AXE027 Cable interferes with daclevel?

[RonHackett]

I've been using a simple 08M2 program to measure the voltage levels produced by the daclevel command. The circuit uses a +5V regulated power supply (actual V+ = 5.08V), and the AXE027 cable for programming the 08M2. I'm not using an op amp - just a DVM to measure the voltage at the DAC pin. For example, to determine the maximum voltage output (at DAC level 31), I used the following code:

  dacsetup %10100000
  daclevel 31
  stop

When I first ran the program, I obtained a voltage reading of 2.60V - far below the 4.8V or 4.9V output that I expected to see. I re-ran the program several times with different DAC levels - the majority of the results were way below what I thought they should be. Then I remembered the two resistors that are attached to the TXD line in the AXE027 circuitry. To determine whether they were affecting the results, I disconnected the AXE027, and again measured the DAC voltage. This time, at DAC level 31 the voltage was 4.90V. I repeated the measurements at several DAC levels, with the AXE027 in the circuit, and without it. Here are the results:

DAC| Cable | Cable
Lev| <Out> | <In>
------------------
00 | 0.02V | 0.08V
01 | 0.18V | 0.18V
02 | 0.34V | 0.25V
10 | 1.59V | 0.49V
20 | 3.17V | 0.78V
30 | 4.75V | 2.15V
31 | 4.90V | 2.60V

Just to be sure all this isn't due to the fact that I wasn't using an op amp in the circuit, I added an MCP6021 op amp, and did another set of measurements, this time on the op amp output voltage. The results were similar: At the majority of DAC levels, whenever the AXE027 cable was connected, the voltage was dramatically lower than it was with the cable disconnected. (Levels 0 and 1 were the only exceptions.)

I did a search of the Forum, and couldn't find any mention of an issue like this, so I'm hoping someone can shed some light on this situation. Is the AXE027 cable the culprit here, or am I missing something?

 

[KeithRB]

Since the DAC pin is shared with the serial out pin (which is connected to the download cable)...

Check out the 08M2 pinout diagram in Manual 1.

 

[Technical]

DAC is on output C.0, which is also the serial output whilst programming.
So yes, the C.0 pin will be electronically connected to the circuit inside the AXE027. As DAC is a voltage reference only and has no direct drive capability, any other connection apart from the recommended op-amp will affect the DAC reading.
In this situation you are better off with a jumper link to switch between the two functions as required.

Or use an 18M2, where it is a different pin.

 

[RonHackett]

Yes, I know C.0 is also the Serial Out pin - that's what made me think of the resistors in the AXE027 cable. Since the documentation in Section 2 of the Manual doesn't mention the need to disconnect the programming cable, I was hoping there might be another option that I wasn't aware of.