AC0RN TECHNICAL NANUAL



Econet Circuit Boards. 200,024 and 201,002
Introduction	page	2
Econet Eurocard	200,024
  Parts list	page	3
  Component layout	page	4
  Circuit diagram	page	5
  Installation	page	6
  Software	page	6
Econet ATOM card	201,002
  Parts list	page	7
  Component layout	page	8
  Circuit diagram	page	9
   Installation	page	10
   Software	page	10
Circuit description	page	11
Connectidns	page	14
Termination	page	14



C  Copyright Acorn Computers Ltd,  1981



Issue 1, March 1981




1
Introduction

	The Econet system allows a number of computing machines to communicate with each other and to share resources. It is a local network intended for use in situations ranging from one room to a small group of buildings. Typically the Econet will be used in applications where each user station is a low cost machine, for instance an Acorn ATOM, and all of the stations are connected together by the network. Also on the network one would expect some stations to be machines with extra facilities, say disk file storage and a hard copy printer, these would normally be Acorn system 3 or 4 computers. Using a communication media such as the Econet these facilities may be made available to the user stations, indeed it is possible for it to appear to the users that their machines each have their own disk units etc. Taking into consideration the high cost of such peripherals in relation to the ATOM personal computer and the small time for which they are required by each user, the Econet provides savings in cost and increases in operational efficiency. Of course the Econet may also be used to transfer data or programs between stations and any one station can send information to all of the other stations. Connection to the network by a user is always optional and so users retain complete control over their own machines. Compare this with a time sharing system where all the remote terminals cease to function when the central machine goes down. Thus a large number of machines, varying in complexity and cost, may be connected together to great advantage in the classroom, laboratory or office using the Econet
	The Econet is a bus network using cable transmission over two pairs of wires which go from one end of the network to the other connecting to all stations on the way. Information travels in a serial bit form over one pair of wires and the other pair carry the network clock. The Econet gives each station a fast access to any other station and it continues to operate if other stations are not working. The bus approach choosen for the Econet allows stations to communicate without involving a third party and the interface hardware is simple, it operates at high speed and the total length of cable required is minimised .
	This manual describes the hardware required to impliment the Econet, a Eurocard for the Acorn System 3 or 4 computers and a specially engineered circuit card for the Acorn ATOM personal computer. Also included is information on installing and terminating a network. Users should refer to the Econet Users Manual for a fuller description of local networking and software applications information.



















2
Econet Eurocard parts list. 200,024

Components required on all Econet Eurocards are;-
	PCB		Acorn Computers Ltd part no. 200,024
	IC	1	MC6854P	Advanced Data Link Controller
	IC	2, 3	2 off LM3l9N	High speed dual comparitor
	IC	4	SN75159N	Dual differential line driver
	IC	5	74LS123N	Dual retriggerable monostable
	IC	8	DM81LS95 or 97	Tri-state octal buffer
	IC	9	74LS132N	Quad 2 input schmitt NAND gate
	IC	10	74LS14N	Hex schmitt inverter
	IC	11, 12	2 off 74LS138N	3 to 8 line decoder
	R 9, 10, 11 and 12	4 off lOOK  resistors
	R 13, 15, 16 and	17	4 off lOOK
	R 14, 18 and 21		3 off 2M2
	R 19, 20, 22 and	23	4 off 10K
	R 24, 25 and 26		3 off 1K
	R 27		39K
	R 28 and 29		2 off 10K
	R 35 to 42		8 off 47K
	C 5		lOnF disc capacitor
	C6		lnF disc
	C7		10 to 47uF	electrolytic
	C 11, 12 and 13		3 off 47nF	discs
	C 14		10 to 47uF	electrolytic

1 off 28 way DIL IC socket
1 off 20 way DIL IC socket
3 off 16 way DIL IC sockets
5 off 14 way DIL IC sockets
	1 off 64 way Eurocard plug DIN 41612	(for acorn bus)

For stations generating the network clock add:-
	IC 6	74LS74N	Dual D type flip-flop
	IC 7	CD4Ol7B	Decoded decade counter
	R 30		4K7
	R 31		1K2 or 1K5,  S.O.T.
	R 32	and 33	2 off 1K
	C 9	and 10	2 off l5OpF discs

1 off 16 pin DIL IC socket
1 off 14 pin DIL IC socket

For terminating stations at the end of a network refer to the terminating instructions which specify some off;R 1 to 8
C 1 to 4
D 1 to 6







3
Installing the Econet Eurocard

	The Econet Eurocard is designed to fit in a standard Acorn system 2, 3 or 4 card frame. The interrupt line from the Eurocard must be connected to the NIRQ line on the 6502 CPU card, this wire should be linked across on side B of the connectors on the Acorn backplane to card slots that require it. This interrupt connection is normally on pin 28 B of the connectors. The card is enabled by address decoding on it into locations 1940 to 197F (hex) in the system memory map.

