Rabbit OP7100 User Manual download pdf (Page 159)

User’s Manual 153

D.2 Physical Implementation

There are four signaling functions associated with a RabbitNet connection. From the mas-

ter’s point of view, the transmit function carries information and commands to the periph-

eral board. The receive function is used to read back information sent to the master by the

peripheral board. A clock is used to synchronize data going between the two devices at

high speed. The master is the source of this clock. A slave select (SS) function originates

at the master, and when detected by a peripheral board causes it to become selected and

respond to commands received from the master.

The signals themselves are differential RS-422, which are series-terminated at the source.

With this type of termination, the maximum frequency is limited by the round-trip delay

time of the cable. Although a peripheral board could theoretically be up to 45 m (150 ft)

from the master for a data rate of 1 MHz, Rabbit recommends a practical limit of 10 m

(33 ft).

Connections between peripheral boards and masters are done using standard 8-conductor

Ethernet cables. Masters and peripheral cards are equipped with RJ-45 8-pin female con-

nectors. The cables are nonpolarized in that they may be swapped end for end without

affecting any functionality.

D.2.1 Control and Routing

Control starts at the master when the master asserts the slave select signal (SS). Then it

simultaneously sends a serial command and clock. The first byte of a command contains

the address of the peripheral card if more than one peripheral card is connected.

A peripheral card assumes it is selected as soon as it receives the select signal. For direct

master-to-peripheral-card connections, this is as soon as the master asserts the select

signal. The connection is established once the select signal reaches the addressed slave. At

this point communication between the master and the selected peripheral card is estab-

lished, and data can flow in both directions simultaneously. The connection is maintained

so long as the master asserts the select signal.

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Rabbit OP7100 User Manual Page 159