Cheap Idea for a DRO

[ecat]

Thanks again Bill, I'll check the speed and try the pullup.

The Zx reply was quite easy to work out :) As you can see in from the two sample replies in my previous post, Rx: appears as 0018 3A and Ix, appears as 0918 08. 'x' appears consistent, returning 18h, ':' is 3A which is correct, ',' appears as 08h.

Consistent, by this I mean that consecutive replies to a given request are often the same, or they fall into a pattern. Over time the values and patterns do change, with the possible exception of the ':'. Looks like a baud rate or level issue, or my Prolific is marginal.

No scale connected until I stop messing around and verify 1.9v operation, too costly to risk. What the PIC thinks it can see is another matter, stroking the component side of the board is enough to fool it into thinking something is there.

Can you confirm the value of the LED series resistor in the 1.5v supply part of the circuit ?

[BillTodd]

Double post caused by this bloody snow!

[BillTodd]

R? is selected to give about 1v5 across the red LED (probably in the region of 330R) this only necessary because the 1v5 scales have the cell +ve connected to the metal body, so to avoid grounding problems I've shifted the PICs Vss supply (and the scales own Vss) 1v5 below the system (PC and Machine) ground.

If your scale has a 3v lithium cell you may have to rethink the power supply, because I'm not sure if the 3v scales have their metal body connected to the +ve end of the cell or not (none of mine are 3v) .

The other LEDs in the outputs are just to limit the voltage to the scale when the PIC output commands.

[ecat]

k,

All callipers are 1.5v.
Maybe I have some wack LEDs, don't think I've measured less than 1.7v across the one in the power section and that was with >1k resistor, not good for maintaining voltage stability at those values. I thing the vf of my reds is around 2v

[BillTodd]

Try a couple of forward biased silicon diodes (e.g. 1n4148, or even 1n4000 series would do), they should give you about 0.7v each .

[ecat]

YaY

Progress of sorts.... Or maybe just chance!

43 48 3A 49 58 0C 30 0C 44 41 43 49 4D 41 4C 0C CH:IX.0.DACIMAL.
49 4E 43 48 3A 49 58 2C 30 2C 44 65 63 69 6D 61 INCH:IX,0,Decima
6C 2C 49 6E 63 68 3A 00 18 3A 03 18 3A 02 18 3A l,Inch:

May not look like much bit it makes me happy...

Had a dodgy scope and a very old Thurlby storage adaptor on it, it looked very much like the transistor was slow in turning off. The above is the first time I've seen anything approaching English, trick was to half the value of the 3k3 collector resistor.

Unfortunately the above was looking like a 1 off :(

Undeterred I added a 2nd and 3rd parallel 3k3 ending up with 825r collector resistance. heh. It's alive :)

49 58 0C 30 0C 44 45 43 49 4D 41 4C 0C 49 4E 43 IX.0.DECIMAL.INC
48 3A H:

My Prolific dongle is only giving about -4v on DTR, problem?

[BillTodd]

Hmm. -4v seems low, but if it works... I know I'm abusing the serial port, but I've used this circuit many times may be the prolific device is different to mine???

BTW are you driving it from a PC or laptop? (laptop USB ports seem to lack the ooomph required for most USB devices)


You could try sticking a 10u cap between the DTR line and Vss just to stiffen the -ve rail a bit.

Bill

[ecat]

More progress \o/

The sharp eyed would spot the problem in my previous post, I had no commas! That's right, every comma, 2Ch, was coming through as 0Ch which is form feed :( I tried reducing the collector resistance a little more and the comma appeared but so did the occasional funny in the rest of the message :( :(

A little reading showed me another trick, adding a resister between base and emitter is a common way to speed things up. 3k3 between b and e and things appear to be working beautifully :) :)

Hmm. -4v seems low, but if it works... I know I'm abusing the serial port, but I've used this circuit many times may be the prolific device is different to mine???

BTW are you driving it from a PC or laptop? (laptop USB ports seem to lack the ooomph required for most USB devices)


You could try sticking a 10u cap between the DTR line and Vss just to stiffen the -ve rail a bit.

Bill

Indeed Bill, -4v does look poor, not much margin for error. I'm connecting to a PC but via a powered hub. Another experiment perhaps.

The DTR line looks quite solid, at least it did before my latest changes, heh. A cap certainly wouldn't hurt, thanks for the tip.

As of now your PC app is mostly happy with the board. A couple of range errors when I simulate the calliper with a finger swipe but no data errors so far. Thanks again for your help :)

[ecat]

Not wanting to damage my actual scales I decided to risk my cheap but incredibly useful callipers at 1.88v. On the up side I know the meter I was using reads about 0.03v over actual, on the down side 1.85v is still around 20% above the voltage of typical fresh SR44 type battery . I'd normally consider 10% over to be a maximum safe(-ish) voltage but so far so good , I have the PC app working and I've set the scale to -6.66mm for luck. I'll leave it running for a couple of days and see how it goes.

Thanks again Bill, elegant circuit and nicely crafted software

[irving2008]

Gosh you are a busy bee Sean!...