46
RS-232↔IEEE 488 INTERFACE CONVERTER
When more than 10 queues become available, it asserts RTS or issues “X-ON.”
The IEEE bus input signals that the IEEE input (or serial output) buffer is full
when the number of queues available drops below 10 (1280 character
locations left). When the number of available queues drops to 4 or less (512
character locations left), the IEEE interface of the interface converter stops
accepting data from the bus. This bus hold-off will only occur until additional
queues (more than 4) become available. Then the interface will resume
accepting bus data.
5.3 IEEE Data Transfers
The following methods may be used by the IEEE controller when sending
data to the interface converter:
5.3.1 B
LIND
B
US
D
ATA
T
RANSFERS
If the IEEE controller does not mind waiting an indefinite time for data space
in the buffer to become available, the data can simply be sent to the interface
converter. This is referred to as “blind data transfer,” because the IEEE
controller is blind as to whether or not the interface converter is capable
of accepting data. In this case, the bus controller’s output data transfer will
be held off by the interface converter if it is unable to buffer the data. It will
resume accepting IEEE input data when memory becomes available. This
type of control might be appropriate in a single-user environment.
To illustrate how this would appear, let’s assume the interface converter is
connected to a serial device which will accept data at 1200 baud or 110 bytes
per second. The IEEE bus controller is capable of sending data to the
interface converter at a rate of 5000 bytes per second. The data would be
transferred on the bus at 5000 characters per second for slightly over six
seconds, filling over 31,000 locations. At that time, the IEEE input would
hold off additional data transfers until 128 characters are sent out the serial
port at rate of 110 characters per second. This 110 cps would then become
the average bus data acceptance rate of the interface converter.
If the controller is set to detect a data time-out error, then it will do so if the
interface converter holds off IEEE input data transfers for too long. The error
can be used to alert the operator to the problem, such as a printer out of
paper, so that it can be corrected. If the controller then restarts transmission
exactly where it left off, no data will be lost.
If data is requested by the controller and no serial input data is available in
the interface converter, the bus will hang until serial data is received. If no
serial data is received, it will hang forever until the controller times out.