Step 1 : The Standard Parallel Port
To build a parallel port control box we must first understand how the PC's parallel port works. In this How To we will only discuss the Standard Parallel Port
. There are also Enhanced Parallel Port (EPP) and Extended Capability Port (ECP) modes on newer ports. If you have a newer parallel port that can be set to specific modes, be sure to set it to Standard mode when following this How To.
There are several pins that are very important to us to control our Christmas lights (or anything else). Most people focus on the 8 data pins and suggest that you can only control 8 circuits with each parallel port for a total of 24 circuits if you max out your PC with three ports. This is only partially true. For the simple schematic, 8 is about all you can do from the data bits
without using multi-write hardware/software. However, we do still have 4 control bits
we can use as well. These bits can be used the same as the data bits for a total of 12 circuits for each port. All 12 of these bits are latched. This means they stay "on" or "off" until they are set differently which makes very simple circuits for controlling our lights. The control bits
are a little bit more complicated than the data bits
since three of them are inverted. This means that if you want the signal to go high (+5V), you must send a zero (0V) to the bit and it will automatically be inverted (notted) and output a 1 (+5V). You are also using a slightly different base address to access the control bits
. The parallel port base address for the first port is typically 0x378 (in hex) and the control address is base+2. Still, in the scheme of things, it is still pretty easy and you gain 50 percent more circuits! So, you can have 36 total circuits with three parallel ports and no additional hardware. This is still a bit limited and we are going to look at a multi-write solution to have MANY circuits controlled from one parallel port with a little bit of hardware added.
Here we go.....