Multilink Network

Multilink was an early networking technology, developed by Nine Tiles Computer Systems. Among the systems it supported are Amstrad PCWs and CPCs, the BBC Micro, and Apricot and IBM-compatible PCs. There's very little information about it online, and this page is an attempt to collect what there is.

In March 2019, there were a fair few articles reporting that Nine Tiles had donated a prototype Spectrum to the Centre for Computing History at Cambridge. In fact, they donated a number of items, including several Multilink interfaces, and an Amstore network-attached storage device (labelled as a 'pre-production unit').

More recently, two Multilink interfaces for the Amstrad PCW were sold on eBay; I'd guess that these were also part of the Nine Tiles collection, and deemed surplus to requirements. I bought one, and so am in a position to examine it.

I've since also attended a lecture by John Grant of Nine Tiles, in which he described the system and its operation.

The hardware

RS232 interface

In its original incarnation, a Multilink node is a standalone box designed to connect to its host computer or dumb terminal by RS232. When used with a terminal, the node is controlled by ASCII commands from the terminal.

The PCW card

In due course, Multilink interfaces were made which attached directly to the host computer's bus, such as this one for the PCW:

From looking at this card and the Multilink cards pictured on the CCH website, it's possible to come to some conclusions on the nature of the network:

• Each card carries a Z80 CPU, 8K ROM, 2K RAM and a ULA labelled 5C124J or 5C124J4. These four components form a computer roughly equivalent to a ZX81 in power; they implement the network protocol and deal with transmitting and receiving messages. (The earliest cards used discrete logic instead of a ULA, just like the ZX80 as opposed to the ZX81).

• The cards also contain a relay and a 75116 differential transceiver. When power is switched off, the relay bypasses the card's components, connecting the 'in' signals from the network directly to the 'out' signals. Thus turning a computer off would not break the ring.

  Uniquely, the PCW card does not include these two components, though it is silkscreened for them.

• The physical connection seems to vary; some interfaces have DE9 connectors, others 6-way or 4-way terminal blocks. The PCW interface is silkscreened for both, but only the 6-way terminal block is populated (attached to a cable with a BT631W type plug). The six connections are 0V, +5V, OUT-, OUT+, IN- and IN+. Those interfaces with 4-way connections omit the +5V and 0V lines.

  In light of the presence of the BT-style plug on the PCW interface, it's interesting to note a BT-style socket on the preserved Amstore unit. Possibly the two were connected together, at a short enough distance that differential signalling was not necessary.

  If the DE9 socket was in use, it would be connected to a cable with a GPO 420 plug at the other end. The GPO 420 socket is configured so that when no plug is present, the 'input' lines connect directly to the 'output' lines, ensuring there is no break in the ring.

By way of comparison, here's a card for the PC, with relay, transceiver and DE9 socket in position. The firmware ROM is the same version as for the PCW, 5.10. The DIP switches control the I/O, interrupt and DMA settings.

I/O

To its host, a Multilink card is designed to look something like an 8251 serial port. In the case of the PCW, it is mapped at ports 0A6h (status / control) and 0A7h (data). It will thus conflict with the AMX mouse, the Electric Studio light pen, and Cirtech / ASD hard drives.

Status port
Bit	Meaning on read
7	Reserved: Appears always to be 1
6-2	Reserved: Appears always to be 0
1	Set if a byte can be read from the data port
0	Set if a byte can be written to the data port

Since I have no network to connect the card to, mine sends the byte sequence 00 90 99 00 on power-up, meaning 'break in the ring at this station'.

Firmware

I have been able to dump and briefly examine the firmware on the PCW card. The ROM is mapped into the card Z80's address space at 0000h (as expected) with the RAM at 0F800h.

I/O is memory-mapped; the firmware ROM accesses memory locations at 4000h, 4001h, and 8000h-8007h. 4000h / 4001h appear to be the channel used for communication with the host PC, and 8000h-8007h to control the network interface.

Network signalling

A packet has a 6-byte header followed by a variable-length trailer:

	DEFB	destination	;Destination node number, 0-127.
	DEFB	time-to-live	;Number of nodes this packet can pass through
	DEFB	length		;Length of remainder of packet (number of data 
				;bytes + 3).
	DEFB	checksum	;Packet checksum.
	DEFB	source		;Source node number.
	DEFB	type		;Packet type / flags
	DEFB	channel		;Channel number (optional; a bit in flags
				;indicates its presence)
	DEFB	xor		;XOR key (optional; a bit in flags indicates
				;its presence)
	DEFB	data		;Packet data (may not contain 0xFF)
	DEFB	0xFF		;Stop byte

Channel numbers are used to multiplex streams of data across the ring. Multilink is a connection-oriented system, so to connect to two or more remote systems at the same time, the host must open two channels, one to each, and switch between them as it requires.

Since 0xFF terminates the packet data, it can't be included within the packet. If a 0xFF byte would be included, the node generates a key such that, xored with each byte in the data, none of the result bytes is 0xFF. This key is then included at the start of the packet, with a bit set in 'type' to indicate its presence.

Client software

I do not have any drivers for the card, but I've seen a screenshot; the redirector is called DIVERT.COM, and allows CP/M drives to be assigned to directories on the fileserver. The redirector signs on as 'SimpleNet diverter for CP/M+, Vn 1.2'.

The Amstrad Multilink Network Interface (presumably targeting the CPC rather than the PCW) has two ROMs; I'd guess that the second one is a network redirector for AMSDOS.

