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Building Serial Cables

This section describes how to interface your RS-232 device to the MIT RJ-11 serial standard. The MIT RJ-11 serial standard works for:

Most people will want to start with the commonly available DB-25 and DB-9 to RJ-11 converters (generally available from DigiKey or Jameco). The part numbers for DigiKey are included at the end of this section. If you buy one that fits your computer, the rest of the guide will tell you how to connect its internals to match the MIT RJ-11 standard.

Serial ports on various machines

IBM PC and compatibles

IBM PC or compatible may have either a male DB-25 or a male DB-9 coming out the back. (A female DB-25 is typically a parallel printer port, while a female DB-9 is typically for an old EGA-style monitor).


Macintosh computers have two 8-pin mini DIN connectors on the back. One is labeled with a phone icon and is called a modem port. The other is labeled with a printer and is called a printer port. For our purposes, the hardware for these ports are identical.

The best way to hook up to a Macintosh is to buy a "Haye's compatible modem cable" which acts as a mini DIN to Male DB25 adapter. The Male DB25 works just like IBM and compatable DB25, and should be wired the same. Macintosh "Printer Cable"s are identical except that they have pins 2 and 3 swapped. (Often, Mac MiniDIN-8 to DB25's aren't labeled, so you won't know which you have. Either experiment, or try adding a null modem to make yours work).


SPARCstation 1's have two female DB-25's coming out of the back of the box (`/dev/ttya' and `/dev/ttyb'). Use the instructions for making a phone to male DB25 converter below.

Other SPARCstations have only one female DB-25 for serial, labeled "A/B". The standard male DB25 adapter pinout will let you hook up to port "A" (`/dev/ttya'). Port B uses a disjoint set of pins on the same DB-25, so you'd probably want to make some sort of custom Y-adapter to get access to both ports at the same time. (This document doesn't currently contain the pins of this DB-25 used for port "B").

Still other SPARCstations have 8-pin mini DIN connectors (they are round with 8 small pins). These connectors are the same as Macintosh serial connectors, and the same instructions should be used.


The VAXstation 2000 has a male DB-9 coming out the back (`/dev/ttys1'). *WARNING* This is a non-standard pinout -- most DB-9 serial hardware you'll find uses the IBM-PC standard. Use the VAXstation 2000s instructions below, and be suspicious of random DB9 hardware you find.

The VAXstation 3100 has two sort-of phone-like connectors coming out the back (arrows icon: `/dev/ttys0' and printer icon: `/dev/ttys1'. On athena, `/dev/ttys0' doesn't seem to be usable). The connector is very similar to the phone connector that are used on the robots, except that the springy tab is offset. When you find the right male connector, you'll need to make a phone cable. Because the VAX uses opposite pinout of our RJ11 standard, you'll need to make the cable with the wires STRAIGHT THROUGH; the colors should not swap sides, unlike the normal phone cables used elsewhere. But don't worry, you can't confuse a VAX cable with a normal cable because the special VAX connector can't be plugged in to standard RJ11 jacks.


The DECstation 3100 has two sort-of phone-like connectors coming out the back (arrows icon: `/dev/tty00' and printer icon: `/dev/tty01'). See VAXstation 3100 below for how to wire up to these connectors.

DECstation 5000 has two IBM-standard male DB-25's coming out of the back of the box (`//dev/tty00' and `/dev/tty01'), and you can use the standard instructions below.

Serial adapter construction

*********************** WARNING WARNING WARNING ***********************

Different brand phone adapters use different color schemes!! Read the following paragraph to find out which type of phone adapter you have. (Depending on which kind you have, you may need to swap "Yellow" and "Black" in the following instructions).

Some phone adapters have 4 wires, some have 6. For those that have 6, ignore the outside two (the blue and the white). You can cut them off if you want.

Your adapter should have red and green wires in the middle, and black and yellow wires towards outside of these two. But different adapters swap the order of the red/green and black/yellow pairs!

Pick up the phone plug portion of the phone adapter you need for your machine. Orient it so that the individual colored wires are coming out towards you, and so that they are coming from the lower half. You should see that the black wire is on the left and the yellow wire is on the right. IF THIS IS NOT THE CASE (it isn't for some brands of adapters), PLEASE REVERSE BLACK AND YELLOW IN THE FOLLOWING INSTRUCTIONS.

If you have to make a phone cable, be sure to wire your phone cables like the phone company does, which means ALL WIRES ARE SWAPPED. There is typically a ridge on one of the flat faces of the cable. If you put the ridge in the same direction with respect to each plug, the wires will get swapped.

Phone to female DB25 adapter (IBM, Mac "Modem",DECstation 5000):

Phone to male DB25 adapter (SPARCstation 1):

Phone to female DB25 adapter (Mac "Printer" cable):

Phone to female DB9 adapter (IBM PC and compatibles):

Phone to female DB9 adapter (VAXstation 2000):

Ordering the adapters

The specification of male or female relates to the gender of the adapter itself. For example, if you have a SPARCstation with a female DB25 on the back, you would order 046-0008-ND, the male DB25 to RJ11 adapter.

If you want to order parts from Digikey to build the adapter, you can call 1-800-DIGI-KEY and give them the appropriate part number. We aren't associated with Digikey in any way, but they are one of the few vendors that will deal with small parts orders, so I include their part numbers here.

046-0002-ND     Male DB9-RJ11 adapter
046-0003-ND     Female DB9-RJ11 adapter
046-0008-ND     Male DB25-RJ11 adapter
046-0009-ND     Female DB25-RJ11 adapter

The RJ11 cable is just the same as you use to hook your phone to the wall and can be purchased much more cheaply at your local hardware store than from Digikey.

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