User Manual DDS200 Digital Demodulation System 070-9952-00

Remote ControlOutput UnitThe output unit collects the information requested by the controller from the dataset management. It processes the information in conformance with the SCPIrules and makes it available in the output buffer. The output buffer has a capacityof 1024 characters. If the requested information is longer, it will be madeavailable in portions in a way that is not noticeable to the controller.If the device is addressed as a talker and the output buffer does not contain anydata or expect data from the data set management, the output unit sends the errormessage “Query UNTERMINATED” to the status reporting system. No data willbe sent on the IEC/IEEE bus and the controller will wait for timeout. Thisprocedure is prescribed by SCPI.The output unit is cleared as soon as the command line has reached the outputunit. It is written with the new data resulting from this line. The output unit iscleared in particular when there is a line between the command for reading theoutput unit (for example, “FREQ?”) and fetching this value.Command Sequence andCommandSynchronizationIt is evident from the discussions above that overlapping execution is possiblewith all commands. Setting commands within a command line are not necessarilyexecuted in the sequence that the commands are received.To ensure that commands will be executed in a certain sequence, each commandmust be sent in its own command line with an IBWRT() request.To avoid overlapping execution of commands, one of the commands *OPC,*OPC? or *WAI must be used. All three commands have the effect that a certainaction will only be triggered after the hardware has been set and has settled. Bysuitable programming the controller can be forced to wait for the respectiveaction (see Table 3–1).Table 3–1: Synchronization with *OPC, *OPC? and *WAICommand Action after hardware settling Programming of controller*OPC Setting the Operation-Complete bit inthe ESR– Setting bit 0 in the ESE– Setting bit 5 in the SRE– Waiting for service request (SRQ)*OPC? Writing a “1” into the output buffer Addressing the device as a talker*WAIContinuing the IEC/IEEE-bus handshakeSending the next commandAn example of command synchronization is given in Appendix D: ProgramExamples.3–18 DDS200 Digital Demodulation System User Manual