Micro­power

  

August 1981, Volume 1, Number 1








Page 14 of 33











The first 2K is part of the operating system of the printer (the rest of this is located in a ROM contained in the 8049 processor chip which controls the printer); the second 2K contains the data for the characters. The character is defined by 9 bytes in which. Just to be awkward, a 1 means leave a space and a 0 means print a dot; each bit controls one of the matrix needles. So, what about the ninth needle – after all the adverts make great play of the fact that this is a 9x9 printer. Well in fact the ninth needle is only used for letters with descenders (g,p,q,y); the bytes which define these letters are applied to the bottom eight needles. The letter J uses the eighth needle, and all the other ASCII characters use only the top seven.

The pixels are not stored in the graphics section of the ROM, but are printed, as blocks of 3 x 4 dots, by a special routine in the operating system. To print pixels the machine does two passes per line, and also prints in one direction only, so it is four times as slow as the bidirectional printing of ASCII characters. If you program your own characters into the machine, you can access them as codes £80 – £BF and design your output routine so that when it receives a character in this range it prefixes ESCAPE/5, while a pixel character is prefixed by ESCAPE/4; both characters should then be converted to the range that the printer is selected to accept.

The printer has quite a range of control codes with which you can select the print size, print density, line spacing, form length, and horizontal and vertical tabs. Using pixels in the ‘condensed print’ mode you have a resolution of 264 points across the 8 inch printing width of the Epson. However, there is a high-resolution graphics version of this printer – the MX80 F/T 2. The software for this version is contained in three 2716s, the third one replacing the 8049’s internal ROM! To do this a single wire link is cut on the P.C.B., which pulls pin 7 of the 8049, the External Access pin, to +5v. This forces the 8049 to ignore the internal ROM and read program data from an external device. The F/T 2 has normal density graphics, with a resolution of 480 dots/line, and double density, 960 dots/line. When printing in high-resolution mode each bit of the data received controls one of the top eight needles, with the most significant bit uppermost.

The F/T 2 has a couple of features which I would like to see on the F/T 1; it deletes single characters on receipt of a Backspace (you can only clear the whole buffer with the F/T 1) and it has automatic ‘ skip over perforation’ (particularly useful with Zeap, which has no paging capability). Unfortunately, you lose many of the print options which are needed for ‘correspondence quality’ text, and you also have no pixel characters. It would be nice to have a machine with the best features of both versions without having to swap ROMs.

The MX80 is a very well designed machine – a great improvement in appearance and performance on the TX 80. In fact the most serious faults I can find are the DIP selection switches, which are only accessible by removing the whole upper case, and the buzzer which indicates that the printer is out of paper – this goes on for a full 30 seconds, and it can be infuriating (this fault has been rectified on the F/T 2).

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This is an OCR’d version of the scanned page and likely contains recognition errors.








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