Telex ADAM DBX, Руководство пользователя

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ADAM Dual Bus Expander
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General Description of DBX Communication Scheme and Failure Modes
DBX Wiring and Communications
The DBX cards in all system are wired such that if you could unplug the DBX cards with their port wiring intact
and lay them out on the ground untangled, the DBX cards and the wiring connections would form a giant circle.
If all the links in a DBX system are up and running, it is possible for a DBX card to send a message out its
PORT A, and receive the same message back on its PORT B after the message had passed through every
other DBX card in the system (assuming that every card that received a message on one port then transmitted
the message on its other port).
The purpose of this configuration is to allow any DBX card to have at least two paths to every other DBX card
using coax only. A card can transmit out either of its ports to reach another card as long as all links are up. If
a single link goes down (a link being a pair of coax cables for TX and RX), every card can still reach every
other card using coax only by going out either its PORT A or its PORT B since a broken circle still has all the
points connected by a single line.
Only when two or more links are broken is it possible to isolate one or more DBX cards from other cards on
the coax, but even in this case it is often possible to reach the isolated cards (which will be described later).
A DBX card can transmit and/or receive messages on either of its ports, as well as the control bus. However,
a DBX card only transmits on the control bus to the destination card (the card who is the ultimate target of the
message). All messages transmitted by a DBX card go out one of its ports, unless the target card is in the
same frame as the DBX card, in which case the DBX card will use the control bus to make final delivery of the
message.
This means that the “active” DBX card in the first frame can send a message to any DBX or AIO card in the
system, as long as it has a coaxial path to the destination frame. If the “active” DBX card can reach the
destination frame via coax hops, then the message can be delivered on the control bus by the DBX card in the
destination frame.
This does NOT guarantee, however, that the destination card can send a message back to the “active” DBX
card in the first frame. Each slave frame has an “active” DBX card who is responsible for polling the AIO cards
and reporting status changes to the “active” DBX card in the first frame. All the AIO cards in a slave frame
keep track of the “active” DBX card in their frame, and messages that need to go to the first frame are sent to
the “active” DBX card in the slave frame for forwarding.
The asymmetry in message paths occurs because the DBX card in the slave frame, which delivered a mes-
sage to the AIO card, may not be the “active” DBX card. The “active” DBX card may not have a coaxial path
to the first frame, even though the “active” DBX card in the first frame does have a coaxial path to the destina-
tion frame. For this to occur, there has to be more than one link failure.
It might be possible to allow intermediary control bus hops in routing a message, however, the number of
possible message routes becomes enormous, and it requires that every DBX card be aware of the link status
of every other DBX card which becomes extremely difficult to do when you are trying to use link status to
determine message routing, but you need to use message routing to pass link status back and forth.
An even worse consequence of allowing arbitrary message routing is that it becomes difficult to predict the
order of arrival of transmitted message. In the case of crosspoint messages, the order is critical; suppose two
messages are sent, one to turn a crosspoint on, the other to turn it off; now suppose the messages end up
taking different routes because of a link status change, and the second message arrives first. The AIO card
that receives the messages turns the crosspoint off (it already was off), and then turns it on, and the crosspoint
is left in the wrong state!