National Instruments GPIB-140A User Manual

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National Instruments GPIB-140A User Manual Manual pdf 18 page image

© National Instruments | 3-3GPIB-140A User ManualTo enable HS488, set DIP switch 2 to the ON position, as shown in Figure 3-2. To disableHS488, set DIP switch 2 to the OFF position.Figure 3-2. DIP Switch Setting for Enabled HS488Parallel Poll Response ModesAccording to IEEE 488, devices must respond to a parallel poll within 200 ns after theController-In-Charge (CIC) asserts the Identify (IDY) message—Attention (ATN) and End orIdentify (EOI). The CIC waits at least 2 μs before reading the Parallel Poll Response (PPR). Inmany cases, a remote device on an extended system cannot respond to parallel polls this quicklybecause of cable propagation delays. To solve this problem, use one of the following twosolutions in your application:• If possible, specify in your application that the CIC must allow enough time to receive theresponse. For more information, refer to the following section, Immediate PPR Mode. Ifyou are using the NI-488.2 software, you can use the NI-488.2 Configuration utility to setthe amount of time that the CIC waits.• Execute two consecutive parallel polls and use the second response. For more information,refer to the Latched PPR Mode section later in this chapter.Immediate PPR ModeIn immediate PPR mode, the GPIB extenders do not use the internal PPR data register. When aController on the local system asserts IDY, the local extender sends the IDY message to theremote bus and the response is returned as fast as propagation delays permit. Your applicationmust allow enough time to receive the response.Latched PPR ModeIn latched PPR mode, the GPIB extenders use an internal PPR data register. When a Controlleron the local system asserts IDY, the local extender sends the contents of the PPR data register tothe local data lines. At the same time, a parallel poll message is sent to the remote bus. When thelocal system unasserts IDY, the PPR from the remote system is loaded into the internal PPR dataregister. Consequently, the register always contains the response of the previous complete poll.To obtain the response of both local and remote systems, your application should execute twoconsecutive parallel polls and use the second response.= Not used to set HS488 modePARALLEL POLL IMMEDIATEHS488 ENABLEBUFFERED TRANSFEROFFON