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Step 2: Hardware Modification: Soldering

By unscrewing the 3 screws on the rear side, one can open the top cover and expose the main circuit board (make sure the plotter is unplugged from the mains).

There are only 4 lines that will be needed: RX, TX, GND and RTS.

Since these plotters have very limited memory, it is very easy for data to be lost and have the plotter do unexpected things, therefore hardware handshaking will be required. This just means that the plotter will tell the computer over the RTS pin when it has enough memory available for new incoming data.

Solder 4 wires directly to pin 1, 2, 5 and 6 of the plotter's USART chip (GND, RX, RTS and TX of the chip respectively) and run them out the back to a header pin strip for the RS232 module.

I added a cable tie to prevent the wires accidentally being ripped out.

Gently close the top cover paying attention to the paper lever.

JesusB594 months ago
Good work, may i ask you a question ?
i have a SEKONIC XY PLOTTER SPL 430. It look like the HP 7475 but the mother card is not the same so i can't connect the FTDI Basic breakout at the same place like you.
Th mother card of SEKONIC PLOTTER is : H PCB E 144 TK 00081-A
Have you an idea of a side i can plug the FTI circuit ?
Thank's for answering