If it responds with a Serial Poll, it will read the binary value
01010000, i.e. the R0S bit and the OPERATION COMPLETE bit. Upon
execution of the Serial Poll, these two bits are reset in STB I.
However, the 9400A remembers that another bit, capable of setting ROS,
was set, in this case the ERROR bit. Thus, it generates a second
Service Request. When the host computer executes a second Serial Poll,
it will read the binary value 01100000, i.e. the ROS bit and the ERROR
bit. Upon execution of the second Serial Poll, STB 1 will be cleared
completely. Thus, 2 different bits of STB I (when demasked) always
generate two different Service Requests, provided that STB 1 is read by
Serial Poll. This follows the principle that SRQ should be generated by
one event (at the level of STB I) at a time.
If the host computer responds by reading
the status byte(s) with "STB
1,?" or "STB ?", it will read the binary
value 01110000 in STB I. The
Main Status Byte (STB I) is cleared, and
no more Service Requests are
generated. Thus, STB I, when read explicitly, keeps accumulating other
status bits, and there might be fewer Service Requests than events (at
the level of STB I).
Another Example:
MASK 1932
Sets the mask of the Main Status Byte (STB i) to the
binary value 00100000, allowing the ERROR bit to generate
an R0S bit, i.e. to generate a service request.
MASK 6,1
Sets the mask of the ERROR byte (STB 6) to a non-zero
value, allowing the propagation of errors to the ERROR bit
of STB I.
TIME/DIV?
This legal command generates a response message which can
(and should) be read by the host computer. In addition,
the MESSAGE READY bit is set in STB i. Because this bit is
not demasked it does not generate an SRQ. However, if STB
1 is now read explicitly or by Serial Poll, the binary
value I0000000 (= 80 in hexadecimal) would be read.
AAA? This illegal command generates a syntax error and sets the
ERROR byte (STB 6) to the value II. Simultaneously, the
ERROR bit of STB I is set. Since this bit is demasked in
STB I, it sets the R0S bit and generates a service request
SR0.
CALL HOST
This command is illegal, since the required parameter is
missing. It generates a semantic error and sets the ERROR
byte (STB 6) to the value 40. This error should again
propagated to the ERROR bit of STB l, but it cannot
generate a service request, since it is still pending.
7-54
Remote Operations
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