EASY-TO-USE DOUBLE SPECTRUM INTERFACE

Morex has produced a Centronics and a proper RS232 interface for the Spectrum. Both are contained in a small black box which plugs into the back of the Spectrum. Only 850 bytes of machine code are required to operate the interface and it is located below the USR-definable graphics section of the 16K or 48K RAM - both versions are supplied on the same tape.

Both interfaces will operate via the LLISIT and LPRINT commands once a call to the machine code has been made via the USR command. The RS232 will also allow INKEY$ from an RS232 device. The code sent can either be all codes from 0 to 255 or they can be sent as TEXT which would normally appear on the screen.

Various speeds can be set on the RS232 to a maximum of 600 baud on a 16K machine - it can be set higher but the manual warns it can be unreliable - or 4,800 baud on the 48K. The baud rate cannot be split, to work Prestel for instance, at 1,200 one way and 75 the other.

The RS232 has the same handshaking as most professional devices - CTS, RTS and TX, RX data - on a 7-pin DIN socket. No plug is supplied.

The only difficulty was finding some device with which to try it. It will not work with non-standard devices as it requires a negative voltage to register as the LOW condition. With a BBC Micro Model B it worked very well; as the BBC allows the RS232 to act as the keyboard, the whole machine could be controlled from the Spectrum, which should be very useful for schools.

The RS232 is also a both-way device, so PRINT statements and programs can be fed into the Spectrum as well. It is simple to use and very well-documented in the manual supplied, including a list of system variables for machine code users.

The Centronics interface has a 26-way IDC connector - the same type as the BBC - so there should be no difficulty in obtaining a printer cable. A graphics tape is also included for Seikosha, NEC or Epson printers to permit a screen COPY to be done in two sizes - but not using the COPY command.

The edge connector is also extended through to the back for any other add-ons and the port addresses used are 254 and 127.

The Morex interface costs £45.95 from Morex Peripherals ltd. An RS232 lead to a 25 D-plug costs £13.45. It will also work with a version of Tasword-2, the Spectrum word processor.

GETTING INTO PRINT

Good as it is for the price, many users are bound to be disappointed with the output from Sinclair Research's own printing device. For those toying with the idea of upgrading their Spectrum system with a professional printer, Henry Budgett takes an in-depth look at the technology involved and the plethora of interface devices now available.

Regardless of what one actually thinks about Sinclair Research's ZX Printer, it's impossible to ignore the fact that it really does work. For the price and performance that it offers there's no equal anywhere. But despite that, there are users who need something a little more professional. Software houses developing programs, small businesses using the ZX Spectrum or just a serious user who demands something better than a sheet of aluminised paper that is, at best, just legible after a few days' handling.

We're fortunate in having a number of commercially available interface units that allow the ZX Spectrum to be connected to the growing range of low-cost printers. In the main, these units convert the signals provided on the expansion connector into those which conform to the industry standard Centronics interface. There are two stages to this conversion: that of altering the electrical characteristics of the output from the ZX Spectrum and that of converting the character codes from Sinclair Research's own internal format to true ASCII.

The alterations to the electrical signals are handled in a variety of ways by the interfaces reviewed here; some use very sophisticated chips to handle the work, others tackle the task by using more of the simple ICs. Only one of the interfaces in this review offered the possibility of connecting to a serial printer but, in theory at least, the problems involved are tackled in much the same way.

All the interfaces were supplied with software drivers of one sort or another (see the individual panels for further details) and in general, these all work by intercepting the ZX Spectrum's printer routine vector. This is the location in memory that stores the address of the routine which controls data being sent to the printer by LLIST and LPRINT. By changing this address to that of the new driver all the data is re-routed through one of the OUT ports to the interface. On their way, each of the characters passes through a 'code converting' routine held somewhere in memory. While the normal alphanumeric codes closely follow the standard ASCII set, all the control codes, predefined graphics and Basic tokens must be intercepted. In the case of the tokens, these are converted back into strings of ASCII letters before being listed. Because many of the currently available printers use sequences of control codes to change character fonts or to select graphics, provision must also be made to allow these codes to be sent without being trapped.

