186
edits
7000 series readout system: Difference between revisions
→Character generation
| Line 55: | Line 55: | ||
====Character generation==== | ====Character generation==== | ||
[[File:7000-readout-chargen-positions.jpg|thumb|300px|right|Character generator matrix positions in 7000 series readout analog character generator ROM]] | [[File:7000-readout-chargen-positions.jpg|thumb|300px|right|Character generator matrix positions in 7000 series readout analog character generator ROM]] | ||
The character generators are custom analog chips that provide X and Y output currents for ten characters of up to seven strokes (eight X/Y coordinate pairs) inscribing each character. The in-character coordinate points are selected through a triangular analog signal, not a digital code, which in turn activates one of eight groups of three transistors each generating the X/Y output currents. Each of these groups is fed a constant emitter current (I<sub>E</sub>), and the distribution of this current to the three transistors is controlled by their different emitter areas. The first and second collectors provide X and Y currents respectively, the third ("Z") directs current to the substrate (I<sub>E</sub> - I<sub>X</sub> - I<sub>Y</sub> - I<sub>Z</sub> = 0). Actually, the emitter area itself is not varied in the mask, but out of a fixed number of emitters of equal size, a certain number is connected on each transistor: [[File:7000-readout-chargen-circuit-1.jpg|300px|thumb|ROM circuit for one character (digit "3"). Bases are scanned in turn. The numbers under the emitters indicate how many of the emitters are connected to the common rail. Ten such character circuits plus the scanning (base drive) circuit are integrated in each ROM chip. All X and Y collector outputs are connected across characters and chips.]] | The character generators are custom analog chips that provide X and Y output currents for ten characters of up to seven strokes (eight X/Y coordinate pairs) inscribing each character. The in-character coordinate points are selected through a triangular analog character scan signal, not a digital code, which in turn activates one of eight groups of three transistors each generating the X/Y output currents. Each of these groups is fed a constant emitter current (I<sub>E</sub>), and the distribution of this current to the three transistors is controlled by their different emitter areas. The first and second collectors provide X and Y currents respectively, the third ("Z") directs current to the substrate (I<sub>E</sub> - I<sub>X</sub> - I<sub>Y</sub> - I<sub>Z</sub> = 0). Actually, the emitter area itself is not varied in the mask, but out of a fixed number of emitters of equal size, a certain number is connected on each transistor: [[File:7000-readout-chargen-circuit-1.jpg|300px|thumb|ROM circuit for one character (digit "3"). Bases are scanned in turn. The numbers under the emitters indicate how many of the emitters are connected to the common rail. Ten such character circuits plus the scanning (base drive) circuit are integrated in each ROM chip. All X and Y collector outputs are connected across characters and chips.]] | ||
If the bases were controlled by a digital decoder, only one triplet of transistors would be active at any time and only points would be generated on the display. Instead, through an analog driving circuit, gradual cut-over is achieved, i.e. current from one group is reduced while that from the next group is increased, and a linear transition from the first to the second X/Y coordinate pair is displayed as a stroke on the CRT. The actual shape of the base drive voltages has great influence on the proper character rendering (this is explained at length in Gilbert's 1971 SSC paper). | If the bases were controlled by a digital decoder, only one triplet of transistors would be active at any time and only points would be generated on the display. Instead, through an analog driving circuit, gradual cut-over is achieved, i.e. current from one group is reduced while that from the next group is increased, and a linear transition from the first to the second X/Y coordinate pair is displayed as a stroke on the CRT. The actual shape of the base drive voltages has great influence on the proper character rendering (this is explained at length in Gilbert's 1971 SSC paper). | ||