Acorn BBC Master 128, Руководство по обслуживанию

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Master Series Service Manual
Analogue Port:- This 15 way D-type connector provides access to an NEC
uPD7002, 4 channel, 10 bit analogue to digital converter. The sampled
input is compared to a 1.8V reference derived from three small signal
diodes in series.
A tracked link may be cut to deselect this reference. The user may
then solder in a two pin precision reference in the holes provided or
supply an external reference. Any user supplied reference should have a
maximum voltage of 2.5V.
Conversion:- An input voltage on any one of the 4 channels wil be
digitised when the A/D control register is so instructed. Conversions
are in the range 0 to 1.8V.
The voltage reference is made available at the connector. Provision is
made on the board for an additional high stability reference, if
required.
A link will have to be made for the additionl reference to be used.
Conversions take place in 5mS and the "end of conversion" pulse causes
an IRQ to be generated by the system VIA.
Auxiliary Connections:- Two "fire buttons" are provided for with the
connections IO, These are connected to the system VIA and cause
interrupts (as IRQ) to be generated.
A light pen may be connected to the signal LPSTB. This also causes the
system VIA to generate an IRQ (if enabled). It also causes the 6845
CRTC to latch the address of the currently selected video data byte.
This may not be the same as the displayed byte, and some software
correction may be necessary. Factors such as phosphor characteristics,
light pen response and the angle at which the pen is used, may all
affect the correction needed.
Serial Ports - Cassette and RS423:- Much circuitry used to provide the
RS423 port also generates cassette interface signals. For this reason,
these will be described together, with the differences where
appropriate.
U.A.R.T.:- The device responsible for providing most of the serial
port functions is the 6850 UART. This has all the receive/transmit and
data formatting/error checking that is neccessary for both systems.It
is fully described in the March 1983 edition of the Hitachi
Microcomputer Databook.
SERPROC:- The ACORN proprietary part, the SERPROC is effectively a
multiplexer and baud rate generator for the 6850. It also generates
the phase-continuous transmission circuitry for use with the cassette
interface.
Buffer Components:- The RS423 transmit data and CTS lines are buffered
by an AM26LS30, or equivalent. This provides a single ended
transmission with slew rate limited output.
RS423 receive data and RTS is buffered by a uA9637AC or equivalent.
Both buffers are connected with single ended input configurations.
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