Decoder Circuitry

Figure 2 shows a more detailed drawing of the decoder circuit. When the TRS-80 is either first turned on or the reset button is pressed, a low pulse is sent out on the system reset (SYSRES) lead. The interface unit buffers this signal with IC10F and then sends this low pulse on out to the CLEAR (CLR) leads of figure 3, figure 4, and figure 6.

Of the 40 leads from the '80 only eight data lines (D0-D7), eight address lines (A0-A7), system reset (SYSRES), IN, OUT, and the Ground (GND) leads are utilized. The eight data lines are sent to NAND gates IC1 and IC5. Inverters (IC3 and IC4) are used where necessary, so as to cause IC1 to output a low signal when the decimal number 1 is on the data lines. IC5 outputs a low signal only when the data leads contain a decimal number 2. The outputs of IC1 and IC5 are tied to one leg of IC9A and IC9B respectively.

Dropping down to the address lines for a moment, IC2 is connected so as to output a low signal when address 252 in binary form is received. This low from IC2 goes to the OR gate (pin 13), IC9D. The low signal on this pin 13 will not be passed on to the output until a low pulse is also received on the OUT lead from the TRS-80. When this happens, a lowsignal is output from pin 11 of IC9Dtoboth OR gates IC9A and 9B (pins 2 and 4).

Now back to the earlier discussion on the data lines. If a number 1 was received, IC1 will output a low on pin 8, which is connected to IC9A (pin 1). So, with a low on both inputs the OR gate IC9A sends out a low pulse to the CLOCK lead of the relay circuit (figure 4). This in turn will

operate relay B, with the first pulse and release it with the next. The command that does this in Ba-" j is: OUT 252,1 If the data lines hold a binary number 2, then IC5 will be outputting the low signal and sending it to pin 5 of IC9B. In this case OR gate IC9B will have the low on both of it's inputs, causing it's output to send a low pulse to the CLOCK lead of the AC Circuit (figure 3). This first pulse will provide AC to the front panel AC outlet, and the next pulse on this CLOCK lead will terminate it. The Basic command that does this is: OUT 252,2

When the address code 253 is received on the address lines, IC6 responds by outputting a low on it's pin 8, which in turn goes to pin 9 of IC9C and also pin 1 of IC10E. If a low is now received on the IN lead from the '80, IC9C outputs a low to the eight input circuitry (READ lead) of figure 5. Or, if a low is received on the OUT lead from the

'80, a low pulse is sent to the eight output circuit (WRITE lead) of figure 6. The command to input from the eight input circuitry in Basic is: INP 253. The command to output to the eight output circuits in Basic is: OUT 253,A where A is any number containing the binary configuration desired to be sent to the eight output terminals.

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