4 Links

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4

Links

INMOS bi-directional serial links provide synchronized communication between transputer products and

with the outside world. Each link comprises an input channel and output channel. A link between two de-

vices is implemented by connecting a link interface on one device to a link interface on the other device.

Every byte of data sent on a link is acknowledged on the input of the same link, thus each signal line carries

both data and control information.

The quiescent state of a link output is low. Each data byte is transmitted as a high start bit followed by a

one bit followed by eight data bits followed by a low stop bit. The least significant bit of data is transmitted

first. After transmitting a data byte the sender waits for the acknowledge, which consists of a high start bit

followed by a zero bit. The acknowledge signifies both that a process was able to receive the acknowl-

edged data byte and that the receiving link is able to receive another byte.

Links are not synchronised with ClockIn and are insensitive to its phase. Thus links from independently

clocked systems may communicate, providing only that the clocks are nominally identical and within speci-

fication.

Links are TTL compatible and intended to be used in electrically quiet environments, between devices on

a single printed circuit board or between two boards via a backplane. Direct connection may be made be-

tween devices separated by a distance of less than 300 millimetres. For longer distances a matched

100 ohm transmission line should be used with series matching resistors RM. When this is done the line

delay should be less than 0.4 bit time to ensure that the reflection returns before the next data bit is sent.

Buffers may be used for very long transmissions. If so, their overall propagation delay should be stable

within the skew tolerance of the link, although the absolute value of the delay is immaterial.

The IMS C011 link supports the standard INMOS communication speed of 10 Mbits/sec. In addition it can

be used at 20 Mbits/sec. Link speed can be selected in one of two ways. In Mode 1 it is altered by Separa-

teIQ (page 6). In Mode 2 it is selected by LinkSpeed; when the LinkSpeed pin is low, the link operates

at the standard 10 Mbits/sec; when high it operates at 20 Mbits/sec.

012

3

4

5

6

7

Data

Ack

HH

L

L

H

Figure 4.1

IMS C011 link data and acknowledge packets