Tuesday, January 22, 2008

Make: An RJ45-DB25 Serial Connection

The next device that I will make is an RJ45-DB25 serial connection. To the best of my knowledge, DB25 ports can be built to accommodate three types of connections - serial, SCSI (pronounced "scuzzy"), and parallel printer ports for IBM-compatible PCs. The differences are in which pin is a given signal or voltage sent or received. The number of pins vary, as well.

The website Nullmodem.com has a page that lists the pin configurations for all three DB25 connections. Ossmann's instructions for the 5-in-1 cable also includes how to build an RJ45-DB25 serial plug. Scroll down to 'Extras'.

Comparing Ossmann's connection diagram to the one shown on Nullmodem, I see one DB25 pin missing from Ossmann's diagram. That is pin 22, Ring Indicator. The Ring Indicator is, according to Lammertbies.nl, "used by modems to signal an incoming call to the attached computer". Maybe this signal is not necessary nor used if one is in a situation where an RJ45-DB25 connector is useful?

In any case, when it comes to the other DB25 connections, SCSI and parallel printer, neither can pin out to RJ45 easily, if at all. Both use more than 8 pins to communicate.

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