Opposite of "*/" ??

Turnbull2112

How do I work the */ operator backwards? I have an 8 bit reading from an ADC that I'm adjusting with a multiplier and shifting back out a DAC. I need to get my adjusted value back to an 8 bit number (0-255). This is probably a stupid and obvious solution but I'm stuck.

Thanks,
Code attachedExcerpt ADC issue.bs2
Rob

Tracy Allen

I don't see the problem. The snippet you posted does not change the O2result that comes from the ADC. The */ SPAN operator is applied to get a second variable, O2conc, but it is O2result that gets shifted out to somewhere but needs to be changed.

Could you please explain a little more what you are trying to do?

Turnbull2112

Sorry about that, before I added the "RCTIME/SPAN" code is was just using O2conc = O2result */ 1000 to get my numbers. Now that I'm using the "SPAN" to adjust O2result, what would I SHIFTOUT to the DAC? If I use "O2result" the signal generated by the DAC will be uncorrected. I need to SHIFTOUT a 0-255 result to the DAC. How would I get "O2result */SPAN back into a 0-255 number to send to DAC? If I use "O2conc" the DAC sends out crazy values. Sorry if I'm not making much sense, I'm fringing on the limit of my programming vocabulary.... lol

Thanks,

Tracy Allen

What value of O2result (or O2conc) should cause a value of 255 to be SHIFTed out? That will determine the next scale factor. For me, the issue is not so much programming, rather, a description of the project itself.

Turnbull2112

I'm reading an oxygen sensor that gives me a 0-5v output for 0-100% O2. I'm reading that through an ADC0831. I'm displaying the concentration on a serial LCD and also sending the value out of a 4-20mA control loop, 0 = 4mA and 255 = 20mA ( 0-100%). The AD420 which drives the loop is 16 bit but I'm sending it 8 bits, my resolution is a little blocky but not an issue. The O2conc is what's displayed (max value is 1000 to get tenths) but I'm sending 0-255 to the AD420 for remote scaling in a SCADA or PLC. How would I include the trimmed value so that the AD420 can interpret it correctly?

Tracy Allen

Okay, the full scale range is supposed to be 4mA loop output when O2conc=0, and 20 mA when O2conc=1000 (100.0%).

Into the DAC, that requires input codes from 0 to 65535. It is best if you scale up so that you can send a full 16 bit quantity to the DAC (but still it will have only 256 steps, which you say is okay).

[B]LOW[/B] latch
[COLOR=#999999][COLOR=#000000][B]SHIFTOUT[/B] datain, clock, [B]MSBFIRST[/B], [(O2conc */ 16777) \ 16]  [/COLOR]'****new approach[/COLOR]
[B]HIGH[/B] latch
[B]DEBUG[/B] ? O2conc */ 16777

The math (O2conc */ 16777) will scale up to 65535 when O2conc=1000. Note the \16, which causes the SHIFTOUT to send all 16 bits.

Turnbull2112

Excellent! I will give this a try and post my results. Thanks Tracy!

Rob