Other sources of information

• Networking micros - principles and practice by John D. Rice gives an overview:

  The Nine Tiles Multilink network, manufactured by Hawker Siddeley, has network interfaces for the Epson QX10, Apple II, Multibus, Q-BUS, S100 bus, UNIBUS and BBC Micro, with a connection cost of under £100 for the BBC Micro. CP/Net and a network filing system are available. This new development is clearly an indication that network interfaces at about one tenth of the cost of the microcomputer to which they connect are likely to become commonplace in the very near future.

• On the management and performance of a class of local area networks by Mohammed Mahboob Yasin describes Multilink as a Distributed Loop Computer Network (DLCN), which uses a ring topology:

  The Computing Discipline of the Mathematics Faculty at the Open University, Milton Keynes, U.K., where the research work reported on here was carried out, has a variant of the DLCN network. This network is called the Multilink network. It is manufactured by the Nine Tiles Computer Systems of Cambridge, England and marketed by Hawker Siddeley Dynamics Ltd. of U.K.

  Multilink is a local area network primarily intended for connecting terminals into the central computing facilities within a building. It has totally distributed network control, and hence it does not require any special control node. Therefore, it does not require huge initial investment, except for laying cables. This makes it particularly suitable for small applications where only a few devices are to be networked. The per connection cost is also small, a stand alone network access unit, called a station in Multilink terminology, costs under £400. Cheaper still, and more convenient, is so-called chip-set version which fits inside a micro-computer and costs around £100.

  Multilink uses the dynamic register-insertion mechanism of the DLCN network. It uses a removal by destination policy, although it avoids the address checking delay by performing an address check on the fly. When a node detects its own address as the destination address of a message being relayed, it terminates the on-going transmission by transmitting null characters for the duration of the period needed to receive the message. This results in a truncated message on the output of the destination node. This message is smaller than the smallest message allowed on the ring. All such messages are discarded by any downstream node which needs to buffer its input. In the Multlink ring network, therefore, these messages do not consume any useful network resources. The result is that the substantial per node delay incurred in case of a DLCN network is eliminated by using a special message removal technique.

• Expanding and Networking Microcomputers by Dennis Longley and Michael Shain has a brief mention:

  Multilink Interface Board

  Capable of up to 10,000 characters per second, file transfers of 1Mb in two minutes are possible. An on-board processor handles all protocols on the network independently of the Apple and will allow remote computers to be accessed using the Apple as a terminal.

  No special software is required and the computer may be used as a mailbox.

  Manufacturer: Nine Tiles Computer Systems Ltd.

  Price: £150

• A letter in the Computer Weekly Supplement of 1983:

  Comparing raw data rates is absurd

  I OBSERVED with interest Dr. K. H. Benner's letter (Computer Weekly, October 20) in which he states that the inter-machine transfer rate over Keele's Cambridge ring is only about 1.5 Kbytes per second.

  It emphasises the absurdity of the common practice of comparing local area networks on their raw data rate. Our own local area network, Multilink, has a raw speed only one fortieth of that of the Cambridge Ring, but transfer rates of up to six times that reported at Keele are easily achievable.

  The high speed of file transfer using Multilink is achieved because the protocols (including packet assembly, acknowledgement, etc) are handled in the node. All the sending and receiving devices have to do (once the connection has been made) is to move the stream of bytes across the interface to or from the node. If appropriate, this can be done by DMA so that the host CPU has no work to do at all.

  This also gives the lie to the idea prevalent in the LAN industry that connectionless protocols, in which the sender has to divide the data stream into blocks, each of which includes full addressing information, are more efficient than those involving virtual circuits.

  Multilink is firmly in the class of "micronets" and yet apparently outperforms some of the more "heavyweight" networks; surely performance criteria rather than raw speed should form the basis of informed comparisons of networks?

  DR C. R. GRANT
  Nine Tiles Computer Systems
  Cambridge.

• The Complete Guide to the Amstrad PCW 8256 and 8512 describes the Amstore drive, giving a price of £1400. It also mentions a competitor, the HM Systems Amstrel, "a version of their Minstrel". The Minstrel, at least in its initial versions, was a 16-bit TurboDOS system combining the features of MP/M and CP/Net. Its client PCWs would most likely have been attached through serial links rather than dedicated hardware.

  Northern Computers' Amstore provides just such a system. If you're feeling really ambitious you can couple 120 Amstrads, or other computers, to the one network. This allows you to add more or less sophisticated micros, according to usage.

  At Amstore's centre is a 20Mb Winchester, which connects up to a ring circuit, with sockets for each micro, printer, extra drive or whatever. These are known as nodes, and Amstore allows you to change and expand the system at will.

  Amstore is also intelligent. Communicating such large amounts of information around a network calls for some clever programming to ensure that everything reaches its correct destination. The demands made on the user by Amstore are very few. A little care may have to be taken with security though, to ensure that files are only accessed by authorised users, and that certain uses are prioritised.

  Expansion holds vast potential, from a shared, quality printer to communications. In this last respect, Northern Computers is producing a telex node. And if the thought of trusting all that information to one disc doesn't seem wise, they are currently developing a streamer, to let you back up files on tape.

  The system is cheap and easy to install, with its neat junction boxes. Obviously to realise its potential to the full, and to justify its cost, it's best suited to larger businesses, for whom it could be a godsend.

• Jackson, Christopher (1994) Performance of buffer insertion LANs. Masters thesis, Durham University describes later versions of the ring network (Superlink and Swiftlink), stating that the network uses packets of variable length, with a six-byte header and up to 80 bytes body.
• CPCWiki has a brief page on the Amstore, including an advertisement depicting the drive and an article briefly describing it (and the Amstrel).
• The ACM Digital Library includes a reference to a 1985 article describing Multilink, but the article itself does not appear to be downloadable.

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John Elliott 3 November 2019.