ok, i think i am about to start a pretty big project, i just hope i'm ready for it. with the help of a very experienced programmer, i'm (hopefully) going to make a circuit (and the software which will support it) that will allow me to control 2 circuits simultaneously through one parallel port. i have only ideas going right now, but i'm hoping that i can get some more ideas and opinions on it from you guys. my first ideas revolved around 2 devices- an LCD and a LEDMeter. After some more thought, i could maybe control any 2 devices... as long as they don't need more than 4 control lines combined. the software would have to recognize /identify the devices. i think the HD44780 LCDs use 3 of the control lines when interfaced to the port, so that leaves 1 inverted control line for another device. i assume the status pins on the port are for "printer busy" "printer on-line/offline", right? data input pins.... maybe use some 74 chips like AND Gates to switch between the control lines???
You could always try this: You can have 64 individually controlled outputs using 8 74HC573 latches being enabled with a 3:8 decoder (74HC138). The black line is a data bus (all D0 connected together, all D1 connected together, etc etc) and each latch is enabled by the demux by setting the 3 output bits (14, 16, 17) and enabling the demux with pin 1. Ooops, just realized that the 74HC138 is active low, so all of its outputs will need to be inverted. But hopefully you get the idea.
yeah of course i could. but that's too easy! actually, the reason i want to do it like this is because i sell these lcds to people, and i started also making the LED meters... hard to sell both to the same people since that would require them to buy the extra add-on card. i would really like to sell the two as a package... and leave open the option for different devices later on.
Hazer, i think i follow you... the 74hc238 has non-inverting outputs and has the same functionality... btw- your db25 pins are upside down... that's just the sort of stuff i was hoping for though, thanks Hazer!
There's also the DiscoLitez method that gives 8, 16, 24 or 32 outputs and this Computer Christmas how-to goes to 320 from 1 PP.
thanks guys.... i have another question.... when i wire up my lcds to the port, i am using 3 of the 4 control lines... i don't think i need all 3 though. the control lines on the port are pins 1, 14, 16, and 17. in the lcd's circuit i have the pins connected like this: Parallel LCD 1............. 6 14........... 5 16........... 4 17........... (unused) according to this site: http://www.repairfaq.org/filipg/LINK/F_LCD_progr.html, the control lines are used for the lcd as follows... "Pin 4 is register select. When high you are writing data (characters to the display) and when low you are writing to control registers. Pin 5 is Read/Write. Can be tied low since a basic implementation has no need to read from the display registers - but is useful in more advanced applications. Pin 6 is Enable. The data on pins 7-14 is latched on the falling edge of this signal. Just strobe it low after you have set up data on the data pins." so i can tie pin 5 on the lcd to ground instead of using the control line on the port.... right?
Yes. But, if your using LCD software that was downlaoded, it may use the read function and mess things up. If your using your own code, then you can tie it to ground.
i am using the 573 chip as mostly a buffer, so i can get rid of it altogether and use the ULN2803 instead, (thanks hazer, cpemma!), BUT, i still need a conditional gate or latch... i need it because i plan on eventually having another circuit hooked up to two of the control lines on the PP which will be directing the data to two or more devices... here's the schematic- none of the control lines have been connected yet, only the data pins, but eventually, i will have two of them connected to the hc573... they will enable or disable the latch... i think. comments?
i found a guide here that shows how to build an ISA card that i think will do what i am wantng to do. (which is controlling more than one device simultaneously). how difficult would it be to figure out how to make this card PCI?
ISA != PCI! They aren't even similar. PCI is higher speed and has much, much, much tighter specs. It would be very, very, very difficult for you to do this.
as nleachim said the PCI specs are too hard to be able to make something function on them easily. When PCI first came out there were such strange errors (mostly caused by voltage/signal degrigation). The LPT port is easy, some machines present the I2C SMB, which is fairly easy to tap into (by present i mean have a header there begging for you to plug into). programming with the SMB isn't *THAT* hard either.
i always wondered what the heck those headers were for on my mobo. in my mobo's manual, it has a pinout.... i'll be googling to find more.
