Parallel port 3.3v, any way around this?

I have a Hermes Vanguard 7200 I'm converting to use linuxCNC. Because the machine was given to me and I don't have a lot of cash to toss around I'm trying to use as much of the hardware that came with the machine as possible, including the proprietary driver board. I've done quite a bit of tracing and probing on the Hermes driver board and I have found the step and direction pins for each axis, as well as the limit switch pins. I thought I had everything put together and was ready to start running the machine but I've arrived at a small problem.

The driver board has opto-isolators, which is really nice and I'd like to use them, the input circuit was well thought out and uses a common +5v so the parallel port is sinking current from the opto-isolators. Unfortunately when I connect the board to the parallel port I am not able to get the desired results. I've confirmed that the parallel port is in fact working, I am able to flash an LED connected to the parallel port between pin 2 and ground (with a current limiting resistor of course) using PTEST. Unfortunately when I connect the same LED applying +5v from a USB port to the anode and connect the cathode to pin 2 so that the parallel port is sinking current, the LED illuminates when the pin is low and becomes dim when the pin is high, instead of turning off completely. I believe that the LEDs in the opto-isolators are acting like the LED in my test circuit and only becoming dim with the high output signal, the transistors that make up the other half of the opto-isolator never go low so the stepper motors never turn.

So after more probing and more head scratching it appears my parallel port is only outputting 3.3v on a high signal, so it makes sense that with 5v applied to the LED there is enough current flowing through the LED to illuminate it dimly. I'm not an electronics guy, I'm a mechanical guy who has figured out enough electronics to be dangerous, I want to believe that I can put a resistor on the +5v line going to the opto-isolators to drop the voltage to somewhere near 3.3v and things will start working like magic, but I feel like that's not going to work. Here's the datasheet for the optocouplers pdf1.alldatasheet.com/datasheet-pdf/view/82579/TI/TIL193B.html I'm not very good at reading datasheets for electronic components I'm not even sure if the optocouplers will work with less than 5v, can someone look it over and give me some ideas? Is there any other way to get this thing going or will I have to buy a PCI parallel card that outputs 5v?

Hi

You should be able to use a buffer board to up the parport signals to 5v

www.cnc4pc.com/Store/osc/product_info.php?products_id=203

A PCI parport is likely to be 3.3v as well, everything has been for 10 - 12 years

regards

A very simple patch is to power the BOB from 5V fed through 2 1N4001 Diodes
(giving very roughly 3.7V which -3.3V = 0.4V which is not enough to turn on the OPTOs LEDs)

andypugh wrote:

kornphlake wrote:

when I connect the same LED applying +5v from a USB port to the anode and connect the cathode to pin 2 so that the parallel port is sinking current, the LED illuminates when the pin is low and becomes dim when the pin is high

That surprises me, I wouldn't expect the parport to be able to sink current when high. (In fact I thought that was all that allowed the 3.3V ones to work as parports).

Is there any scope for running the input optos from a common 3.3V supply rather than 5V?

Nope, no surprise. The parallel port output pins are clamped at 3.3 V, so if you have 5 V on the LED anode, then current will flow from the 5 V source to the 3.3 V clamp of the parallel port. 5 - 3.3 = 1.7 V, and is enough to turn on a red (or infra-red) LED. If you lowered the 5 V a bit, then the LED should not turn on, and maybe that would work in your application.

Jon

andypugh wrote:

jmelson wrote:

andypugh wrote:

Nope, no surprise. The parallel port output pins are clamped at 3.3 V, so if you have 5 V on the LED anode, then current will flow from the 5 V source to the 3.3 V clamp of the parallel port.

OK, I haven't bothered with parports for years

But I had the impression that they were pulled-up open collector.

How do 3.3V P-Ports manage to drive 5V printers. Or don't they?

TTL logic levels have a decision point somewhere between 1.6 and 2 V, so a 3.3 V high level is quite adequate for MOST devices.

Jon

I pulled 3.3v from one of the orange wires coming out of the PC power supply and used that for a common and the driver board is working as expected. I'm not entirely happy with the alligator clips I used to connect it though, I'd prefer to have a standard connector like USB that I can get power from, I don't really want to have to make any customizations to the PC, it should be easily replaceable down the road. I am thinking something like this www.ebay.com/itm/AMS1117-3-3V-DC-DC-Step...&hash=item51d147ef22 might be the best solution and will integrate with my break out board pretty easily.

Now to figure out my limit switch problems... but that's a discussion for another thread.