More recent PCs also have more programmable function keys, fifteen or twenty now being common.

Some, though by no means all, recent applications software expects to take advantage of these extra keys and, unless such software is configurable for earlier PCs, it will be difficult, if not impossible, to run satisfactorily in the 512.

Some of the extra keys provided by PCs are included for highly specialised purposes, directly related to hardware, or more properly firmware functions of other parts of the machine. In the BBC Micro no such hardware to firmware dialogue is provided, all keyboard scanning being entirely under the control of the host's MOS. To illustrate, the nearest equivalent operation in the BBC Micro is the effect of the shift and control keys which, when pressed simultaneously will normally prevent the screen from scrolling regardless of the application used.

In addition, the method of translating keyboard hardware matrix scans to an internal key representation is entirely different in PCs. Even for those keys that are common to both the BBC Micro and PCs there are differences because the actual internal number produced by most key presses is different. For the 512 system all key-presses passed across the tube must first be translated by software to the required IBM code before being passed onto DOS applications.

It follows that an application expecting to detect an IBM key at source by reading the keyboard hardware matrix will fail, but so will any package which expects to respond to one of the extra IBM keys, even if the legal DOS keyboard facilities are used.

For example, in most PCs the left and right SHIFT keys generate two different internal key numbers. While entering text into a word processor there may seem to be no operational difference between these two keys, but at other times they can serve entirely different purposes. Both keys can be detected and distinguished separately in PCs either by scanning the keyboard hardware matrix (strictly speaking illegal, but it doesn't matter in a PC) or by reading the internal key number using legal DOS calls. Applications that use the first of these methods will always fail, while those that require the extra keys will fail to respond correctly (or at all) in the 512.

This type of low-level non-ASCII key detection is not possible with the BBC Micro because the keyboard matrix is much simpler and does not provide the fall range of PC keys, nor does it provide any hard-wired functions. Also the keyboard is on the wrong side of the tube for direct access by DOS or any application, therefore the keyboard is not directly addressable as it is in true PCs.

Model B or B+ Micros have additional problems in that they use an even more simple keyboard than the Master. Even for those PC keys which are emulated on the Master's numeric keypad when used in conjunction with SHIFT-LOCK, the results may not always be reliable because they depend on the method of access employed by the application as well as the key number expected.

The Western Digital 1770 or 1772 FDC (Floppy Disc Controller) employed by Acorn for ADFS is eminently suitable for use with DOS disc formats but there are one or two specific points to note.

DOS expects two disc drives for a practical usable system. It is possible to run DOS Plus with only a single drive, provided that all applications leave enough free RAM for an adequate memory disc. In practice this would be highly restrictive, therefore to all intents and purposes two physical drives should be regarded as the minimum.

For floppy disc drives both must be 80-track double sided. There is no longer such a thing as a single sided DOS disc drive, although the ancient and rarely seen 160k discs were single sided 40-track devices. Although IBM 360k formats use 40 tracks, it should be noted that, even if switchable drives are fitted to the host, they should always be set to 80-track operation as all format detection and track double stepping is catered for in the 512's software.

Fortunately printers hold few problems, either for DOS or the 512. The vast majority of recent printers provide either an optional IBM mode (usually IBM Proprinter X24 emulation) or they provide adequate IBM block graphics character set support. The standard DOS Plus background print utility operates quite satisfactorily in the 512, with the small condition that you disable the printer's automatic line feed if you wish to avoid double spacing.

Whether you do this by means of the printer's DIP switches or the host's printer ignore character is a matter of personal preference, though regular users of the 512 will find it more convenient to disable the line feed in the printer's hardware, than issue (or configure)

*FX6,0

in the BBC host in native mode, or *CONFIGURE IGNORE 0 on a Master. This avoids the need for use the relatively tedious and slow STAR command when the 512 is active.

The standard printer connection for DOS Systems is a Centronics (or parallel) interface. In DOS Plus the logical print device is PRN, while in MS-DOS or PC-DOS it is usually LPT1. This may give rise to problems with some software in the 512, for example, GW-BASIC.

Within applications, owing to the proliferation of PC compatibles, a printer configuration section is included in the installation process of most packages. Generally such packages write to the printer legally, as it is a slow speed device, and leave the logical to physical device assignment to the operating system. However, in a limited number of cases it may be necessary to change the printer device assignment within the application. If so this should be set to PRN (or LPT1 if PRN is not accepted). The package should then print correctly.

A useful tip with GW-BASIC in which the LPRINT command will not work on a 512, is instead to 'open' the printer with the command

OPEN "PRN" FOR OUTPUT AS #1

at the start of the program, and then to use PRINT #1,a$ instead of LPRINT a$ when you need to print to the printer. If your program uses files, then the channel number may have to be changed from 1 to another free channel number.

The more capable packages also will include a list of different printers from which your model, or a 100% control code compatible alternative can be selected, ensuring correct control codes will be sent for each of the various typeface effects and enhancements. More recent packages will also cater for a range of laser printers too.

Serial connection of printers is not directly supported, either by DOS or by applications software. As is normally required when in native mode, the speed should be set correctly, either by using STAR or the MODE command. Also printer output should be directed to the serial port either by means of STAR again, or by logical to physical device assignment using DOS's DEVICE command.

If printing from an application proves impossible with a serial printer, you may find the package provides a print to file or a save ASCII data option. If no such option is provided, it may be possible to use DOS's re-direction facilities to achieve effectively the same result. In either case the resulting file can be printed afterwards, by means of the standard DOS Plus PRINT command. This last 'trick' can also solve problems with some packages that refuse to communicate with 'PRN' or 'LPT1'. If this proves unacceptably inconvenient or impossible, the only alternative is to change the printer's interface to parallel.

Like the printer's Centronics interface, the RS232 serial port is a standard communications connection, which has been adopted by micros, rather than the reverse. There are few problems concerned with the 512's serial port except in the area of proprietary PC software.

Support for the RS232 port is notably weak in MS-DOS and PC-DOS, and access is very slow via the legal calls. In consequence, most communications software authors find it necessary to write their own hardware driver code. Because of this, virtually all DOS communications software uses illegal direct hardware addressing to provide more sophisticated facilities and to permit the use of the higher speeds. Since the 512's serial port is located on the far side of the tube it is impossible to use such techniques in the 512 as they are guaranteed to fail. There is no possible fix for this problem, so do not invest in DOS communications software, except as mentioned below, or which you have seen working in a 512.