After a lengthy struggle with a C-11 with no resolution, I went with a PMDX-106 which worked flawlessly from the get-go.
Great support too.....so they say. Didn't NEED any.
RC
Hi RC,
The Step/Dir to analog converters based on the LM2907 chip should not be better/worse than a PWM solution. As I produce both types, I have done a comparison of both. THE Step/Dir and PWM both produce very linear outputs with respect to their inputs. The Step/Dir has a slightly better response due to the faster update rate.
That said you need to understand how the LM2907 works to apply it correctly. Just implementing the datasheet application note does not really cut it.
The LM2907 converts the energy in the step pluses to a voltage using a charge pump. Basically, the more step pulses or the bigger(longer) the step pulses, the higher the voltage output. Therefore to get an accurate output voltage you need a number of things;
1. Regulated power supply. The power supply for the LM2907 needs to be consistent with the minimum of noise. VFDs are usually OK at suppling an adequate 10V supply. Many controllers such as the KBIC120 style are marginal. They use a 15V zener shunt power supply designed to supply a couple of mA to a 5K or 10K potentiometer. If it can't provide enough current, then the voltage supply will start to sag.
To make things worse, the controllers also have a max speed trimpot. This is just a resistor in series with the power supply. The voltage across this resistor is dependant on the current being drawn by the LM2907. The problem is that the current it draws changes depending on what voltage the LM2907 is producing. The result of this is that the power supply to the LM2907 sags, resulting in linearity problems with the output voltage.
The DC-06 overcomes this problem by providing an optional DC/DC converter that produces a consistant and regulated powersupply for the LM2907.
2. Constant and accurate step frequency. - Mach3 provides this.
3. Consistent and known step pulse width. - This is where most let them selves down. If you don't know what pulse width you are dealing with then you can't size the charge pump components correctly. The result of this is that if the step pulse is too narrow, not enough energy is being provided to the charge pump in the LM2907. If the pulse is too wide, then the chargepump ends up being saturated.
Having to adjust the step pulse width in Mach to get the charge pump in the LM2907 to work correctly, is poor design. The adjustment is there to cater for different types on interfaces to drives, opto isolated, bufferred etc.
The DC-06 contains a monostable that produces a constant width pulse to the charge pump in the LM2907. This occurs irrespective of the step pulse width input. That way, the energy per step pulse is known and the charge pump components can be sized correctly.
It basically comes done to understanding what you are designing and the environment that it will be working in. The datasheet applications are meant to be a starting point, not a finished design.
While I'm explaining the features of the DC-06. It also has a jumper to select the polarity of the input signals as some people use active hi signals and others use active lo.
I hope the above helps a bit.
Cheers,
Peter.
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