The System Manual of SOL-20 - History of Computers

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PROCESSOR TECHNOLOGY CORPORATION Sol THEORY OF OPERATION SECTION VIII Figure 8-5. 6574 Character Generator ROM pattern. "C" and "T" were stored in the RAM in the proper character positions in the third character row. After the first two character rows have been displayed, the Scan Counter (U40) is reset to a binary count of 15 (1111) and the Character and Line Address Multiplexers (U30, U32 and U12) call up the "C" in the Display RAM. The Scan Counter output specifies line 15 in the Character Generator ROM on RSI through RS4. As previously mentioned, this line in the ROM is blank. Thus, the first scan line of the third character row is blank. The 7-bit ASCII code for the "C" (1000011) is input from the Display RAM to address the Character Generator ROM by way of the Dat. Latches (U26 and U27). This address is applied to ROM inputs A7 through Al (A6 through A0 in Figures 8-5 and 8-6). The Scan Counter changes to a count of zero which specifies scan line R0 in the Chara' ter Generator ROM. As shown in Figures 8-5 and 8-6, the ROM in turn outputs a 7-bit word, 0011110, on D6 through D0 respectively (B7 through B1 on the schematic). VIII-30
Sol THEORY OF OPERATION PROCESSOR TECHNOLOGY CORPORATION SECTION VIII Figure 8-6. 6575 Character Generator ROM pattern. For the second character position the Character and Line Address Multiplexers call up the "T" in the Display RAM. The resulting ASCII code for a "T" (1010100) ultimately appears on the address inputs to the Character Generator ROM. Since the Scan Counter is still at a count of zero, the ROM outputs 1111111. This process continues for the balance of the displayable portion of the video scan line. At the end of the horizontal scan line, the Scan Counter changes to a binary count of 0001 which specifies scan line R1 in the Character Generator ROM. The "C" and "T" are again called up from the Display RAM for the first and second character position respectively. The ROM consequently outputs 0100001 and then 0001000. This sequence continues through scan line R8 when the Scan Counter is at a count of 8 (1000) to produce the "C" and "T". As discussed earlier, the Scan Counter cycles through 13 counts or scan lines. For the "C" and "T" in our example, the Scan Counter has counted ten lines (15, 0, 1, 2, 3, 4, 5, 6, 7 and 8). The remaining three scan lines are not used in forming the "C" or "T", so on counts 9, 10 and 11 of the Scan Counter the Character VIII-31
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Sol SYSTEMS MANUAL, 6200 Hollis Str
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CONTENTS OUTILNE Detailed contents
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Sol TERMINAL COMPUTERTM FIGURE 7-5
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