Getting Started
3-18
3.5.5 System expansion issues
Matrix expansion by Model 708A Switching Systems affects
system specifications and speed. The extent of affect de-
pends on the size and configuration of the switching system.
Backplane row expansion decreases isolation among like
cards and increases offset current. Isolation relays (on the
Model 7072) and backplane jumpers (for general purpose
rows) help decrease these effects.
Expansion of units along rows or columns also degrades the
isolation and offset current specifications because of the
number of parallel paths and relays on each signal line.
Issues that affect system speed include:
Relay settling time
— Each matrix card has a predefined re-
lay settling time. When card types are mixed in a system, the
longest settling time is in effect.
Bus communication
— A master/slave setup responds
slower to bus commands because all communication is
through the master unit and the data transmission among the
units is verified with handshaking. Table 3-8 compares some
typical response times.
3.5.6 Documenting system configuration
With the connection flexibility of the matrix topology and
the expansion/isolation options of the Model 708A, docu-
mentation of the system configuration is important.
An example table for tracking card connections and expan-
sion is shown in Table 3-9. Use the top portion of the table to
note system operation and size, the FROM/TO portion to list
card row and column connections, and the lower portion for
notes concerning expansion and operation.
Table 3-8
Response time comparison
Action
Stand-alone
Master with
four slaves
Respond to bus command
to close single relay.
Download one setup to
708A.
<15ms
50ms typical
<55ms
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