LPTX CONFIGURATION PARAMETERS:

• Parameter Overview
• Address
• Bits Preset
• Channel Preset
• Help File Omit
• IRQ (Interrupt Request)
• Model
• Parallel Port - Stimulus DigOut
  • Using an 8255 Port
  • Using an Extra LPT Port
• Input Range
  • Digital Input Range
• Conversion Speed
  • External Pacer
• SAR Settle Wait Delay Preset

PARAMETER OVERVIEW:

  LPTX.ADC    A:1  M:SAR  R:2.5

You can check to see which parameters have been set, either via the configuration line or by default, by checking the board information summary available via the ALT-H Plug-In Help system. This also shows a summary of the default values for all parameters. There is no need to enter any parameter that is the same as the default value assumed by the driver.

A: ADDRESS

  LPTX.ADC    A:1

If the number you enter is not in the 1 to 4 range, Daqarta will exit immediately with:

  'LPT value out of range.'

Most computer systems come with a single standard printer port, and if you need more than that you will have to install a separate board. All DOS systems can support ports LPT1 through LPT3 "naturally", without special software. The system BIOS keeps a record of the location of each LPT that it recognizes, and that record is consulted to validate the LPT value that you use here. If there is no entry for the LPT number you give (L:3 in this example), Daqarta exits with a message like:

 'LPT3 not recognized by BIOS.'
 'BIOS recognizes only LPT1 to LPT2.'

Problems may be due to adding a new printer port board and getting the jumpers or switches wrong. Most boards allow designation as LPT1 or LPT2. If you need to add LPT3, be sure the board allows it before you buy. Some boards allow LPT4 use, but not all systems support this. For example, IBM PS/2 systems use the LPT4 location in the BIOS data area for other purposes.

You can use the LTEST.EXE utility supplied with Daqarta to see the port addresses corresponding to each LPT, as read from the BIOS data area just like this driver does. A typical display would be:

  Printer Port assignments:
     LPT1: 0378h
     LPT2: 0000h
     LPT3: 0000h
     LPT4: 9FC0h

Even if LTEST shows that the LPT4 location is available to install a new port, you may need to run special software (supplied with your port board) to make use of it... not all BIOSes will assign this automatically on start-up.

B: BITS PRESET

The SAR defaults to an 8-bit ADC configuration, but can be set via this parameter (or dynamically under menu control) to use anywhere from 1 to 8 bits. You might want to use less than 8 bits for faster sample rates on slow systems.

Initially, you will probably want to omit this parameter and just experiment with the menu control. Use the B: parameter if you find that you want to run with less bits most of the time... you can always change the setting via the menu.

C: CHANNEL PRESET

  LPTX    Status   DB25    Printer
  Chan    Bit      Pin     Function
   0       3       15       Error
   1       4       13       Select
   2       5       12       Paper Out
   3       6       10       Acknowledge
   4       7       11       Busy

  LPTX.ADC    C:1

If you have selected DigOut (default) or DAC via the M: Model parameter, channel 4 is reserved for External Trigger use. In this case, selecting 4 (default) shows as "All 4", which means that all 4 channels (0-3) are combined in a weighted fashion, as though they were the output of a 4-bit ADC. Depending on what digital inputs are active at any given sample interval, the trace will show one of 16 levels.

For the SAR or MUX model, the channel usage is a bit different: The SAR consists of a DAC whose output is combined with the analog input signal and connected to one of the digital inputs. The C: parameter (or the Channel menu item) simply tells the driver which digital input you have chosen... it doesn't select from among different analog input sources as with other board drivers. (In theory, you could drive multiple DACs with each connected to a different analog signal and digital input, or you could use a single DAC with an output buffer for each analog input.)

Thus for the SAR or MUX model, there is no "All 4" combination of inputs, and the menu item normally allows selection of only Channels 0-3, with 3 as the default. Channel 4 is reserved for External Trigger as for the other models, but if you use the C:4 parameter you may specify it as the SAR input and forgo External Trigger operation. In this case, the menu item will of course be preset to Channel 4, but you may then change it over the 0-4 range... useful during hardware development, where you are testing to see which input works best with the SAR. (Some inputs may require different Wait settings, or have different thresholds that affect passive SAR design.)

If you select External Pacer operation via the S:En Speed parameter option, then Channel 3 is dedicated to that use. In that case, DigOut or DAC models only allow channels 0-2 to be set individually, and attempting to set Channel 3 (either via the C: parameter or menu option) will result in an "All 3" combination of channels 0-2. Note that the weighting will be such that channel 2 is as large as channel 3 under the "All 4" combination... everything is scaled up by a factor of 2.

