[boyvox]

Does the parallel port itself determine what the voltage of High and Low logic signals are? My parallel in my PC is putting out 3.4-3.5V on the High signal and .3 on the low. Those are not working out well with my drive. It would help me a lot if the 3.5V High was more like 4V or up. Will a different parallel port operate differently (and perhaps give me a stronger High signal) or are they all the same? How do PCI slot parallel ports generally differ from the built in ones. Does the PCI slot offer any advantages?

[ger21]

Newer PC's use lower voltages on the parallel port. A PCI card should give you the higher voltage you need. You can get them on Ebay for about $10

[gar]

051010-1549 EST USA

boyvox:

In the late 1960s TTL (Transistor Transistor Logic) became commerically available and replaced RTL. Most circuitry over the 30 years from the origin of TTL has been designed to be compatible with TTL signal levels.

Roughly speaking the input specifications on TTL are (see TI "The TTL Data Book"): minimum input voltage for a logic 1 input is 2 v, and the maximum input voltage for a logic 0 is 0.8 v. The specifications for output voltage are based on a maximum load on the output that varies depending upon the series (74S, 74, 74LS, 74L). The maximum logic 0 output level is 0.4 v (thus 0.4 v below the maximum input for a logic 0), and the minimum logic 1 output level is 2.4 v (thus 0.4 v above the minimum input for logic 1). These values differ slightly by series.

Many chips used in parallel ports are designed around these specifications. Thus, the voltages you mentioned are to be expected.

There are lots of considerations in connecting something to a parallel port.

It would be an advantage to you and many others to get a good understanding of the devices you are working with. The voltage differences between logic 0 and 1 are relatively small and thus susceptable to small noise voltages. To build circuits that are going to work with good reliability you need to understand the operation of these circuits.

.

[boyvox]

Thanks. I picked up a PCI slot parallel port card today and it had 5 volt High Signals which solved my problem since the parallel port installed on the motherboard only had 3.5V High signals. I also found this page

http://www.epanorama.net/circuits/parallel_output.html

that has a bit of info on using parallel ports.

Thanks guys.

[greybeard]

Thanks for that parallel port link Bovox. Very useful resourse.

[ElectronicsEng]

Thanks. I picked up a PCI slot parallel port card today and it had 5 volt High Signals which solved my problem since the parallel port installed on the motherboard only had 3.5V High signals. I also found this page

http://www.epanorama.net/circuits/parallel_output.html

that has a bit of info on using parallel ports.

Thanks guys.

The parallel port on your motherboard was most is most liekly using apretty modern driver chip which is running off 3.3V supply, hence the lower maximum output voltage. Orginally these ports all used TTL and in fact the output could be as low as 2.4V, or possibly even lower and the things attached to it would still work perfectly. The card you bought is probably using somewhat older (and thus cheaper) integrated circuits whch are running on 5V and so produce closer to that with lightly loaded outputs.

The big culprit here is probably the input device in your peripheral. Parallel ports were intended to drive TTL inputs. If you external device has CMOS type imputs running on 5V then it possibly needs as much as 3.1 to 3.4 volts to be seen as a valid high level signal. It SHOULD in fact be designed to see a valid high at 1.7V if my stale old TTL memory serves me correctly. But as most of the modern parallel ports use MOSFET type outputs with pull-up resistors at low current they will deliver near rail voltage. But watch out of you draw more than a nominal load from them! The voltage will plummet rapidly.

..john

[smarbaga]

most third party products (like the pci port u got) are designed to function in the upper spectrum of the specifications they are intended for because the developer never knows what kind of use of a system they will be placed into. this gives the manufacturer an advantage over oem parts . they need all the advantages they can get to sell there product over oem stuff. it doesn't cost (dollar wise) any more to have a logic 1 at 5 volt than it does to have it at 3.5v , it costs in current and battery life of the original equipment it is being used in.