Omega CL3001, User Manual, Page: 60 / 104

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 placed in the error queue where it
can be read by the FAULT? command. Reading the first error
with the FAULT? command 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
contains up to 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 later errors are
discarded until the queue is at least partially read. The first
errors are kept, because if many errors occur before the user
can acknowledge and read them, the earliest errors are the most
likely to point to the problem. The later errors are usually
repetitions or consequences of the original problem.
11) Input Buffer Operation
As the CL3001 receives each data byte from the controller, it
places the byte in a portion of memory called the input buffer.
The input buffer holds up to 250 data bytes and operates in a
first in, first out fashion.
IEEE-488 interface:
The CL3001 treats the IEEE-488 EOI control line as a separate
data byte and inserts it into the input buffer if it is encountered as
part of a message terminator. Input buffer operation is
transparent to the program running on the controller. If the
controller sends commands faster than the CL3001 can process
them, the input buffer fills to capacity. When the input buffer is
full, the CL3001 holds off the IEEE-488 bus with the NRFD (Not
Ready For Data) handshake line. When the CL3001 has
processed a data byte from the full input buffer, it then completes
the handshake, allowing the controller to send another data byte.
The calibrator clears the input buffer on power-up and on
receiving the DCL (Device Clear) or SDC (Selected Device Clear)
message from the controller.
RS-232 interface:
The CL3001 uses the RS-232-C Xon/Xoff protocol to control buffer
overflow. The CL3001 sends a Xoff (Ctrl S) character when the
input buffer becomes 80% full, and sends a Xon (Ctrl Q) character
when it has processed enough of the input buffer so that it is less
than 40% full.
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