Contemporary Control Systems EXP Expansion Modules User Manual - Manual (Page 26)

3
Operation
3.1 Theory of Operation
Hub States
When the MOD HUB 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 hub 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 hub times
the absence of an ARCNET logic “1.” Once the unlatch delay
time is exceeded (typically 5.9 ms), the hub reverts back to the
IDLE state.
Signal Regeneration
To generate an ARCNET signal requires the synthesis of signals
P1 and P2. These 100 ns non-overlapping pulses in turn drive
the various transceivers on each of the expansion modules. 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 expansion modules incorporate a non-return to zero
(NRZ) signalling scheme with a logic “1” signal equivalent to
the logical OR of P1 and P2. The expansion modules
TD401600-0M
O
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