Page 359 - Introduction to Information Optics

344 6. Interconnection with Optics

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'Electrical vt«s"sftow«| vewe«1jii^lll0ii lSfolv!iig'multiple polyimide layers.
Fig, 6.41. Electrical vias showing vertical integration involving multiple polyimide layers.

These problems become more significant when the linear dimension of a board
increases.
Propagation time does not affect the maximum data rate of an uncompelled
asynchronous block transfer (such as the source-synchronous block transfer
[SSBLT] proposed for addition to the VME bus standard). However, it does
limit other types of bus transactions: address transfers, handshake single-word
transfers, bus contention, and so on. Estimates have been made by Sweazey
[44] of the sustained throughput; i.e., the data transfer rate averaged over a
time that is long compared to the duration of a single transaction. Assuming
that bus overhead is 200 ns per read and 100 ns per write operation, and
assuming reads outnumber writes by 2 to 1, Sweazey calculated the sustained
throughput as a function of block transfer speed (burst speed) and of the
number of bytes per transfer. For 64-byte transfers, the calculated sustained
throughput is 196 MB/sec for a burst rate of 400 MB/sec, and 384 MB/sec for
infinitely rapid block transfers. The propagation speed for the electronic bus is
at present greatest for backplane-transceiver logic (BTL) backplanes such as
FutureBus: about 0.18 c (c is the speed of light in vacuum), giving a 15 ns
round-trip time for a 40 cm backplane. This cannot decrease by much, since it
is based on the extremely low driver capacitance of 5 pf/driver provided by
BTL.
Uncompelled block transfers are limited by bus line skew. The principal
cause of this is speed variations (time jitter) between transceiver chips. This
jitter is at least 5 ns, even for a well-designed set of transceivers. This means
that there will be a total skew between data lines and strobes of up to 20 ns