Eurocard software

	The software to required to drive network communications consists of a 4K machine code program, this normally resides in a system at address AOOO (hex). For an Acorn system 2, 3 or 4 the software may be supplied in EPROM or on some other media. If supplied in EPROM the IC should be fitted in a memory card of issue 3 or later and enabled at AOOO (hex). If supplied on other media the software must be loaded into a 4K RAM space at AOOO (hex), this will often necessitate the fitting of an extra RAM card. Other memory requirements (for block zero byte allocations etc) are detailed in the network software users manual .
6	.
Econet ATOM card parts list. 202,002

Components required on all ATOM network stations are;-
	PCB		Acorn Computers Ltd part no. 202,002
	IC	1	MC6854P	Advanced Data Link Controller
	IC	2, 3	2 off LM3l9N	High speed dual comparitor
	IC	4	SN75159N	Dual differential line driver
	IC	5	74LS123N	Dual retriggerable monostable
	IC	8	DM81L595 or 97	Tri-state octal buffer
	IC	9	74LS132N	Quad 2 input schmitt NAND gate
	IC	10	74LS14N	Hex schmitt inverter
	R 9, 10, 11 and 12	4 off lOOK  resistors
	R 13, 15, 16 and		17	4 off lOOK
	R 14, 18 and 21			3 off 2M2
	R 19, 20, 22 and		23	4 off 10K
	R 24, 25 and 26			3 off 1K
	R 27			39K
	R 28	and 29		2 off 10K
	R 34			10K
	R 35	to 42		8 off 47K
	Cs			lOnF disc capacitor
	C6			lnF disc
	C7			10 to 47uF	electrolytic
	C 11, 12 and 13			3 off 47nF	discs
	C 14			10 to 47uF	electrolytic
	Q1			BC239 npn transistor

1 off 28 way DIL IC socket
1 off 20 way DIL IC socket
1 off 16 way DIL IC socket
5 off 14 way DIL IC sockets
2 off 10 way Molex bottom entry sockets 4455-B

For stations generating the network clock add:-
	IC 6	74LS74N	Dual D type flip-flop
	IC 7	CD4Ol7B	Decoded decade counter
	R 30		4K7
	R 31		1K2 or 1K5,  S.O.T.
	R 32 and 33		2 off 1K
	C 9 and 10		2 off l5OpF discs

1 off 16 pin DIL IC socket
1 off 14 pin DIL IC socket

C 8 is not required on the card

For terminating stations at the end of a network refer to the terminating instructions which specify some off;R 1 to 8
C 1 to 4
D 1 to 6





7
Installing the ATOM Econet card

	The ATOM Econet card is fitted within the ATOM case on the upper side of the ATOM Printed Circuit Board adjacent to the keyboard. The case shuold be opened and the ATOM PCB removed. The Econet card is connected to the ATOM PCB by two rows of pins in the position marked Skt 8. These pins mate with the 20 way bottortr entry socket on the Econet card. The card is connected to the network by a five core cable terminated in a 5 pin, 180 degree DIN plug. This plug may be on the end of a lengh of cable taken out via the rear of the case (a small recess may be filed in the case at the exit point) or it can be fitted into the case bottom. The card is enabled into addresses B400 to B7FF (hex) in the ATOM memory map by a signal on Skt 8. For the ATOM Econet card the interrupt line from the board should be linked on the ATOM PCB by making link 3.
Econet ATOM software

	The software required to drive network communications consists of a 4K machine code program which resides in the ATOM at address AOOO (hex). The software is supplied in a Read Only Memory which plugs in to the utility ROM socket on the ATOM PCB (socket marked IC24).
10	.
Econet hardware circuit description

	Apart from differences in physical size the circuits for all Econet interfaces are largely the same and a generalised circuit description is given here with component references taken from the Eurocard circuit diagram. When fully populated, Econet interface cards consume around 250 mA from a single +5 volt power supply. This supply is normally provided on the card connector in the computer being interfaced to the network. All Econet interfaces are based upon the MC6854 Advanced Data Link Controller IC, other support functions for it are network line drivers and receivers, network clock generators, network collision detectors and station identifiers.

