Hal Communications DKB-2010 Instruction Manual Download

Page: 29 / 101

The case-change code is coupled through a NAND gate (part of IC-25) and an inverter to IC-33, where it

is used in place of the shift register output to control the keying circuit.

When the keyboard is switched to the Morse mode, the

M/R

bus goes low. Since this signal is coupled to

pin 10 of IC-33 and Pin 10 of IC-55, the outputs of both these gates remain high. Clock pulses are thus

allowed to flow to the keying circuit, and the RTTY loop is held in the mark state. One section of the mode
switch, S301B, also shorts across the loop keying contacts in the Morse mode as a means of completing the

loop circuit when the keyboard power is switched off.

4.10 RTTY Timing Chain

Accurate timing pulses for RTTY operation are derived from four crystal-controlled oscillators, one for

each RTTY speed, as shown in Figure 8.8. The outputs of the oscillators are fed to four separate NAND
gates. Only one of the gates is enabled at any time, as determined by the setting of the mode switch,
S301A.

The gate outputs are coupled through a NOR gate, composed of four open collector inverters (part of IC-

30), to the input of a sixteen-bit binary counter. Consequently, the clock signal appropriate to the RTTY

operating speed chosen is supplied to the counter.

The first four stages of the counter, which divide the input frequency by a factor of sixteen, drive the

VHØ clock line. This signal, in the range from about 375 to 610 kHz (depending on the operating speed), is

used in the RTTY loop interface (Figure 8.7) to drive the isolation circuit during mark pulses. It is also
supplied to the quick brown fox generator to clear the circuit rapidly in the Morse mode.

In the next four counter stages, the frequency is again divided by sixteen to obtain the HØ clock signal,

which toggles at 512 times the baud rate. The HØ clock line synchronizes operation of the keyboard encoder
(Figure 8.3) with the buffer control and the shift register control circuits (Figures 8.4 and 8.6, respectively).

The HØ clock signal is again divided by the remaining eight counter stages to produce the Ø clock signal

at twice the baud rate. The reset terminals of the last four stages are controlled by the

RTTY START

line,

which clears the flip-flops between characters and allows clock pulses to flow only after a character has
been transferred from the storage buffer to the shift register. The reset feature also ensures that the first
shift of the register will occur exactly one select-pulse duration after the beginning of the start pulse.

Table 4.1 lists the frequencies of the four oscillators, as well as the corresponding VHØ, HØ and Ø clock

frequencies for each RTTY speed.

Also shown in Figure 8.8 is the circuit which drives the M/R and the

M/R

lines. In the RTTY mode, the

input to an inverter (pin 9 of IC-30) is pulled high by resistor R21, which is connected to the +5 Volt supply

bus. The

M/R

line is therefore high and the M/R bus is driven low. Conversely, when the switch is moved_to

the Morse position, the inverter input is grounded, pulling the

M/R

bus low and allowing the M/R line to go

high.

Table 4.1: RTTY Timing Chain Output Frequencies

RTTY

Operating

Speed (WpM)

Oscillator

Frequency

(kHz)

VHØ

Clock Rate

(kHz)

HØ

Clock Rate

(kHz)

Clock Rate

(kHz)

100

9730.660

608.166

38.010

148.478

7459.988

466.249

29.141

113.830

6553.600

409.600

25.600

100.000

5957.818

372.364

23.273

90.909

Supplied to other portions of the keyboard circuitry, these signals activate the RTTY control and encoder

circuits in the RTTY mode, and the Morse character generator and output control circuits in the Morse mode.
They also control the input to the shift register, determine the character code produced in the ROM code
converter, and inhibit the operation of the quick brown fox generator in the Morse mode.