This comes about because the Manchester II system is edge sensitive and it depends
upon whether the cassette recorder inverts the data out relative to the data in (some
do). If tapes recorded on different recorders are to be replayed, it might be
advisable to fit a switch to select the appropriate Q output.
The encoder does not require any adjustment, and correct operation can be
checked either with a ’scope or by recording a block of memory filled with code 03H,
in which case the recorded data will have an even tone when listened to. The only
decoder adjustment is the preset pot P, which may be achieved either with a ’scope by
connecting the output of the encoder to the decoder input and then adjusting for
correct decoding by displaying both the input and the output; or without a ’scope by
first recording several K’s worth of 03H and coarse adjusting for correct reloading on
replay. Fine adjustment is then made by recording the system monitor several times
over, and adjusting for correct reloading.
Figure 3 shows a small modification which may be added to reduce the tendency
to pick up spurious noise. Pin 2 of the 74LS00 is connected to pin 3 of G1 of the tape
control circuit published in ‘Computing Today”, February 1980, or to pin 12 of IC41 if
the former is not installed.
The whole circuit may be connected to the Nascom 1 using the switching system
illustrated in fig. 4. The four pole four way switch gives the following functions:
|1||– 3900 baud normal polarity|
|2||– 3900 baud inverse polarity|
|3||– 488 baud (twice Nascom 1 normal)|
|4||– 244 baud (Nascom 1 normal)|