Sinclair ZX Spectrum Service Manual
Spectrum For Everyone https://spectrumforeveryone.com/
15
For example, pressing TRUE VIDEO closes the switch contact at row 1, column 6 on the upper membrane
(CAPS SHIFT) and row 3, column 1 on the lower membrane (TRUE VIDEO).
Similarly, pressing any of the following keys results in the corresponding switch contact on the lower
membrane closing as well as the SYMBOL SHIFT switch on the upper membrane:
, (comma)
. (full-stop)
; (semi-colon)
" (quotes)
For example, pressing full stop closes the switch contact at row 2, column 8 on the upper membrane
(SYMBOL SHIFT) and row 3, column 8 on the lower membrane (full stop).
1.6 TAPE INTERFACE
When LOADing or SAVEing programs using a cassette recorder, the ULA transfers information between
the MIC and EAR sockets and the data bus, performing A/D and D/A conversions as required. During the
LOAD operation the CPU executes successive I/O read cycles to I/O port address 254, reading the EAR
input off bus line D6. When performing a SAVE operation, the CPU executes successive I/O write cycles
to I/O port address 254, this time writing data to the MIC output via bus line D3.
To ensure that I/O cycles are correctly implemented, the IORQ line supplying the ULA is gated with
address line AO via TR6. Thus, if any memory transactions occur when AO is high (i.e. not port address
254) then the IORQ input is forced high inhibiting any attempt to perform the I/O cycle.
1.6.1 ULA Sound Output
It should be noted that while SAVEing, the level of the MIC output is barely sufficient to modulate the
sound carrier to IC38. However, during the execution of a BEEP instruction the CPU writes instead to
port 254 on bus line D4. This effectively boosts the MIC output, modulating the sound carrier so that the
BEEP tone can be easily heard.
To Next Page
Loading...
Need help?
General
