Filter
Top Products
47
CHAPTER 5: Peripheral Operation
5.3.2 C
ONTROLLED
B
US
D
ATA
T
RANSFERS
If the controller must avoid waiting for the serial device, it can “serial-poll” the
interface converter. Serial polling is a method by which the controller can
inquire the internal status of the interface without disturbing any data being
transferred, slowing data transfers, or locking up the bus. You should refer to
the programming manual of your controller to determine the method of
performing serial polls.
When serial-polled, the interface converter provides eight bits of status
information to the controller. The most significant bit (DIO8) of the
interface converter’s serial poll byte is set to a logic “1” when the IEEE input
buffer is NOT EMPTY. The term NOT EMPTY is used to signify that not all
of the previous data sent to the interface has been transmitted to the serial
device. If it is NOT EMPTY, the controller may avoid sending any more data
to the interface converter. If this bit is a logic “0,” then the serial device has
accepted all previous data and the IEEE controller may send more.
Another bit (DIO4) of the Serial Poll byte is used to indicate additional
information concerning the IEEE input buffer. This bit is set to a logic “1”
when there is 1280 or less locations in the buffer for data. It is cleared, set
to a logic “0”, when there is greater than 1280 locations available. This bit is
referred to as the IEEE input buffer FULL bit.
When serial data is received, DIO5 of the Serial Poll byte is set to “1”, to
indicate to the IEEE controller that the serial input buffer is NOT EMPTY. If
this bit is set, it indicates that at least one character is available in the serial
input buffer to be read by the IEEE controller. Once all of the serial input
data is read by the IEEE controller this bit is reset.
The interface converter can generate a request for service on the bus when it
receives the last serial terminator. To enable this feature, the Peripheral SRQ
switch, located on the internal switch bank of SW1, must be enabled. When
SRQ is enabled, the interface converter will assert the IEEE bus SRQ line and
set serial poll status bits DIO7 and DIO3 when the last serial terminator is
detected. The IEEE controller must perform a serial poll on the interface to
clear the SRQ. If the Peripheral SRQ switch is in the disabled position, there
will not be any indication in the serial-poll status byte that a serial terminator
has been received.