Parallel port circuit

[Kerri Ann]

i would like to be able to build a circuit that allows 2 or more devices to be controlled at the same time with the same parallel port... i.e.- a hd44780 character lcd and an led display, (or whatever).
i don't have any experience with programming, so i'd like the whole project to be primarily hardware based.

here are some of the thoughts i have had so far-

the pins on the parallel port consist of this:
8 data out pins
5 input/status pins
4 control lines
(then the rest are ground pins)

when interfacing the hd44780 to the parallel port, all 8 data pins are used, and 3 of the control lines. BUT, depending on the software used, one of those control lines is not needed by the lcd.
so, here's what i'm thinking- i can somehow use the 2 left over control lines as device selectors. i.e.- maybe there could be x number of devices at one time, each having it's own address. i'm not sure how i would go about addressing the devices yet, but i have a huge assortment of 74 series ICs to play with...
does anyone have any ideas about what i could use to make this circuit?

the reason i am wanting to do this is because i sell small character lcds that i modify for use with the pc, and i have just recently started to build some led circuits that are also controlled with the PP, soooo.... it kind of makes it difficult to sell the led meters to my customers when my lcds are already occupying their PP!

[ViperTX]

Well you're on the right track...using the the control lines as device selectors...you could use a 2 to 4 mux to increase the number of devices.....you could even piggy back them....boy then it would be shy of a night mare.

Good to see a woman on this site!

[H500]

If you need more than 4 devices, you could also use a serial-in parallel-out shift register. On chip will give to 8 addresses. You can daisy-chain them to get more.

[Kerri Ann]

thanks for the help guys. i am having trouble now with something....

i downloaded a nifty little app called parmon- it lets you see each parallel port pin's high/low status in real time. you can also toggle each pin (hi-lo)...
i got this app so i could see which of the various apps i use for controlling the lcds leaves 2 control lines open.
if i try to use the parmon app to toggle (what should be) an unused pin while the any of the lcd applications are running, after a second or two, the pin goes back to it's original state, like the lcd app is refreshing the state of unused pins or something??? is that what it is?
is the software possibly controlling the unused pins even though it doesn't actually need them?
i was hoping i could use existing software to control the lcds, and use my own software in addition to the lcd app for the selection of devices. i hope i don't have to write the code for all of it!

H500- ok, ive got here a CD74HCT164. SIPO Shift register...
so- would i need to use latches or something for each device? the 8 outputs on the 154 chip would be hooked up to the latch enables for each device...
like this...

[Kerri Ann]

ummm... i can't view the full sized image??? am i the only one?

- nevermind

[ViperTX]

Well you have to remember that the control pins are only active during the time something is being output...some of the control pins are bi-directional...I forget what each pin is called and don't have a parallel port assignment sheet that's readily accessible..

[ViperTX]

http://www.beyondlogic.org/spp/parallel.htm#6

Check this out for some hints.

[WayneHill]

Here is great source for information on interfacing parallel ports, Serial ports, USB Etc...

www.lvr.com

[H500]

If the lcds have a chip select pin, you would just need something like a hc244 to buffer the parallel port. Then the output can drive several lcd's in parallel. In other words, data will be sent to all devices at once. The shift reg is used to assert the chip select of the device you want to address. The unselected devices will just ignore the data.

[Kerri Ann]

Originally Posted by H500 If the lcds have a chip select pin, you would just need something like a hc244 to buffer the parallel port. Then the output can drive several lcd's in parallel. In other words, data will be sent to all devices at once. The shift reg is used to assert the chip select of the device you want to address. The unselected devices will just ignore the data.

so you're saying that instead of the 573 chips in my schem... i should just use something like the 244.
but all of the devices i will have hooked up in parallel will not have a chip select pin (although the lcds do).

here's something else that i am confused about...
lets say that i have an LCD which is showing the time (hh:mm:ss), and a LED meter (just 8 leds hooked up thru a buffer chip to the data out pins) showing a vu meter type display thru a winamp plugin, both devices hooked up at the same time. when the software gives the port the data, then thru the control lines selects the device it should be received by, we'll say it's the led meter this time... what will the lcd be displaying? is the port fast enough to send data to both devices? what about 4 or 8 devices?

[abasir]

Originally Posted by Kerri Ann here's something else that i am confused about... lets say that i have an LCD which is showing the time (hh:mm:ss), and a LED meter (just 8 leds hooked up thru a buffer chip to the data out pins) showing a vu meter type display thru a winamp plugin, both devices hooked up at the same time. when the software gives the port the data, then thru the control lines selects the device it should be received by, we'll say it's the led meter this time... what will the lcd be displaying? is the port fast enough to send data to both devices? what about 4 or 8 devices?

The LCD display is persistent, i.e., if you send data to it, it will display it until new data is sent. Assuming you're displaying time, the LCD data will only need to be updated every 1 second and it takes only take about 1/1000s to do it.

I'm both a programmer and circuit designer (by hobby not profession ), IMHO, if you're planning for two devices only, somehow monitor the LCD Chip Select lines as the demux signal, i.e., when CS is high... signal for LCD, when it's low... signal for the 2nd device. My 2 cents

[Kerri Ann]

that would be simple... i would kick myself later though if i end up making another circuit for the PP!

well, my brain is starting to hurt right now, so i'm giving this one a rest until later in the weekend. on sunday i will hopefully have some time to make a quick circuit and test it out with that parmon utility...
here's hoping i don't catch anything on fire! (yeah, ive done it before)