Contemporary Control Systems AI Active Hubs - Manual (Page 29)

TD675100-0M
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3
Operation
3.1 Theory of Operation
3.1.1 AI States
When the AI has installed expansion modules, the timing
module waits for the first instance of an ARCNET signal on any
one of its ports. During this time, the hub is in IDLE mode with
all port receivers enabled and all port transmitters disabled. As
soon as the first port senses an ARCNET signal (there should
only be one in a normally operating ARCNET system), the hub
enters the ACTIVE state with the receiving port left enabled and
all other receivers disabled. During this state all transmitters are
enabled with the receiving port’s transmitter disabled. This
allows all nodes on the network to hear a particular node which
has momentary control of the network while squelching any
echoes from unterminated lines (open or shorted cables). The AI
remains in the ACTIVE mode until the last ARCNET signal is
received by the originating port. To determine if the last signal
has been sent, the AI times the absence of an ARCNET logic
“1.” Once the unlatch delay time is exceeded (typically 5.9
µ
s
when operating at 2.5 Mbps), the AI reverts back to the IDLE
state.
3.1.2 Signal Regeneration
To generate an ARCNET signal requires the synthesis of signals
P1 and P2. These 100 ns non-overlapping pulses (when
operating at 2.5 Mbps) in turn drive the various transceivers on
each of the ports. A precision delay line gated oscillator forms
the basis of the regeneration circuitry and was chosen because
of the predictable delay experienced from this type of oscillator
which is important in reducing bit jitter.
EIA-485 ports incorporate a return to zero (RZ) signaling
scheme with a logic “1” signal equivalent to the logical OR of