printer port buffer.

[jim]

Hi,

I have 2 Velleman instruments (a 'scope and signal generator) to use with a computer via the parallel printer port. These work fine with a normal desktop PC however with a laptop they refuse to work properly.
I contacted Velleman and they said this is due to the low power printer port used on laptop's. I thought about building some kind of buffer so I can use the instruments with my laptop.
Does some one have experience with this kind of designs ?

Thanks in advance,
Jim

[gmarsh]

Buffering a "classic" printer port is straightforward - you can use a 74xx244/245/373/573 IC to buffer the D0-D7 lines, and power the IC with a 5 volt supply taken from a USB or game port.

There's no need to buffer any lines other than the data lines, as control lines are generally open collector and are pulled internally to 5V using 4.7K resistors, and hardware designers should know better than to draw power from them.

If the equipment you're using does bidirectional transfers on the printer port's data lines, then building a buffer is a lot more complicated.

[jim]

gmarsh,

I think it works bidirectional, but only (I guess) for the 'scopes data which does work with a laptop. I forgot to mention that only the signal generator doesn't work properly.
So that means there's no easy solution ?

Thanks anyway,
Jim

[gmarsh]

do you know if the scope uses EPP or ECP mode?

For anything other than the scope (eg, a signal generator) then using a TTL chip to buffer the data lines as described above should work.

[jim]

Hi Gmarsh,

I received the schematics from Velleman a few yesterday and as far I can see it doesn't work bidirectional i.e. the datalines are used to set up the sig. generator and the scope whereas the data from the scope is sent to the computer via the "Acknowledge" line (pin 10) and "Paper Empty" (pin 12).
So I think it must work buffering the data lines.

I've made a buffer which I'm going to try this evening.
Thanks for your help so far.

Jim

[jim]

Hi,

Just tried out the buffer powered from the usb-port.
Now both instruments work on my laptop.

Thanks again,
Jim

[jackinnj]

here's a truely isolated parallel port interface -- I have used it -- you can crush a computer's circuitry with a bad ground:

[jim]

The problem with the printerport of my laptop is that it can't drive all the optocouplers used in the Velleman scope and signal generator. The laptop is isolated from the measuring equipment with those optocouplers so I don't think there's a problem here as I use the +5V power from the USB-port.

Greetz,
Jim

[mevangel]

Hi Jim,
can you report your experiences with those Velleman PC based instrum ents? Do you find them satisfactory for serious diy jobs? I am thinking to buy the PCS500 because the price is very attractive for a 50 MHz DSO + 25 MHz spectrum analyzer...
Thanks.
Marco

[jim]

Originally posted by mevangel
Hi Jim,
Do you find them satisfactory for serious diy jobs? I am thinking to buy the PCS500 because the price is very attractive for a 50 MHz DSO + 25 MHz spectrum analyzer...
Thanks.
Marco

As far as I can tell, yes !!
Maybe not far very high frequency-stuff but very satisfactory for my needs.
If you use it in combination with the velleman signal-generator you can also use the nice bode-plot function.

Greetz,
Jim