Product Manual

7526A
Users Manual
5-16
characters. The controller reads it with a statement such as a BASIC INPUT
statement. What is read is removed from the queue. If the queue is empty, the
Product does not answer to the INPUT statement from the controller. The Message
Available (MAV) bit in the Serial Poll Status Byte is 1 if there is something in the
output queue and 0 if the output queue is empty.
10. Error Queue
When a command error, execution error, or device-dependent error occurs, its error
code is put in the error queue where it can be read by the FAULT? command. If you
read the first error with the FAULT? Command, this removes that error from the
queue. A response of 0 means the error queue is empty. The error queue is cleared
when you turn off the power, and when you use the *CLS (Clear Status) common
command. The error queue holds A maximum of 15 entries. If more than 15 errors
occur, only the first 15 errors are kept in the queue. A 16th entry in the queue is
always an "error queue overflow" error, and all new errors are discarded until part of
the queue is read. The first errors are kept, because if many errors occur before the
user can acknowledge and read them, the first errors are most likely to point to the
problem. The newest errors are usually repetitions or consequences of the initial
problem.
11. Input Buffer Operation
As the Product gets each data byte from the controller, it puts the byte in a part of
memory named the input buffer. The input buffer holds a maximum of 250 data bytes
and operates in a first in, first out procedure.
IEEE-488 interface:
The Product works with the IEEE-488 EOI control line as an isolated data byte
and puts it into the input buffer if it is found as part of a message terminator.
Input buffer operation is transparent to the program on the controller. If the
controller sends commands faster than the Product can do them, the input buffer
fills to capacity. When the input buffer is full, the Product holds off the IEEE-488
bus with the NRFD (Not Ready For Data) handshake line. When the Product has
processed a data byte from the full input buffer, it then completes the handshake.
This lets the controller send a different data byte. The calibrator clears the input
buffer on power-up and when receiving the DCL (Device Clear) or SDC
(Selected Device Clear) message from the controller.
RS-232 interface:
The Product uses the RS-232-C Xon/Xoff protocol to control buffer overflow.
The Product sends a Xoff (Ctrl S) character when the input buffer becomes 80 %
full, and sends a Xon (Ctrl Q) character when it has processed a sufficient part of
the input buffer so that it is less than 40 % full