For an SAR model with External Pacer, you may only use inputs corresponding to channels to 0-2. Trying to set a higher channel via the C: parameter will not produce an error, it will just set Channel 2.

F: HELP FILE OMIT

Note that ALL Help files, including main, can be omitted via an F:0 parameter on the INIT line (first line) of the DQA.CFG file. If this is present, no individual module Help will be loaded.

I: IRQ (Interrupt Request)

  LPTX.ADC    S:E20  I:7

If your printer card doesn't support interrupts, or if there is some system conflict that prevents proper use, you can still use the External Pacer option by simply omitting the I: parameter. However, you will be limited to Sequential mode only... the RTime key option will be shown crossed out.

If you give an I: parameter but no corresponding S:E to set External Pacer mode, Daqarta will exit with:

  'I: IRQ parameter only for S:En External Pacer mode.'

M: MODEL

    LPTX.ADC    M:DAC

• DigOut assumes this will be 8 Digital Output lines for use by the STIM3 Stimulus Generator module.
• DAC assumes a Digital to Analog Converter for use by STIM3, typically made by a simple resistor network.
• SAR assumes that the DAC output is combined with an analog input signal and fed to a digital input on the status port. The driver will control this to form a simple Successive Approximation Register ADC. No STIM3 outputs are available from the printer port.
• MUX uses special circuitry to switch the printer port between DAC and SAR modes on each sample. It allows full ADC input and STIM3 DAC output.

P: PARALLEL PORT ADDRESS FOR STIMULUS USE

The built-in printer data port is used for this by default if the DigOut Model is selected (default M: setting). You can also use a separate 8255-type port board, or an additional LPT printer port. This will allow you to use the main LPT for DAC or SAR operation, yet still provide digital stimulus output.

If you only need a single digital output bit, see if the Trigger Pulse output option available on all LPTX Model configurations is adequate before adding another board.

8255 PORT FOR DIGITAL STIMULUS OUTPUT:

An 8255-type port occupies 4 consecutive addresses: Port A, Port B, Port C, and a Control Port. The Port A address is always used to indicate the base 8255 address. You should give the address in hexadecimal notation by preceding it with an 'h', as in:

  LPTX.ADC    P:h310

Note that only the selected A, B, or C port is set to be an output... the other two ports are set as inputs. This allows other equipment to stay wired to them for other applications, since the inputs will just be ignored here. If the other application expects these to be inputs and there are active device outputs connected to them, then of course you don't want them to be forced to any possibly conflicting state.

However, suppose the other application uses those ports as outputs, and you want to leave device inputs connected to them while you run the current application. In this case, you can't assume that those device inputs will never be activated: The pins of the 8255, even though set as inputs, will have some indeterminate high or low voltage state unless they are loaded with around 5000 ohms or less. If your device inputs don't provide this load, you could be in for a nasty surprise when the other device activates unintentionally.

CAUTION:
To avoid unexpected missiles launches, third stage booster explosive squib detonation, ten-ton hydraulic ram activation, or even just a simple guinea pig food pellet dispenser cycle, be SURE to provide a few thousand ohms load to any critical device inputs that you must leave enabled but don't want to activate. Connect the load resistor from the device input to ground or +5 Volts, as appropriate to insure the desired state of the device.

When the other application later makes the port an output, it will have no trouble driving the device to either state with this "pull-up" or "pull-down" resistor present. Of course, if you are SURE that your device inputs already provide adequate loads, go ahead and skip the resistors...

LPT PRINTER PORT FOR DIGITAL STIMULUS OUTPUT:

For example, to specify LPT2 with the P: parameter, you would use:

  LPTX.ADC    P:L2

R: INPUT RANGE

To specify a range such as +/- 2.34 Volts, use:

  LPTX.ADC    M:SAR   R:2.34

  LPTX.ADC    M:SAR   R:1m23

  LPTX.ADC    M:SAR   R:100k

  LPTX.ADC    M:SAR   R:U1m23

Note that Unipolar operation requires that the Trigger Level be set above zero for Norm trigger mode. For Auto mode, the trigger always behaves in an AC-coupled manner, such that when Trigger Level is set to zero, the trigger point will be half of the peak-to-peak span of the input. For Unipolar data this will be the midpoint of the positive input.

DIGITAL INPUT RANGE

This seemingly unusual format is slightly faster to compute, but, more importantly, it is actually the "natural" format of a single-bit ADC. This keeps it compatible with the "All 4" Channel setting, which emulates a 4-bit ADC.

