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21020285 D User’s Guide
Chapter 7 Board-control Commands
Page 7 - 16 General Options (07H)
Bus Throttle
Note
The adapter will enforce
a minimum burst size
equal to the transfer
width. The adapter will
also enforce a maximum
burst for Block Mode
transfers, in order to
ensure adherence to the
VME specification—
rules 2.12 and 2.75—
related to address
boundaries.
The bus throttle control allows you to regulate the flow of SCSI data on the
VMEbus. The flow of data is regulated by limiting the number of bytes that the
adapter is allowed to transfer each time it acquires the bus. The adapter transfers
the data in bursts. You may specify the number of bytes that the adapter
transfers in a burst by placing the appropriate value in the Bus Throttle field.
The number of bytes in a burst is expressed as a power of 2. The possible values
are 0 though 0BH.
The table below translates the Bus Throttle field values into number of bytes
per burst.
The default setting for this field is 0BH, 2048 bytes per burst.
Select Flags
The Select Flags are used to determine several operating factors for the RF3880
adapter. Use of Block Mode transfers by the adapter is affected by Select Flags.
Other factors pertain to the SCSI bus: parity checking and disconnect/
reconnect.
The format of the Select Flags byte is as follows:
DIS Disconnect/Reconnect - With this bit you are able to select whether
the RF3880 adapter will allow peripherals to disconnect from the SCSI bus
while performing a command.
0 ⇒ Disallow Disconnect/Reconnect.
1 ⇒ Allow Disconnect/Reconnect.
The default value for this bit is one — allow disconnects on all commands.
Value Represents Number of Bytes per Burst
00H 2
0
1
01H 2
1
2
02H 2
2
4
03H 2
3
8
04H 2
4
16
05H 2
5
32
06H 2
6
64
07H 2
7
128
08H 2
8
256
09H 2
9
512
0AH 2
10
1024
0BH 2
11
2048
Table 72: Bus Throttle Field Values
BITS
76543210
0 0 0 0 X BMT PAR DIS
Table 73: Select Flags Byte