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The Memotech MTX Series

The Memotech Node Ring Network



The Memotech Node system, also called the Oxford Ring, is a simple type of network called a Token Ring. This page only provides a very high level overview of token ring networks, for more detail, the Wikipedia entry is a good starting point. The purpose of this page is to provide details on Memotech's implementation using the Node system. Specific details of the Memotech Node system can be found in the documents on the manuals page. The Node software is well documented in the manual, but the hardware, specifically, the interconnection between the nodes is not described in any detail, hopefully, the notes on this page will provide the missing details

Note: I want to set up a working Node system using my Memotech equipment, but so far, I have been able to get a working Node ROM attached to my MTXs so have been unable to verify the notes on this page.

In Memotech computers equipped with the Node ROM, a ring network was formed by daisy chaining connections between RS232 port "0" on all computers making up the ring. When running the Node software, the port is operating at 19.2 kbaud, according to the Node manual, this gives a maximum data transfer rate of  about 30000 bytes per minute! (Node Ring networks were later defined in IEEE 802.5)


Node Ring Overview

As the name suggests, in a Token Ring network, the members (nodes) are connected in a physical ring and a token passes between the members, controlling the network traffic.

Only when a node receives the token is it able to transmit data onto the network

This diagram, describing IEEE 802.5 is also helpful in visualising the Memotech Node ring.

All nodes in listen mode receive the data, but only the target node acts on the information received.

The node with the token is allowed to enter transmit mode and put data onto the network.


A detailed discussion of the RS232 protocol is beyond the scope of this webpage, the Wikipedia RS232 entry provides a basic introduction should you require further details.

RS232 is designed for point-to-point communications between an item of data terminal equipment (DTE), e.g., a computer and an item of data communication equipment (DCE), e.g., a modem.

In an MTX computer fitted with the RS232 board, RS232-0 is wired as DCE and RS232-1 is wired as DTE.

[RS232-0 only has a reduced set of the control lines that are present on RS232-1, but they are not required for Node.]

Connector Channel Type



RS232-0 A DCE Transmit 3
Receive 2
RS232-1 B DTE Transmit 2
Receive 3
Both - - Ground 7


When connecting two computers together using the RS232 protocol, the normal method is to use a null modem cable.

As the diagram shows, pins 2 and 3 are the Transmit and Receive data lines, these are crossed over and along with the ground connection, make up the most basic of interface cables (the remaining pins are used for handshaking / flow control and are not always necessary, so are usually loop-backed).

The Node ring uses RS232-0 for each computer attached to the ring, the connections are daisy chained as shown, i.e., the transmit data from one node is connected to receive to the next node in the ring.

Using typical serial cable, the length of the cable for RS232 is limited by the cable capacitance and usually taken to be about 15 metres at 9600 baud.

RS232 links can be extended by using higher specification cables such as RG-6 coaxial cable. Memotech used coax cable for the Node ring but I have been unable to establish how the cables were connected to the RS232 ports. The sketch shows how I assume the cables were connected, i.e., using a simple adapter module to break out the coax into a pair of wires, plus ground, for the RS232 connector.

Fault Tolerance

Using a simple passive connector, such as the one illustrated above, means that the ring is not fault tolerant in any way. If one node is powered off or disconnected from the ring, then the network will fail.

On the other hand, a very simple modification to add a relay would provide some level of protection from node failure. I can find no reference to this in any Memotech documents, so I assume that the Node ring did have this protection

The diagrams show a normally closed relay across the transmit and receive data lines and assume that the relay is energised when the node is turned on and healthy.

With the node powered off, the relay would deenergise and the relay contacts would close, bypassing the nodes RS232 interface. When the node was powered on, the relay contacts would open, allowing the ring traffic to pass through the node.

A Minimal Node Ring

A minimal logical Node ring only requires two nodes to be connected.

Although a logical ring would still exist, the two nodes can be directly connected using a simple crossover cable, requiring pins 2 & 3 to be crossed and ground (pin 7) connected.

Wiring diagram for a bare minimum cable to provide 1 : 1 connectivity between two MTX computers (only) running Node

A standard null-modem cable can also be used

For three or more nodes, if coax cables are not being used, then small rings could be established by making up custom cables as shown.

This would be impractical for more than, say, two or three nodes, after that, it would be more efficient to make up adapters and use coax cable.



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