Measurement Computing Serial488A - Manual (Page 51)

4.2
When the serial input buffer requests one of the last 10 queues (1280 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 Serial488A 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.
4.3 IEEE Data Transfers
The following methods may be used by the IEEE controller when sending data
to the Serial488A:
4.3.1 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 Serial488A. This is referred to as blind data transfers because the IEEE
controller is blind as to whether or not the Serial488A is capable of
accepting data. In this case, the bus controller's output data transfer will be
held off by the Serial488A 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 Serial488A 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
Serial488A 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 Serial488A.
If the controller is set to detect a data time-out error, then it will do so
if the Serial488A holds off IEEE input data transfers for too long. The