80-Bus News

  

January-March 1982, Volume 1, Issue 1











Page 8 of 55











N2 VIDEO

REVIEW OF SCREEN WEAVE & SCREEN FLASH ELIMINATORS FOR NASCOM 2 by R.Dowling

SCREEN WEAVE ELIMINATOR

WHAT YOU GET : A small fibre glass board approx. 1.5 xl inch comes ready built comprising of one i.c-(74LS90), one socket and one 16 pin plug to attach to the Nascom as a piggy-back board.

FITTING : Full instructions are supplied and the unit was found to be simplicity itself in fitting. One i.c. is removed from the main board and housed in the empty socket provided on the small board which is then plugged into the consequent empty socket on the main board. One other connection has to be made to the main board from a terminal pin on the piggy-back board.

UNIT IN USE : On power up it was found that screen weave had disappeared totally and the resultant monitor display was very pleasing. On disconnecting the wire link to the main board, the display was exactly how it was before fitting the unit. However it was discovered at a later date that the unit is not compatible with the well published modification to recover the "lost" bottom two lines of graphics characters which is necessary if fitting a programable graphics generator. The result was screen weave at twice the intensity originally experienced. As the graphics mod. also has the benefit of stabilising the screen as a whole, removal of the screen weave eliminator produced a stable display with only very slight weave noticible if looking closely at the screen.

CONCLUSIONS : A well documented and finished product but almost the same effect can be realised by the above mentioned graphics mod., assuming you don’t mind several wire links running along the top of your main board.

SCREEN FLASH ELIMINATOR

WHAT YOU GET : A fully built fibre glass PCB approx 2.5x2 inches consisting of two IC’s (socketed), an empty socket, a 14 pin plug and ae few discrete components.

FITTING : Instructions supplied with the unit are very easy to follow and consist of removing one IC and fitting this onto the empty socket of the PCB and plugging the PCB into the subsequent empty socket on the motherboard. Two other connections are made from two terminal pins on the PCB to plated through holes near IC7 and IC67. A third terminal pin on the small board enables the user to switch the unit on and off via a switch or port. Most

– users will not require this facility and therefore will leave the unit

permanently working. It is then only necessary to remove the Snowplough (IC58) and put it in the spares draw. The Nascom is then ready to be powered up.

UNIT IN USE : The unit produces wait states but the extra execution time, even for programs that have extensive screen access, is claimed to be less than 1%. It was found that on continuous tabulation to the screen T 0000 FFF8 FFF8 (enter) took 202 seconds on my standard Nascom running at 4MHz with a wait state and with the unit fitted took 203.5 seconds. Tabulating data was found to give a much improved look and on a space invaders program the usual black streaks across large graphic areas had completely disappeared. Running

the Nascom at 2Mhz showed that the unit did not work at this speed and,

indeed, because the Snowplough IC had been removed, produced large areas of white streaks. The manufacturers claim that replacing one of the IC’s on their board from a 74LS74 to a 74874 would cure all. This has not been tried out.

CONCLUSIONS : The screen flash eliminator does exactly what its name suggests and very neatly too! For what you get it does seem overpriced but the results are certainly worth the expense.

Both units are available from Edac Engineering, 257 Orphanage Road, Erdington, Birmingham or dealers at 8.75 & 14.75 exclusive of VAT respectively.


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