Hi Eric,
I replaced the parallel cable with one of the same length made up from cat5 network cable. Even though the network cable has 10 times less resistance the data on the DATAOUT line is still getting corrupted. I think this is because the signal from the ADC is too weak to send down any significant length of cable. I think I'm going to have to use a buffer of some sort. I was thinking of the 4050B or would a darlington array be better suited?

Thanks,
Salvatore.

 

hi,
I would use a HEF4050 non invert buffer, rather than the darlington array .
Its possible that the ADC is unable to drive the ports TTL input directly, as its also got a pullup resistor to +5V.

I use a AD7705 dual 16bit ADC thru the port without problems, but I will have a closer scope view of the ADC Dout signal.
Have you a good cro/scope, so that you could examine the Pin#10 signal?

Regards
Eric

 

Hi Eric,
I have an old beast of an oscilloscope, a large and heavy valve operated tectronix that had served me well, I think remember seeing something very similar in one of the older James Bond films.. . Unfortunatley I think one or more of the valves have gone ( determined by not being able to see as many glowing points of light as before as viewed through the back of the grill ) which means the 'dot' on the screen is now just a fuzzy blob..shame.. .
As an experiment I placed a transistor (2N7000) with a pull-up resistor on the DATAOUT of the ADC and used this to drive the signal down the cable. It inverted the data bits but it worked fine and I was able to use the original parallel cable, so this confirms use of a buffer of some sort for all of the inputs/outputs. You say that a hex buffer would be preferable over the Darlington, I would have thought the Darlington would give the best chance of driving the signal down the cable? Ideally I need a package that will accept TTL/CMOS inputs as well as provide TTL/CMOS outputs, will the 4050 do the job?

Thanks again,
Salvatore.

 

hi Salvatore,
As the ADC is a CMOS device I would use the HEF4050 on the Dout line, its designed to accept CMOS inputs and drive 2 * TTL inputs [fan out]
Refer attachment.

If you would like to study alternatives look at the HCT device range of ic's.

I am currently trying a HEF4050 on my parport project board, it works fine.

Regards
Eric

Attached Files

HEF4050.pdf (28.0 KB, 2 views)

 

Hi Eric,
Looks like the 4050 will do the job for the ADC DATAOUT, and correct me if I'm wrong, although not specified as TTL to CMOS, according the datasheet it looks like it'll do the reverse and accept TTL levels from the parallel port and output CMOS levels for the ADC. If not, do you know of any 'all-in-one' packages that will accept TTL/CMOS inputs and also be able to drive TTL/CMOS?

Salvatore.

 

Hi Eric,
Had a closer look at the 4050 and I'm not sure that it will be safe with a TTL input as the high level input voltage is specified at V2.75. Although my PC's parallel port seems to output a + 4V 'HI' I can't be sure this will be the case on another machine. I found this though: 74HCT241/244 ( see attached datasheet ). Looks like it's designed to do the job both ways in one octal package, what do you think?

Salvatore.

Attached Files

74HCT241.pdf (38.4 KB, 1 views)

 

Hi Salvatore,
I have completed the test of the HEF4050 bfr on the parport with the AD7055 the edges are cleaner/faster.
I would not expect the signal/lines on the parport to much more than +4V, when high.

If you are designing a commercial product and the build budget can absorb the cost of the 74HCT,
it would be wise to try the 74HCT24X.

If you want to keep it simple, using the 2N7000 on the Data line, have a look at pin #11, its inverted within the PC's parport.

For ref, a link. www.interfacebus.com/voltage_threshold.html
Click on the 'voltage translation' link shown in the text

Regards
Eric

 

Hi Eric,
This is purely a personal project, no commercial intent..! Thanks for doing the analysis on the 4050! I'm in two minds as to whether to use a buffer, it would be handy to be a able to utitlise the spare data and status lines for extra input/output switches, but it also depends on how it will perform when sending the signal down a 2 meter length of parallel port cable. The 2N7000 on the inverted status pin is attractive as it is simple ( and works! ) and keeps board space down. This is important as I have decided to use a precision voltage reference IC for both the ADC and DAC reference inputs. This should help cure the slight jitteryness ( several mV, equivalent to about 2-3 LSB's ) on the ADC results, more than likely due to the relatively bad line regulation of the 7805 I'm using to power the ADC and DAC. I was aware of the link you posted, it has been very handy indeed..!

Thanks,
Salvatore.