RS-232 Cabling Tutorial

Transcription

1 RS-232 Cabling Tutorial Marcio Saito, VP Of Technology - Revision 2.2 Mar/2002 Andreas Stempfhuber, Technical Support And Repair Engineer - Revision 2.3 Feb/2003

2 About This Document There is nothing more frustrating than buying the latest and most expensive last-generation super DVD stereo system, getting home, and then not being able to try it out because the cables to connect the speakers are missing. We have a similar feeling when networking equipment doesn t work because an RS-232 cable is missing or because we cannot find a DB-9 to DB-25 adapter. This document is not intended to explain how the stereo system works. It is intended to get it playing DVDs. It has all the information you need to quickly and successfully purchase or build RS-232 cabling. We focus on building cables to connect Cyclades equipment, but most of the information applies to any RS-232 cabling. RS-232: The Basics RS-232C, EIA RS-232, or simply RS-232, refers to the same standard defined by the Electronic Industries Association in 1969 for serial communication. More than 30 year later, we have found more applications for this standard than its creators could ever imagine. Almost any electronic device nowadays has serial communication ports. RS-232 was defined to connect a DTE (Data Terminal Equipment, usually a computer or terminal) to a DCE (Data Communication Equipment, usually a modem): DTE -- RS DCE -- communication line -- DCE -- RS DTE Nowadays, RS-232 is mostly being used to connect DTE devices directly (without modems or communication lines). While that was not the original intention, it is possible with some wiring tricks. The relevant signals (or wires) in a RS-232 cable, from the computer (DTE) standpoint, are: Receive Data (RxD) and Transmit Data (TxD) - The actual data signals Signal Ground (Gnd) - Electrical reference between the two ends Data Terminal Ready (DTR) - Indicates that the computer (DTE) is active Data Set Ready (DSR) - Indicates that the modem (DCE) is active. Data Carrier Ready (DCD) - Indicates that connection over the communication line is active CTS (Clear to Send, an input) - Flow control for data flowing from DTE to DCE RTS (Request to Send, an output) - Flow control for data flowing from DCE to DTE Not all signals are necessary in every application. So, the RS-232 cable may have less than 7 wires. The RS-232 interface defines communication parameters such as parity, number of bits per character, number of stop-bits and the baud-rate. For communication to happen, both sides should be configured with the same parameters. That is the first thing to verify if you think you have the right cable and things still don t work. The most common configuration is 8N1 (8 bits of data per character, no parity bit included with the data, 1 stop-bit to indicate the end of a character). The "baud rate" in a RS-232 line translates directly into the data speed in bits per second (BPS). Usual transmission speeds range from to 9600 and BPS (used in most automation and console applications) to BPS (used by the fastest modems).

3 Cable: How Long, How Good? The original RS-232 specifications were defined to work at a maximum speed of BPS over distances up to 15 meters (or about 50 feet). That was 30 years ago. Today, RS-232 interfaces can drive signals through longer cables and faster. As a general rule, consider: If the speed is lower than 38.4 KBPS, you are safe with any cable up to 30 meters (100 feet) If the speed is 38.4 BPS or higher, keep cables shorter than 10 meters (30ft) If your application is outside the above limits (high speed, long distances), you will need better quality (low-impedance, low-capacitance) cables. Successful RS-232 data transmission depends on many variables that are specific to each environment. The general rules above are empirical and have a lot of safety margins built-in. Connectors, Connectors The connector traditionally used with RS-232 is the 25-pin D-shaped connector (DB-25). Most analog modems and most of the older computers and serial equipment use this connector. RS-232 interface on DB-25 always uses the same standard pin assignment. The 9-pin D-shaped connector (DB-9) saves some space and is also used for RS-232. Most new PC COM ports and serial equipment (specially the ones where compact size is important) uses this connector. RS-232 interfaces on DB-9 always use the same standard pin assignment. The telephone-type modular RJ-45 plug and jack are very compact, inexpensive and compatible with the phone and Ethernet wiring systems present in most buildings and data centers. Most of the networking equipment and newest servers use RJ-45 connectors for serial communication. Unfortunately there is no standard for RS-232 pin assignment on RJ-45 connectors. Every equipment vendor has its pin assignments. Most connectors have two versions. The ones with pins are said to be "male" and the ones with holes said to be "female". RS-232 Signal Name/Function (Input/Output) DB-25 pins (Standard) DB-9 pins (Standard) RJ-45 pins (Cyclades) RJ-45 pins (Netra/Cisco) Chassis Safety Ground 1 Shell Shell Shell Gnd Signal Ground TxD Transmit Data (O) RxD Receive Data (I) DTR Data Terminal Ready (O) DSR Data Set Ready (I) DCD Data Carrier Detect (I) RTS Request To Send (O) CTS Clear To Send (I)

4 Straight Through Or Crossover? The RS-232 interface was originally intended to connect a DTE (computer, printer and other serial devices) to a DCE (modem) using a straigh-though cable (all signals in one side connected to the corresponding signals in the other side in a one-to-one basis). By using some "cabling tricks", we can use RS-232 to connect two DTEs and this is the case in most of the modern applications. A crossover (a.k.a. null-modem) cable is used to connect two DTEs directly, without modems or communication lines in between. They cross transmit and receive data signals between the two sides and there are many variations on how the other control signals are wired. A "complete" crossover cable would connect the TxD with RxD, DTR with DCD/DSR, and RTS with CTS across both sides. A "simplified" crossover cable would cross TxD and RxD and locally short-circuit DTR with DCD/DSR and RTS with CTS.

