Chapter 5
Issuing BlockTransfer Commands
5-9
The factors affecting the time to empty the scanner’s BT queue are as
follows. They are roughly in descending order of importance.
the number of BTs queued to the same adapter
Note: A single adapter, containing more than eight slots and using
single-slot addressing, appears to the scanner as two distinct adapters.
In this case, the lower eight slots belong to the first apparent adapter.
the number of BTs queued to the same module
the number of times an adapter appears in the scanner’s scan list
the length of the scanner’s scan list
the number of words of BT data to be exchanged
the baud rate
Timing Formula
In the following formula, we assume that the module is ready to perform a
transfer when asked, as is frequently true. If a module is not ready to
perform a BT when asked, then the time for it to get ready must be added
to the total time.
T = (Effective Number of Scans) x (Scan List Length) x (Time per Scan)
+ (Number of BTs) x (Time per BT) + ((Number of Words) x (Time per Word))
where T is the time it takes to complete all block transfers that are queued,
given the constraints of the formula.
Effective Number of Scans: If each adapter is in the scanner’s scan list
only once, and no module has more than one BT queued to it, then the
effective number of scans is the largest number of BTs queued to any one
adapter, plus one. For complete details, see the “Number of Scans” section
below.
Scan List Length: The number of adapters in the scan list.
Time per Scan: The time required to scan an adapter depends on the baud
rate. At 57.6K baud, the time is 11 milliseconds; at 115.2K baud, 7 ms; at
230.4K baud, 4 ms.
Number of BTs: The total number of BTs in the scanner’s queue.
Time per BT: At 57.6K baud or 115.2K baud, the time is 5.0 ms.
Number of Words: The total number of words of BT data to be
exchanged. This includes all BTs in the queue.
Time per Word: At 57.6K baud, this time is 0.3 ms; at 115.2K baud, 0.2 ms.
Time to Completion