The standard "offset binary" data format, used by all 8-bit sound cards and by most laboratory acquisition boards, encodes the lowest (most negative) input as all zeroes, zero input as mid scale, and most positive input as all ones. For a 4-bit "board" each binary value is 1/16 of the full-scale value, from 0 to 15/16 of full scale:

  Input:      Binary:   Decimal:  Scaling:    Trace:
  Maximum     1111        15      15/16       Near Top
  Zero        1000        8       8/16        Middle
  Minimum     0000        0       0/16        Bottom

You can tell Daqarta to show the data as positive values only by using the 'R:U' notation just as for SAR analog ranges. For example, if you use:

  LPTX.ADC    M:DAC   R:U10

For the 'All 4' Channel setting, you may want to set R:U16 so that you can read the binary weight of the combined inputs directly from 0 to 15 "Volts". If you are actualy using less than all 4 bits with the 'All 4' setting, adjust the range value accordingly (assuming you use the most significant bits):

  Bits      Binary         Range       Display
  1      0000 or 1000      R:U2        0 or 1
  2      0000 to 1100      R:U4        0 to 3
  3      0000 to 1110      R:U8        0 to 7
  4      0000 to 1111      R:U16       0 to 15

S: CONVERSION SPEED

  LPTX.ADC    S:10

Note, however, that the actual upper limit depends on the speed of your system and the Model configuration you are using. DigOut and DAC models use digital inputs only, so the effective "ADC" is very fast. With a fast Pentium system you may be able to set the S: value as low as 4 µsec or even lower. On the other hand, with the SAR or MUX model you probably can't use anything much lower than 20 µsec.

So how do you determine when you are sampling too fast for your configuration? Typically, you will get an alert and this

  'Sample rate overrun!'

If you activate the Noise, Ramp, or Triangle output modes or use the DAC or DigOut outputs with STIM3, the maximum sample rate will be reduced due to the extra workload.

If you use stimulus oversampling (Stimulus Generator Factor greater than 1), you may be able to have the effective stimulus output rate run faster than the S: limit, since data is not acquired on every output sample.

EXTERNAL PACER:

With the MUX model in Echo mode, the External Pacer allows the signal delay to be varied by an external source for special Flanging or Chorus and Doubling effects.

You must provide an external TTL clock signal at exactly the desired sample rate. Connect it to status input bit 6, which is referred to as the -ACK pin for printer use and is on pin 10 of the DB25 connector. This is normally used by the LPTX driver as digital input Channel 3.

To indicate that this signal should be used as an External Pacer, you need to prefix the conversion Speed parameter value with an 'E', as in

  LPTX.ADC    S:E5

In this example, the E5 indicates an external pacer with a sample period of 5 microseconds, or 200 kHz. This may be way faster than the actual sample rate, but it has the advantage that the frequency axis indicates 0 to 100 kHz, which you can interpret as 0 to 100% of the spectrum.

You can set any integer value from 2 to 65535 here. If your pacer signal is constant, or nearly so, you can just use the closest integer microsecond approximation of its period. If it varies all over the place, you may be better off with a 0 to 100% representation.

For analyzing rotating machinery, sample pulses are typically derived from a magnetic or optical pickup that monitors the passing teeth of a gear. If you want the X-axis to read directly in multiples of the main shaft speed (harmonic numbers), you need to do a little planning.

For example, suppose you put the pickup on a gear with 100 teeth, and that gear is driven 4 times faster than the main shaft. Then the pickup produces pulses 400x faster than the main shaft revolves. If you want the main shaft frequency (fundamental) to show as 1 Hz, you must indicate that the sample rate is 400 Hz (2500 microseconds) via S:E2500. The X-axis will then show 0 to 200 Hz, which you can interpret as 0 to the 200th harmonic.

The value you set is not used for any "real" sampling operations... that's completely controlled by the TTL pacer signal you feed to the pin 10 input. You must insure that this never goes faster than the maximum sample rate for your board, or you will get bad data.

However, the S:E value does affect certain other internal functions that deal with time units. When a Trigger Delay is set, the entered time value is converted internally to an equivalent number of sample periods. The delay operation is then carried out by counting samples. So if the true sample period is different from the S:E value, all trigger delays will be off by the same proportion. Dynamically changing the external sample clock will produce changing delays.

Although the Delay value won't indicate the actual delay in time units, this variable delay operation provides constant delay in terms of rotational degrees. This allows you to start your acquisition sweep at any shaft angle relative to a fixed trigger sensor, for example.

In RTime mode, Cycle times are handled just like Trigger Delay and will show the same sample clock effects. Sequential mode Cycle timing will be correct, since it is done with the system timer using absolute time, not sample counting. The Cycle adjustment control uses the same sample-period increment size in both RTime and Sequential modes, and is limited to a total of 32767 samples maximum, so the S:E value will affect both resolution and range.