5 What Cable Should I Use? First, look up the right cable for your application in the tables below. After that, purchase the standard pre-made cables from a computer store or cable vendor. For custom cables, refer to the cable diagrams to build your own cables or order them from Cyclades or a cable vendor. If your Cyclades product presents the RS-232 interface on a DB-25 connector, refer to table 1. If your Cyclades product presents the RS-232 interface on a RJ-45 jack, refer to table 2. To Connect Cyclades To Use Cable DB-25 Male Standard Cyclom-Yo Cyclom-Ye/DB-25 Cyclades-8Zo DCE DB-25 Female - Analog Modems - ISDN Terminal Adapters DTE DB-25 Male or Female 1 - DB-25 F to DB-25 M straight-through This cable is standard and can be purchased pre-made at computer stores 2 - DB-25 F to DB-25 F/M crossover - Serial Terminals - Old PC COM ports - Most Serial Printers - Some Console Ports - Most automation devices DTE DB-9 Male or Female This cable can be purchased pre-made at computer stores (exact wiring may vary). 3 - DB-25 F to DB-9 F/M crossover - Newer PC COM ports - Most Mice and pointing devices - Some automation devices This cable is standard, but it may be difficult to find it pre-made. You may combine a DB-25 crossover cable with a DB-25 to DB-9 converter. table 1

6 To Connect Cyclades To Use Cable RJ-45 Cyclades Custom Cyclom-Ye/RJ45 Cyclades-Ze PR3000/TS TS Series DCE DB-25 Female - Analog Modems - ISDN Terminal Adapters DTE DB-25 Male or Female - Serial Terminals - Old PC COM ports - Most serial printers - Some Console Ports - Most automation devices DTE DB-9 Male or Female - Newer PC COM ports - Most Mice and pointing devices - Some automation devices DTE RJ-45 Cyclades (custom) 4 RJ-45 to DB-25 M straight-through (Custom) This custom cable can be ordered from Cyclades (a 6ft cable is available - ask for the part number CAB0025) or other cable vendors. A sample is included with the products ("straight-through"). 5 RJ-45 to DB-25 F/M crossover (Custom) This custom cable can be ordered from Cyclades (a 6ft cable is available - ask for the part number CAB0017) or other cable vendors. A sample is included with the products ("Console"). 6 RJ-45 to DB-9 F/M crossover (Custom) This custom cable can be ordered from Cyclades (a 6ft cable is available - ask for the part number CAB0036) or other cable vendors. A sample is included with the products ("Console"). 7 RJ-45 to RJ-45 crossover (Custom) - All Cyclades Console Ports DTE RJ-45 Netra (custom) This custom cable can be ordered from cable vendors using the provided wiring diagram. 8- RJ-45 to RJ-45 crossover (Custom) - Sun Netra Console Ports - Cisco Console Ports This custom cable can be ordered from Cyclades (a 10ft cable is available - ask for the part number CAB0039) or cable vendors using the provided wiring diagram. table 2 Cable Diagrams Before using the following cable diagrams refer to the tables above to select the correct cable for your application. Sometimes, crossover cables are wired slightly differently depending on the application. A "complete" crossover cable would connect the TxD with RxD, DTR with DCD/DSR, and RTS with CTS across both sides. A "simplified" crossover cable would cross TxD and RxD and locally short-circuit DTR with DCD/DSR and RTS with CTS. Most of the diagrams in this document show the "complete" version of the crossover cables, with support for modem control signals and hardware flow control. Applications that do not require such features have just to configure NO hardware flow control and NO DCD detection on their side. Both ends should have the same configuration for better use of the complete version of the cables shown below.

7 Cable #1: DB-25 Female to DB-25 Male, Straight Through DB-25 Female to DB-25 Male, Straight Through Application: It connects Cyclades products (serial ports) to modems and other DCE RS232 devices. Cable #2: DB-25 Female to DB-25 Female/Male, Crossover DB-25 Female to DB-25 Female/Male, Crossover Application: It connects Cyclades products (serial ports) to console ports, terminals, printers and any other DTE RS232 device.

8 Cable #3: DB-25 Female to DB-9 Female, Crossover DB-25 Female to DB-9 Female, Crossover Application: It connects Cyclades products (serial ports) to console ports, terminals, printers and any other DTE RS232 device.

9 Cable #4: Cyclades RJ-45 to DB-25 Male, Straight Through Cyclades RJ45 to DB-25 Male, Straight Through Application: It connects Cyclades products (serial ports) to modems and other DCE RS232 devices.

10 Cable #5: Cyclades RJ-45 to DB-25 Female/Male, Crossover Cyclades RJ45 to DB-25 Female/Male, Crossover Application: It connects Cyclades products (serial ports^) to console ports, terminals, printers and any other DTE RS232 device.

11 Cable #6: Cyclades RJ-45 to DB-9 Female, Crossover Cyclades RJ45 to DB-9 Female, Crossover Application: It connects Cyclades products (serial ports) to console ports, terminals, printers and any other DTE RS232 device.

12 Cable #7: Cyclades RJ-45 to Cyclades RJ-45, Crossover Cyclades RJ45 to Cyclades RJ45, Crossover Application: Usually used to connect two ports of a Cyclades product together ("loopback") for testing purposes.

13 Cable #8: Cyclades RJ-45 to Netra RJ-45, Crossover Usually used in console management applications to connect Cyclades products to a Sun Netra server or to a Cisco product Cyclades Corporation. All Rights Reserved.