The test equipment to which the various interfaces were connected comprised a Tandy CGP-115 four-pen plotter printer and an Epson FX-80 dot matrix printer. Unfortunately, the Epson turned out to be 'Dead on Arrival' and the author's Apple Dot Matrix Printer was substituted. As this printer is based on a TEC chassis, it proved impossible to check the graphics dump facilities on all but one of the interfaces. However, there is no evidence to suggest that the software provided wouldn't operate correctly with the appropriate hardware and indeed, the Kempston interface unit has been seen elsewhere to function perfectly with both the RX-80 and FX-80 printers.

Hardware: The vertically mounted box is functional rather than attractive, and the cut-out appears at first sight to have been made by a hungry rabbit! Even more important than mere aesthetic quality is the fact that the casing partially covers the power socket on the ZX Spectrum; also, the plug has to be forced in and is very prone to dropping out again. As far as design goes, this doesn't. However, the unit was also the only one in the survey to allow other units to be attached in cascade - although the way in which the edge connector is soldered leaves a little to be desired.

Inside, however, the circuitry consists of just four ICs, which is a very compact design considering the inclusion of the serial interface. Both parallel and serial cables are detachable; the parallel cable is keyed but because it's BBC Micro compatible it appears to go in the wrong way. The serial interface socket, a conventional DIN type, is not very firmly anchored to the PCB and heavy use could fracture tracks.

The serial interface is not a full RS232 implementation, although 'handshaking' is provided. While it will suffice to handle printers or display terminals, it's possible that problems may occur with more sophisticated equipment such as modems. The interface performed perfectly with the CGP-115 in serial mode.

Manual: No less than 15 A5 pages accompany this interface. Just about everything is provided: pin-outs of all the connectors, routine addresses within software and hints on getting problematical equipment moving. Also supplied are a couple of example screen prints and a copy of Tasword, probably the most popular ZX Spectrum word-processing package.

Software: As provided, the driving software is configured to produce screen copies on the NEC range of printers; however, a simple POKE will set it up for the Epsons. The LLIST and LPRINT commands work with any of the common devices. Morex also provides the information on enabling or disabling token printing and automatic Line Feeds.

The serial interface can be configured in much the same way with a wide range of baud rates being catered for. Once the software has been adapted the altered program can be SAVEd as a permanent copy.

The screen copy software allows both 'single-size' and 'double-size' copies of the screen to be made; the latter are really very impressive.

Price: £48.60 including one cable. £13.40 for second cable. £4.80 for alternative software.

Summary: Despite its ragged appearance, the Morex unit offers the user both serial and parallel interfaces at a very attractive price. It's also the only unit to offer a really thorough manual and, given that you need both types of connection, this interface rates very highly indeed.

OVERALL SUMMARY

Of the three standard units - Kempston, ADS and LPRINT Mk II - the latter inherently has the advantage of on-board software, giving it greater speed and more flexibility. Between the other two the ADS is slightly the neater but the Kempston is supplied with a better range of driver software and virtually every- thing works with it directly.

The two 'up-market' units both have advantages in their own ways. For serial printers there is no option but to choose the Morex unit. It offers parallel as well and comes complete with an excellent manual; if pushed, it probably represents the 'Best Buy'. It's a shame about the packaging - fit a nice box around it and you'd have a real winner.

The Hilderbay unit stands or falls on its usage. If there is a need in your life for a printer buffer - and you already have a ZX Spectrum - then this interface will satisfy both requirements. It's also rather cheaper than having to fork out £230 for a 64K printer buffer! As a standard interface it offers nothing special, apart from direct compatibility with all of Hilderbay's software.

There are other printer interfaces around - for instance, Softest offer one especially for the Tandy CGP-115. However, as it's a serial-only interface and all the others seem to drive the device quite well, I'm not sure about its value. Unfortunately, the promised review unit failed to appear by the deadline for this survey so no further comment is possible.

The interface you choose should really reflect the uses to which it is going to be put. If all you want is a simple-to-use means of getting a proper printout then the ADS, LPRINT Mk II and Kempston units offer just what you are looking for, provided that the driving software is available. More sophisticated users will find the Morex unit well worth a look and those with other computers to support will find the Hilderbay device attractive from the printer spooler viewpoint.