READ?
Syntax
READ? [(@<ch_list>)]
Description
This command changes the instrument's triggering system from the "idle" state to the "wait-for-trigger" state. Measurements will begin when the specified trigger conditions are satisfied following the receipt of the READ? command. Readings are then sent immediately to reading memory and the instrument's output buffer.
If you omit the optional <ch_list> parameter and a scan list is not currently defined (see ROUTe:SCAN command), the READ? command applies to the internal DMM.
If you omit the optional <ch_list> parameter and a scan list is currently defined, the READ? command performs a scan of the channels in the scan list.
If you specify a <ch_list>, regardless of whether a scan list is currently defined, the READ? command performs a "temporary" scan of the specified channels (independent of the present scan list).
Used With:
34921A through 34925A Multiplexer Modules
34950A Digital I/O Module (digital input and counter channels only)
34952A Multifunction Module (digital input and totalizer channels only)
Parameters
|
Name |
Type |
Range of Values |
Default Value |
|
<ch_list> |
Numeric |
One or more channels in the form (@sccc). |
If <ch_list> is omitted, this command applies to the internal DMM. |
Remarks
Sending the READ? command is similar to sending the INITiate command followed immediately by the FETCh? command.
You can store at least 500,000 readings in memory and all readings are automatically time stamped. If memory overflows, the new readings will overwrite the first (oldest) readings stored; the most recent readings are always preserved. In addition, bit 12 is set in the Questionable Data Register's condition register (see Status System Introduction).
The Analog Bus relays are automatically opened and closed as required during the scan to connect to the internal DMM for the measurement. For example, all 2-wire measurements use the ABus1 (MEAS) relays; for 4-wire measurements, the ABus2 (SENS) relays are used in addition to the ABus1 relays.
For scanning measurements using the multiplexer modules, an error is generated if the internal DMM is disabled (see INSTrument:DMM[:STATe] command) or not installed in the mainframe. The internal DMM is not required for operations on the digital modules.
If you omit the optional <ch_list> parameter and a scan list is currently defined, the READ? command performs a scan of the channels in the scan list. The following rules apply while the scan is running.
When the scan is initiated, the instrument will open all channels in banks that contain one or more channels in the scan list.
In order to guarantee that no signals are connected to the Analog Buses prior to the scan, the instrument will open all ABus1 relays (applies to all banks in all slots). In banks that contain channels in the scan list, the instrument will also open all ABus2 relays (regardless of whether 4-wire measurements are involved). If no channels configured for 4-wire measurements are included in the scan list, the state of the ABus2 relays in the non-scanned banks is not altered.
The state of the ABus3 and ABus4 relays is not altered and these relays remain available for use during the scan. However, be sure to use CAUTION when closing these relays on banks involved in the scan. While the scan is running, any signals present on ABus3 and/or ABus4 will be joined with the scanned measurement on ABus1 and ABus2. If you have any doubt regarding the state of the ABus3 and ABus4 relays, it is recommended that you send a ROUTe:OPEN:ABUS ALL command prior to scanning operations.
While the scan is running, the instrument prevents use of all channels in banks that contain one or more channels in the specified scan list (these channels are dedicated to the scan). In addition, the instrument prevents use of all ABus1 and ABus2 relays on banks containing channels in the scan list. If one or more channels configured for 4-wire measurements are included in the scan list, then the rules for ABus2 relay operations are extended to the non-scanned banks as well.
If the ABus1 relay used for current measurements (channel 931 on 34921A only) is not closed prior to the initiation of the scan, the four current channels (channels 41 through 44) are not affected by the scan. However, if the ABus1 relay is closed, the instrument will open the ABus1 relay as well as the four associated current channels in a make-before-break fashion.
At the end of the scan, the last channel that was scanned will be opened (as well as any Analog Bus relays used during the scan). Any channels that were opened during the scan will remain open at the completion of the scan.
If you specify a <ch_list>, regardless of whether a scan list is currently defined, the READ? command performs a "temporary" scan of the specified channels (independent of the present scan list). The following rules apply while the temporary scan is running (these rules supersede the rules for the pre-existing scan list, but the present scan list is not modified).
When the temporary scan is initiated, the instrument will open all channels in banks that contain one or more channels in the specified <ch_list>. The instrument will also open all ABus1 relays (applies to all banks in all slots). In addition, if one or more channels configured for 4-wire measurements are included in the specified <ch_list>, the instrument will also open all ABus2 relays (applies to all banks in all slots). The state of the ABus3 and ABus4 relays is not altered.
While the temporary scan is running, the instrument prohibits use of all channels in banks that contain one or more channels in the specified <ch_list>. In addition, the instrument prohibits use of all ABus1 relays (and ABus2 relays if applicable) while the temporary scan is running.