Address decoding

	The Econet card for the ATOM is decoded on the ATOM PCB at memory address B400 (hex). The Econet Eurocard has decoding circuits on it which select memory address 1940  (hex). There are then five significant addresses above these bases which contain the following registers: -
				ATOM card	Eurocard
	6854	register	1	B400	1940
	6854	register	2	B401	1941
	6854	register	3	B402	1942
	6854	Tx/Rx	Data reg.	B403	1943
	Station	identification		B404	1944

Network clock generation

	One station per network must generate the network clock which synchronizes all data transfers over the network. The clock generation circuit consists of IC 6, IC 7 and associated passive components. One gate of IC 9 generates a 3 MHz signal locked to the system clock. R 31 should be selected to give a symmetrical 3 MHz signal. The clock transmitter is enabled by linking pin 9 of IC 4 to +5 volts on the card, and it is disabled on all other cards by linking this pin to 0 volts. Refer to the card component layout diagram for the position of these links which must be soldered in during installation.
	The clock frequency must also optimised for the network, this is done by means of links to IC's 6 and 7. These links first select a starting frequency, for instance 500 kHz, and then another link selects either a divide by 2 or a divide by 4, giving 250 or 125 kHz clock speeds. The clock frequency in kHz is also the rate of data transfer over the network in kilo-bits per second or kBaud. The clock frequency of the network is related to the length of the network in that longer networks must have slower clocks due to the finite time signals take to travel over the network and the requirement that all stations on a network are synchronized to the same clock edge. As a guide here are a few network lengths and recommended maximum clock frequencies: -
	Lengh	Frequency
300m	250 kHz
830m	214 kHz
	1.7	km	107 kHz

	In general a lower clock frequency leads to better system noise immunity and improved reliability,  hence in electrically noisy environments a low clock speed should be choosen. There is a limit on

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the speed tolerance of the software, version 1 will not operate above 210 kHz or below 100 kHz. With regard to reliability the section on network termination should be consulted. The following table gives the frequencies provided by IC 7 and the network clock speeds available from them by selecting divide by 2 or divide by 4:-
		/2	/4
	600k	300k	150k
	500k	250k	125k
	428k	214k	107k
	375k	187k	93k
	333k	166k	83k
	300k	150k	75k

Station identification

	The identity number of each station is set up in hardware by links to IC 8. IC 8 is an octal buffer which when enabled feeds the cards station ID to the computer bus. Each link codes a bit in an eight bit binary number allowing any station ID in the range 0 to 255 to be set up. if a link is left open then the bit is a one, when a link is made the bit is a zero. Hence all links open corresponds to station ID 255, and all links made to station ID 0. Each station must have a unique identity and some indentities are associated with specific functions on the network. Station ID zero is reserved for broadcast signals and should not be used. Station ID 255 is reserved at present for the file server, and 235 for the printer server. Wire links must be soldered to each network station card during installation, a sugested scheme for number allocation is to number normal user stations from one upwards and to number special stations and servers from 255 downwards.

MC6854 ADLC

	The Advanced Data Link Controller IC deals with the construction of bit frames to be sent over the net. Frame describes a series of bits which are ordered in a particular way, the bits are grouped into 8 bit bytes and the ADLCs perform parallel to serial and serial to parallel conversions. The Econet software generates packets of information to be transmitted and it communicatens with the ADLC via the computer bus. These packets contain the source and destination station IDs and the data to be transmitted, the ADLC then puts this information packet into a frame of bits which also has start and finish flags and a nCyclic Redundancy Check.

Reset interrupt

	Some systems will require the Econet software to be executed at switch on so that load and save vectors netc are redirected automatically, the ATOM card has this facility. The system reset signal is taken to the ADLC IC and on reset its Not Data Terminal Ready signal goes high. This switches Q1 on, which pulls the NIRQ line low causing an interrupt. On interrupt the ATOM executes its Econet initialisation software from location AOOO (hex). As part of the initialisation the NDTR signal  is switched low which removes this interrupt signal.

Transmitters

	The transmission of data and clock onto the network is via a 75159 dual differential line driver,  these devices are to the RS 422
12	.
specification and they can source or sink 4OmA. Differential line techniques ensure both minimal radiation and high noise immunity for the network. The clock transmitter is enabled by a link as described in the clock details. The data driver is enabled by a Not Request To Send signal from the ADLC when it wishes to transmit. When disabled or powered down the buffers have high impedence outputs and so stations may be left connected to the network even when they are not in use.

Receivers

	The reception of data and clock signals from the network is accomplished using an LM 319 dual fast differential comparator. The receiver circuits are designed to give common mode signal rejection and hysteresis thus providing noise immunity. The network clock is received to clock the ADLC for both reception and transmission of data .
	When no station is driving the network the ADLC needs to be able to reliably detect that the network is undriven and therefore free for use, to this end undriven data lines must appear as a continuous stream of logic ls and not random noise which could be taken for some other stations data. The network terminators bias the lines so that receivers receive a logic 1 in the absence of a logic 0 being transmitted onto the network. ADLCs use a technique called Zero Bit Insertion and when they are transmitting a logic 0 is seen every few bits, this effect is both created and removed by the ADLCs and it need not concern the user. As a consequence of the biased receivers and the ZBI technique a station coming onto the network and wishing to transmit monitors the data line and knows that the network is free if it receives 15 consecutive logic 1s.

