I am working on a project, the door control system I am using is a bit old, and it works through a serial connector on a 4 pin RJ11 line. I am currently trying to wire an old serial wire into its RJ11 wire, so I can get my raspberry pi to work with it. (I am using a USB to serial connector on the pi)

However, the problem is I don't know which wire goes into what. The door controller is one I got was from a friend's deconstruction project. Its a Stanley Access Technologies MC521 controller.

The RJ11 has 4 colors on it, black, red, green, and yellow from order left to right. And has the text RS232 near it. A signal was detected on the red wire when it boots on.

any ideas?


2 Answers 2


You may be able to trace the connections out on the board. If it goes to a standard RS-232 interface chip such as a MAX232 or MC1488/9 then you have at least part of the answer.

If it's a 3-wire interface (pretty common) there will be only three wires required- ground, which you should be able to trace out easily. Data input which will go somewhere on the board (other than ground) and won't measure much, if any, voltage when the unit is powered, and Data output, which should measure significant voltage (mark state/idle voltage on RS-232 is usually -9 to -15V).

That's assuming it's RS232. If it's some other serial protocol, you're not going to have much luck figuring it out, I fear. Also, if it can affect security there will probably some protection against spoofing the signals, cryptographic or otherwise, for obvious reasons.

  • 1
    \$\begingroup\$ It has 4 wires, from left to right. Black, red, green, and yellow. It am detected a signal from the red one when hooking it up to the rx wire in the serial port. \$\endgroup\$ Jun 24, 2015 at 14:21
  • \$\begingroup\$ also it says RS232 next to the port \$\endgroup\$ Jun 24, 2015 at 14:36
  • \$\begingroup\$ Is one of the wires connected to the chassis? \$\endgroup\$ Jun 24, 2015 at 14:53

Look up the technical specifications for the door control system.
There isn't really a standard for RS232 on an RJ11 connector. You just have to see what the manufacturer dreamed up and build a cable to match.

I cannot find a pinout specific to the Stanley unit. You might just google for "RS232 RJ12" and look at the diagrams that come up. I found a couple of different types, so that's a guessing game.

  • \$\begingroup\$ There isn't a technical specification for it, I checked. There were literally specification for everything, but the COM port. The cable was a proprietary cable, any ideas what it might use from the serial port? The cable isn't available from the manufacturer anymore, and I rather narrow down from the 1680 possible configuration it might have. \$\endgroup\$ Jun 24, 2015 at 13:19
  • \$\begingroup\$ Post the manufacturer's name and the model of the system. Maybe someone can find it for you. \$\endgroup\$
    – JRE
    Jun 24, 2015 at 13:21
  • \$\begingroup\$ Stanley Access Technologies, MC521 controller. \$\endgroup\$ Jun 24, 2015 at 13:33
  • \$\begingroup\$ Source for a premade cable: pacificcable.com/Picture_Page.asp?DataName=MC521 Still looking for pinout. \$\endgroup\$
    – JRE
    Jun 24, 2015 at 13:41

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