If you use a varying External Pacer clock frequency during stimulus generation with the STIM3 Stimulus Generator or the Trigger Pulse option, the output frequencies will be scaled accordingly. The sample period established by the S:E value will be used as the basis for STIM3 sine wave calculations when using the DAC outputs. All STIM3 durations, for both DAC sine waves and DigOut digital output pulses, are given in sample periods that may thus be variable in width. These will thus be constant in rotational degrees or phase angle, even though they don't indicate in those units.

When you are using the External Pacer option, the board summary screen will show:

  'External Pacer for Seq and RTime.'

  'System timer 0 used for Seq and RTime pacer.'

  'External Pacer not connected to Channel 3.
   (Status input bit 6 = DB25 pin 10 = -ACK line.)'

  LPTX.ADC   S:e5

If you do use the S:E option, the LPTX driver will next check to see if interrupts can be generated on the IRQ line specified by the I: parameter. If they can't, Daqarta will exit with:

  'External Pacer interrupts not found.'

  'External Pacer for Seq only... no RTime.'

W: SAR SETTLE WAIT DELAY PRESET

However, the DAC output must reach a stable value after each change, before a valid decision can be made. Using either the Wait menu option or the W: parameter, you can specify a delay to allow the new value to settle before any tests are done. You can set separate delays for each bit, or set all bits at once.

The Wait values do not correspond to particular delay values. They are arbitrary, and depend upon the speed of your system. For Wait values from 0 to 8, the delay steps are not equal because each delay is created by a different CPU instruction or string of up to four repeated instructions. For Wait values from 9 to 255, the delays are created by setting the counter of a single-instruction loop from 1 to 247 passes. On fast systems, the Wait = 9 delay may actually be less than Wait = 8, perhaps much less, due to low loop overhead for a single pass at Wait = 9 compared to four in-line instructions at Wait = 8.

When you are first setting up your system, you will probably want to use the menu controls to allow you to change these delays in real time so you can observe the results. But once you find a setting that you want to use in future sessions, the W: parameter can set this automatically at start-up.

To set the delays for All bits to a certain value n, use the format:

  LPTX.ADC    M:SAR   W:A=n

  LPTX.ADC    M:SAR   W:7=5

  LPTX.ADC    M:SAR   W:A=4   W:7=6   W:6=5

  Bit     Wait
  7       6
  6       5
  5       4
  4       4
  3       4
  2       4
  1       4
  0       4

How do you go about determining the optimum Wait delays for your configuration? The simplest method is to use the SAR to monitor a known good triangle, ramp, or sine wave from an external source. This could even be the DAC output of another Daqarta system. A ramp or triangle is best, since you can easily see deviations from a straight line.

Adjust the source amplitude to get near full-scale deflection on the trace, so that the full range of the SAR will be in use. Set the source frequency and the Daqarta sample rate so that you are viewing only a couple of cycles of the wave... you want things spread out enough to see problems in the waveform.

If the Wait delay is inadequate, there will be jagged spots in the trace. These will typically be worse where the more significant bits change, at mid-scale, quarter-scale, and so on. Use the Wait menu option and slowly increase All delays until the waveform looks good. As the Wait delay increases, you may get an alert and 'Sample rate overrun!' message. If so, you should reduce the sample rate until you no longer get this.

When you are satisfied, you may want to reduce individual bit Wait delays to try to get a bit more speed. Depending on your application, you may also want to reduce the number of bits to gain still more speed at the sacrifice of resolution.

How can you reduce the need for Wait delays? By reducing the time for the DAC to settle. The most profound influence on this is the length of the lines between the computer and the DAC, since these add propagation delay due to inductance and capacitance in the cables. The best situation is with the DAC right at the port connector on the back of the computer.

As an example, an entire passive SAR was built into a DB25 connector shell, with the DAC resistors wired directly to the connector pins. The DAC output was wired directly to the Channel 2 input (pin 12). The analog input signal was wired to this summing junction via a selected input resistor (10 k) from the signal connector, forming the complete SAR.

Running on a 33 MHz 486, this SAR system required no Wait delays to give clean waveforms. But adding extension cables between the computer port and the SAR connector made a big difference:

  ---- Connection ----     Wait   Max Sample Rate
  Direct                    0     31.4 kHz
  12 foot ribbon cable      5     19.5
  6 foot round cable        7     11.5

Although not shown above, note that the length of the cable carrying the analog signal from the outside source to the input resistor has no effect on the settling time, whereas any capacitance at the summing junction on the other side of this resistor will make matters worse.

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