Acorn FileStore E01, Service Manual

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Chapter 4 E01 Circuit description
FileStore Service Manual 19
4.8 The Econet port
4.8.1 The Econet network
Econet is the Acorn local area network (LAN) for microcomputers. It uses inexpensive 5 core
screened cable, which carries the differential clock, data lines and ground. Data rates up
to
200Kbps are possible. The clock pair are terminated passively at each end with the TX/RX
clock being centrally connected. The clock data pair passive are passively terminated at each
end with a central active termination or alternatively with a passive at one end and with a
active/ passive at the other.
It is possible to connect up to 254
stations to a network, which may be computers or (in this
case) the E01. It is possible to send messages between stations and access other stations‘
memory. etc. Protection is provided against unwelcome access by a remote station.
4.8.2 Econet module
The majority of the Econet hardware is provided on a removable PCB module which
connects to the main PCB via SK10 and SK1l. SK10 provides an interface to a standard
Econet port, presented as a 5-pin 180 degree DIN socket. The Econet PCB module contains
the 68B54 high speed data link controller and its associated buffer circuitry. The data link
controller and its under the control of the main processor using NMI. The module provides
the following functions:

Data transmission

Data reception

Clock reception and detection

Collision detection

Error detection

On-board circuits provide the Econet clock and termination
4.8.3 Econet clock and active terminator
An on board clock is implemented using the master system clock at 2MHz divided down by
IC16 and ICV28.This is passed to a differential line driver IC26 and thence to the clock lines.
Q6 and Q7 implements an active terminator which generates a 0.2V differential across the
data lines. The control lines for these two circuits are from IC21 PIN 19. The insertion of the
clock and terminator is automated to the extent that if a error in transmission or reception
occurs, the circuit is enabled. It will also be enabled if on power up the processor detects the
absence of the insertion of the DCD signal on the Econet module.
The frequency of the Econet clock is adjusted by links 3 and 4. See the Link Survey for
selection detail.
4.9 The disc interface
The disc interface is controlled by IC20, IC29, IC8, IC30 and IC19. Drive selection is carried
out by IC19. Data request and IRQ lines cause an NMI.
All floppy disc I/O is handled by a 2793 Floppy Disc Controller (IC20). For exact details of
this operation refer to the manufacturer‘s data sheet
In this application it is programmed to support 250Kbps data transfer and MFM encoding.
C14 and C15 are used to adjust the on chip voltage controlled oscillator. The frequency
produced (on TP2) should be 250kHz
+1- 12.5% when the FDC is in test mode (see the
manufacturer‘s data sheet).