At the end of the temporary scan, the last channel that was scanned will be opened (as well as any Analog Bus relays used during the scan). Any channels that were opened during the scan will remain opened at the completion of the scan.
By default, the instrument scans the list of channels in ascending order from slot 1 through slot 8 (channels are reordered as needed). If your application requires non-ordered scanning of the channels in the present scan list, you can use the ROUTe:SCAN:ORDered command to enable the non-sequential scanning mode. In either mode, channels which are not in the scan list are skipped during the scan.
For sequential scanning (default, ROUT:SCAN:ORDERED ON), the channels in the scan list are reordered as needed and duplicate channels are eliminated. For example, (@2001,1003,1001,1003) will be interpreted as (@1001,1003,2001).
For non-sequential scanning (ROUT:SCAN:ORDERED OFF), the channels remain in the order presented in the scan list (see exception below). Multiple occurrences of the same channel are allowed. For example, (@2001,2001,2001) and (@3010,1003,1001,1005) are valid and the channels will be scanned in the order presented.
When you specify a range of channels in the scan list, the channels are always sorted in ascending order, regardless of the ROUTe:SCAN:ORDered setting. Therefore, (@1009:1001) will always be interpreted as 1001, 1002, 1003, etc.
Once you initiate a scan, an error will be generated if you attempt to change any measurement parameters (CONFigure and SENSe commands) or the triggering configuration (TRIGger commands). To abort a scan in progress, send the ABORt command.
The 34923A, 34924A, and 34933A modules use non-latching reed relays. Due to power dissipation issues, there is a limit to the number of relays that can be closed at a time on these modules (you cannot close all channels simultaneously). See the ROUTe:CLOSe command for details.
The Safety Interlock feature prevents connections to the Analog Buses if no terminal block or properly-wired cable is connected to the module. An error will be generated if scanning is performed on banks without a terminal block or properly-wired cable. See the SYSTem:ABUS:INTerlock:SIMulate command to temporarily disable errors generated by the Safety Interlock feature.
The output from this command is affected by the settings of the FORMat:READing commands. Depending on the formats selected, each reading may or may not be stored with measurement units, time stamp, channel number, and alarm status information. If readings are acquired using the internal DMM independently (without a multiplexer scan), the channel number will be logged as channel "0".
The READ? command is not valid with the *TRG command (used with TRIGger:SOURce BUS command for software triggering).
The instrument clears all readings from memory after a Factory Reset (*RST command), after an Instrument Preset (SYSTem:PRESet command), or when mainframe power is cycled.
Return Format
The command sends readings directly to reading memory and the instrument's output buffer (with formatting as set by the FORMat:READing commands). Multiple responses are separated by commas.
Examples
The following program segment shows how to use the READ? command to make a dc voltage measurement using the internal DMM. The CONFigure command configures the internal DMM for a dc voltage measurement (no <ch_list> is specified). The ROUTe:SCAN command is included to clear the scan list. The READ? command (with no <ch_list>) places the internal DMM in the "wait-for-trigger" state and then sends the reading to memory and the instrument's output buffer.
CONF:VOLT:DC !Applies
to the internal DMM
ROUT:SCAN (@) !Clear
the scan list
READ? !Applies
to the internal DMM
Typical Response: +1.26360000E-02
The following program segment shows how to use the READ? command with the CONFigure command. The ROUTe:SCAN command puts the two channels into the scan list (and redefines the scan list). The READ? command (with no <ch_list>) places the instrument in the "wait-for-trigger" state, scans the specified channels when the rear-panel Ext Trig Input line is pulsed low, sends the readings to reading memory and the instrument's output buffer. In this case, the READ? command applies to the two channels in the present scan list.
CONF:VOLT:DC 10,0.003,(@1003,1008)
!Configure
channels
ROUT:SCAN (@1003,1008) !Define
the scan list
TRIG:SOUR EXT
READ? !Applies
to the present scan list
Typical Response: +4.27150000E-03,+1.32130000E-03
The following program segment shows how to use the READ? command with a <ch_list> parameter. The ROUTe:SCAN command puts two channels into the scan list (and redefines the scan list). The READ? command creates a temporary scan (independent of the present scan list) with three channels, places the instrument in the "wait-for-trigger" state, and then transfers the readings to reading memory and the instrument's output buffer.
CONF:VOLT:DC (@2001:2010) !Configure
channels
ROUT:SCAN (@2001,2002) !Define
the scan list
READ? (@2005:2007) !Create
temporary scan list, doesn't modify present scan list
Typical Response: +2.73630000E+00,+1.73730000E-03,+5.00930000E-03