busy indicator for the Junior Computer

71 W. Schaaij RS 232 P86 o„ VIA 1 T1,T2= BC547B RS 232 printer interface and a few minor changes to the Printer Monitor software. As can be seen from the diagram, the additional circuit is not very exten- sive. The input of the circuit is provided by the "busy" signal from the printer which is available at pin 11 of the RS 232 connector. The first transistor, T1, serves as a level interface between the RS 232 and TT L. When the input voltage is positive (logic "0") Ti will conduct and pin PB6 of the VIA will also be at logic "0". When a busy signal is received from the printer, the input wilt become "1", Ti will switch off and the PB6 line is taken to +5 V by R3. Transistor T2 will conduct and light LED D2 provides the "printer busy" indication. Resistor R1 ensures that no busy signal is given to Junior (and the LED cannot light) if the input is open circuited. The modifications to the Printer Monitor EPROM are: busy indicator for the Junior Computer A "busy" indicator can be us...