Measurement Computing Micro 488/EX rev.2.1 - Manual (Page 174)

Section 7 Peripheral Pass-Thru Operation
7.2
When the serial input buffer requests one of the last 10 queues (1270 character
locations left), it signals the serial host that it should stop sending data. This is
accomplished by either un-asserting RTS or issuing XOFF, depending on which serial
handshake control has been switch selected. When more than 10 queues become
available, it asserts RTS or issues XON.
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 Micro488/EX stops accepting data from the bus. This bus
hold-off will only occur until additional queues (greater than 4) become available. At
that time it will resume accepting bus data.
7.3 IEEE Data Transfers
The following methods may be used by the IEEE controller when sending data
to the Micro488/EX:
7.3.1 Blind Bus Data Transfers
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 Micro488/EX. This is referred to as blind data transfers because the
IEEE controller is blind as to whether or not the Micro488/EX is capable of
accepting data. In this case, the bus controller's output data transfer will be
held off by the Micro488/EX 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 Micro488/EX 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
Micro488/EX 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 Micro488/EX.