Collision detection

	One characteristic of broadcast networks like the Econet is that collisions occur when two stations both choose to transmit on to the network at the same time. This is largely avoided by stations testing that no other station is driving the network before commencing transmission, however a period of time exists between a station detecting an undriven network and then enabling its driven r in order to start transmission. During this period another station may begin to transmit and a collision will occur, the 5N75159 driver circuit is not damaged in this event but the data on the network is corrupt. Every station card on the network carries a collision detector circuit which informs its ADLC that a ncollision has ocurred, the station can then abort its attempt to transmit and wait for a period of time before trying again.
	The collision detection circuit  is based on an LM319 dual comparator which is used to compare each of the data signal lines with the common mode signal on the line. When stations collide their differential drivers will short out and the data lines will be forced to the same voltage. If one or the other of the two data wires is above the common mode signal by more than 1 volt the data is assumed valid, but if they both drop below the common mode voltage together a collision is deemed to have occured. The comparators have open collector outputs, the two used in the collision detect circuit have their outputs arranged in a wired or configuration giving the Data Valid signal when there are no collisions.
	This method of collision detection will also flag an invalid data line condition on Data Valid when no station is driving the lines, as the voltage on both of the data wires will drop. However this does not matter as the ADLC only looks for collisions when it is driving the

13
lines.

Clock valid monostable

	The ADLC should only attempt to transmit when the data lines are free and there is a valid clock to clock the data onto the line. The valid clock condition is sensed using a retriggerable monostable which only produces a Data Carrier Detect signal when the card is connected to a clocked network. Data Valid and Data Carrier Detect are anded together to give a Not Clear To Send signal for the ADLC. The ADLC can differentiate between the unconnected network and not data valid conditions as a Not Data Carrier Detect signal is also fed to it from the monostable.

Econet connections

	The network is connected by four core twisted pair cable which may be up to 1 kilometre long. The use of a differential signal on twisted pairs reduces the amount of radiation from the cable and increases the noise immunity of the system. For short networks inexpensive unsheilded cable such as 4-core signal cable available from RS components is suitable, for longer networks a sheilded cable such as BICC type C57219 is recomended. Connection to the cable is via standard 5 pin,  180 deg DIN connectors with the following pin assignments: -
	Pin1	Data	+ve
	Pin4	Data	-ve
	Pin2	Common	 Ov
	Pin5	Clock	+ve
	Pin3	Clock	-ve

The wires from each station are wired in parallel to the network using connectors where convenient. Eurocard stations will generally have a DIN socket mounted on a front panel whilst the ATOM station will have either a DIN socket mounted on the rear of the case bottom or a flying lead terminated in a DIN plug. The cable may go between stations in any order and by any route as long as there is only one path between any two stations. Any discontinuity in the wire such as branches of the wire or the end of the cable may cause reflection and degradation of the signal. For this reason larger or faster networks should be wired electrically in one long line, without spurs or branches, and the ends should be correctly terminated. Shorter networks may have spurs on them, for instance sockets may be networked around a room with a flying lead (spur) from each computer to its socket.

Network termination

	To prevent reflection of the signals at the ends of the network the lines must be correctly terminated. In general the termination required will vary with network geometry, speed and length. On slower or shorter networks the termination is not so critical . When installing long or fast networks it is recommended that the signals on the line are viewed with a differential input oscilloscope while the terminations are being adjusted. The termination is correct when the waveforms on the lines are sharp square waves with no ringing or rounding. The waveforms are most easily observed initially on the clock line and this should be adjusted first. Twisted pair cable as recommended for the network normally has a characteristic impedance in the order of 100 ohms. The line terminators should therefore present

14
100 ohms across the line. There is a further requirement that the data
line when undriven should rest in a state which will be interpreted as
a logic I by the receivers on the network and so the termination
circuit also applies some bias on to the data wire pair.
	The clock line will usually be terminated correctly by linking across positions for R3 and R4 and inserting a 100 ohm resistor in the space marked D 2. The data line will normally be terminated by inserting these components in the marked positions:-
	R 1,	R 2	56 ohm  resistors
	D 1		1N4148  diode
	Cl ,	C 2	10 uf capacitors
	R 5		100 ohm resistor
	D 3		100 ohm resistor
	D 4		wire link














































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