Micro­power

  

Volume 1, Number 1 – August 1981








Page 13 of 33











You will now be able to get hard copy of machine code tabulations, Basic listings, and source and object code from assemblers and disassemblers. If you use the printer with Nas-Dis you will come across a strange problem – the machine goes into doubleprint mode. The reason for this is that after accepting the user options Nas-Dis uses an ESCAPE (£1B) to get to the beginning of a line, and then prompt “Go?”. Unfortunately the printer interprets ESCAPE/G as an instruction to double print. The only cures are to modify Nas-Dis (for example, change to a lower case g), or to use a special printer output routine for Nas-Dis which ignores ESCAPE/G.

If you try to use the pixel set in the Epson you will find that it is not compatible with the Nascom pixels; in fact, it is the TRS 80 pixel set, using characters £80 – £BF or £A0 – £DF (switch selectable). The following section of code carries out the necessary conversion. The effect of each step in the process is listed on the right. An X indicates that the value is not a copy of a specific original bit.

OperationCurrent location of
Original bits
Carry FlagZero Flag
RRCA076543210X
AND A07654321XX
RRAX07654321X
RRA1X0765432X
RRA21X076543X
JR NC, ANDA
SET 5,A213076543X
ANDAAND A21307654XX
RRAX21307654X
BIT 6,AX213076542
JR Z, RES6
SET 7, A2213076542
RES6RES 6, A2X13076542
JR NC, RRCA
SET 6, A2413076542
RRCARRCA524130765X
RRCA652413076X
RRCA765241307X

This completes the bit. manipulation; SUB £20 now shifts the characters to £A0 – £DF, SUB £40 converts them to £80 – £BF.

The graphics ROM in the Epson also contains 64 Japanese characters, which can be accessed when pin 7 of DIP switch 1, at the back of the printer PCB, is set to ON. These characters are coincident with the pixels. If you send control code ESCAPE/5 (i.e., £1B £35 in hex., 27 53 in decimal) subsequent characters in the graphics range will produce the Japanese set; ESCAPE/4 selects the pixel set. Now, you may not want to print in Japanese, but a bit of research into the software inside the machine shows that you could easily reprogram these 64 characters to your own design.

At a first glance at the beautifully made P.C.B. there appear to be some chips missing. There are three 24 pin sockets near the back edge of the board labelled 2716, but only one of these contains a chip, which is a 2332! However, this 4K ROM can be removed and replaced by two 2716s without any modifications to the board.









Page 13 of 33