UNIX System Administration Handbook - Evi Nemeth [66]
The mini DIN-8 variant
Mini DIN-8s are found on Macs and on some laptops and workstations. This almost circular and extremely compact connector provides connections for seven signals. It is illustrated in Exhibit C.
Exhibit C A male mini DIN-8 connector
Neighborhood computer dealers usually carry injection-molded DB-25 to mini DIN-8 converter cables. Don’t try to make them yourself because a mini DIN-8 is so tiny that it defies attempts to secure connections with human fingers. Pin assignments are shown in Table 7.2.
Table 7.2 Pins for a mini DIN-8 to DB-25 straight cable
The DB-9 variant
Commonly found on PCs, this nine-pin connector (which looks like a DB-25 “junior”) provides the eight most commonly used signals.
Exhibit D A male DB-9 connector
PC dealers in your area should carry prefab DB-9 to DB-25 converter cables. Table 7.3 shows the pin assignments.
Table 7.3 Pins for a DB-9 to DB-25 straight cable
The RJ-45 variant
An RJ-45 is an eight-wire modular telephone connector. It’s similar to the standard RJ-11 connector used for telephone wiring in the United States, but an RJ-45 has 8 pins (an RJ-11 has only 4). RJ-45 connectors are most commonly used for Ethernet wiring, but they work fine for serial communication, too.
Exhibit E A male RJ-45 connector
RJ-45 jacks are usually not found on computers or garden-variety serial equipment, but they are often used as intermediate connectors when routing serial lines through patch panels. They can sometimes be found on devices that have many ports in close proximity, such as terminal servers. RJ-45s are often used with flat telephone cable rather than stranded twisted-pair; either is acceptable for serial connections.
RJ-45s are compact, self-securing, and cheap. They are crimped onto the cable with a special tool. It takes less than a minute to attach one. If you are designing a large cabling system from scratch, RJ-45s are a good choice for intermediate connectors.
There are several systems for mapping the pins on an RJ-45 connector to a DB-25. We like Dave Yost’s system, which adds an RJ-45 socket to every device and uses standardized RJ-45 connector cables to allow communion with either DCE or DTE equipment. Dave’s standard is presented in the next section.
The Yost standard for RJ-45 wiring
Here is a scheme that offers solutions to several RS-232 hassles:
• All cable connectors are of the same sex and type (male RJ-45).
• There is no distinction between DTE and DCE.
• You need only one kind of connector cable.
• You can mass-produce cables quickly, using only a crimping tool.
This specification was written by Dave Yost (Dave@Yost.com). He updated it for this book.
Each serial port on every piece of equipment gets its own appropriately wired DB-25 or DB-9 to RJ-45 adaptor. This adaptor is permanently screwed onto the port. The port now presents the same connector interface, female RJ-45, regardless of whether its underlying connector is DB-25 or DB-9, DTE or DCE, male or female. Furthermore, every serial port now transmits and receives data on the same pins.
Once you have put these adaptors on your RS-232 ports, you can connect anything to anything without using null-modems or null-terminals, changing pins on cable connectors, or building special cables. You can connect modem to computer, modem to terminal, terminal to computer, terminal to terminal, computer to computer, etc., all with one kind of cable.
The cables are jacketed, eight-wire ribbon cable. The connectors on each end are squeezed onto the cable with a crimping tool, so there is no soldering or messing with pins.
There are three signal wires (one data and two control) going in each direction, plus a pair of signal grounds. The cables are not wired normally (i.e., with each