The DPLPC board converts between an RS232C serial interface and a Data Products style parallel line printer interface. It allows connecting an old Data Products (or similar) printer to a PC COM port, and performs flow control. I built it in August, 2000, for an Ersatz-11 customer who had bought the similar Black Box product and was having trouble because it has too much buffering. Seriously! It's pretty annoying when the device happily buffers 50 pages of data, which the computer now thinks has been printed successfully, but it turns out there are only ten sheets of paper left, and the other 40 pages never got printed. Or, there can be confusion between whether the printer is actually finished or just paused, and someone wants to change forms. The computer thinks it's finished, but then after you take out your green-bar paper and load your checks and put the printer back online, it prints a zillion pages of compiler listings onto your beautiful checks.
So the customer wanted a nice dumb converter, and that's what I made them. It can just buffer one line, and generates XON/XOFF or RTS/CTS flow control as needed, but that's about it. It's just an i8751 (I used a Dallas DS5000T for development), with a MAX232 for RS232C level conversion, and 7406es and a 74LS14 for driving/receiving the Data Products bus, and a 93C46 serial EEPROM for configuration. That's where I got carried away ... there's a jumper on the board which sets "config" mode, then you can plug an ASCII terminal into the board at 9600,N,8,1 and talk to the command prompt. It has SET and SHOW commands for all the parameters I could think of, and can save or restore the config to/from the EEPROM.
I didn't want to deal with making my own Data Products cables. Those 50-pin Winchester connectors are expensive and hard to find (Black Box was the only place I could find that sells them loose). So I used Black Box's cables, which they make for their own Data Products converter, but they're willing to sell the cable separately for about $40 (at the time, anyway).