Acorn BBC Master 128, Service Manual

Page: 32 / 87

Master Series Service Manual
Cassette data output from the SERPROC is buffered by a single, non-
inverting operational amplifier with a simple, single pole filter; a.
c. coupling capacitor and current limiting output resistor.
2MHz External I/O
Two peripheral devices operate at 2MHz. These are the external second
processor connection and the ECONET connection.
ECONET Module:- Connection is made to the ECONET by a five way DIN
connector mounted on the main circuit board. The interface electronics
including the 681354. line drivers, receivers and chatter disconnect
components are mounted on a separate circuit board. This board has two
connectors:-
a) A 5 way connector which has a one-to-one connection with the DIN
connector.
b) A 15 way connector provides the CPU data bus together with address,
timing reference, chip select and interrupt signals. The main pcb has
two further address connections for future expansion.
External Second Processor:- This interface has a buffered data bus via
the Peripheral Bus Controller (PBC). The EXbus on this component
provides for good data set up and hold times. Together with a limited
degree of line matching, this ensures reliable high speed data
transfer with unspecified cable lengths. A maximum cable length of one
metre is suggested to prevent noise problems.
The interface operates at 2Mhz. This means that if a 1Mhz bus
peripheral is also connected, then the address and data buses on this
connector will appear to perform both 1 and 2MHz cycles.
The connections are:-
DO to D7 Data Bus CMOS levels
AO to A7 Address Bus TTL levels
IRQ Interrupt Request Open collector
TTL levels
nTUBE Parasite chip select TTL levels
Supply +5V
Ground OV
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