The Fibre Channel electrical interface is one of two related Fibre Channel standards that can be used to physically interconnect computer devices. The other standard is a Fibre Channel optical interface, which is not covered in this article.

Signal characteristics

Fibre channel electrical signals are sent over a duplex differential interface. This usually consists of twisted-pair cables with a nominal impedance of 75 ohms (single-ended) or 150 ohms (differential). This is a genuine differential signalling system so no ground reference is carried through the cable, except for the shield. Signalling is AC-coupled, with the series capacitors located at the transmitter end of the link.

The definition of the Fibre Channel signalling voltage is complex. Eye-diagrams are defined for both the transmitter and receiver. There are many eye-diagram parameters that must all be met to be compliant with the standard. In simple terms, the transmitter circuit must output a signal with a minimum of 600 mV peak-to-peak differential, maximum 2000 mV peak-to-peak differential. A good signal looks rather like a sine-wave with a fundamental frequency of half the data rate, so 1 GHz for a typical system running at 2 gigabits per second.

The Bit-Error Rate (BER) objective for Fibre Channel systems is 1 in 1012 (1 bit in 1,000,000,000,000 bits). At 2 Gbit/s this equates to seven errors per hour. Therefore, this is a common event and the receiver circuitry must contain error-handling logic. In order to achieve such a low error-rate, jitter "budgets" are defined for the transmitter and cables.

Connector pinouts

There are various Fibre Channel connectors in use in the computer industry. Details of their pinouts are distributed between different official documents. The following sections describe the most common Fibre Channel pinouts with some comments about the purpose of their electrical signals.

The most familiar Fibre Channel connectors are cable connectors, used for interconnects between initiators and targets (usually disk enclosures). There are also "device connectors" that can be found on Fibre Channel disk-drives and backplanes of enclosures. The device connectors include pins for power and for setting disk options.

DE-9 cable connector

Pin Signal name Comments1+OUTFibre channel output2+5VOptional3Module Fault DetectOptional4Reserved 5+INFibre channel input6-OUTFibre channel output7Output DisableOptional8GNDOptional, return for pin 29-INFibre channel input

Optional pins 2, 3, 7, and 8 are intended for use with an external optical converter. This is often called a Media Interface Assembly (MIA).

Fibre channel DE-9 connectors often have only the 4 required contacts installed. Note that they are the four outermost contacts, i.e., the same as used for Token Ring, this is an easy way to tell a fibre channel cable from an RS-232 cable.

HSSDC cable connector 8-pin High Speed Serial Data Connection (HSSDC)

Pin Signal name Comments1+OUTFibre channel output2GNDOptional, return for pin 73-OUTFibre channel output4Module Fault DetectOptional5Output DisableOptional6-INFibre channel input7+5VOptional8+INFibre channel input

Optional pins 2, 4, 5, and 7 are intended for use with an external optical converter. This is often called a Media Interface Assembly (MIA).

HSSDC2 cable connector 7-pin High Speed Serial Data Connection (HSSDC2) Pin Signal name Comments1GND2-OUTFibre channel output3+OUTFibre channel output4GND5+INFibre channel input6-INFibre channel input7GND

SCA-2 disk connector 40-pin SCA-2 disk connector. Although SCA-2 is the official name for this connector, it is often called SCA-40 to distinguish it by its pin count from other similar connectors.

Pin Signal name Comments1-EN Bypass Port 1Output driven high when port 1 is operating correctly2+12V 3+12V 4+12V 5-Parallel ESIInput to allow ESI operation using the SELx pins6-Drive Present 7ACTLEDOutput to drive the activity LED cathode8Power Control 9START1Input to control spin-up behavior (see the Disk options section)10START2Input to control spin-up behavior (see the Disk options section)11-EN Bypass Port 2Output driven high when port 2 is operating correctly12SEL6Device ID bit 6 / ESI write clock13SEL5Device ID bit 5 / ESI read clock14SEL4Device ID bit 4 / ESI acknowledge clock15SEL3Device ID bit 3 / ESI bit 316FLTLEDOutput to drive the fault LED cathode 17DEVCTRL2Input to control interface speed (see the Disk options section)18DEVCTRL1Input to control interface speed (see the Disk options section)19+5V 20+5V 21+12V Charge 22GND (12V) 23GND (12V) 24+IN1Fibre channel input25-IN1Fibre channel input26GND (12V) 27+IN2Fibre channel input28-IN2Fibre channel input29GND (12V) 30+OUT1Fibre channel output31-OUT1Fibre channel output32GND (5V) 33+OUT2Fibre channel output34-OUT2Fibre channel output35GND (5V) 36SEL2Device ID bit 2 / ESI bit 237SEL1Device ID bit 1 / ESI bit 138SEL0Device ID bit 0 / ESI bit 039DEVCTRL0Input to control interface speed (see the Disk options section)40+5V CHARGE

Disk options

Some of the input signals to a Fibre Channel disk are used to control options, as follows:

START options - if all the disks in an enclosure spin-up immediately on power-on, this can overload the power-supply. Two methods are available to avoid that problem:

START1 START2 Spin-up behaviour00Automatic, immediate01Wait for Start Unit SCSI command10Staggered, delayed depending on loop address11Reserved

DEVCTRL options - a disk that has the wrong interface speed can cause serious problems if it is inserted into an enclosure. That problem is avoided by providing three input pins to the disk that define the interface speed at which it will operate:

DEVCTRLx value Interface speed0-4Reserved54 GB/s62 GB/s71 GB/s

External links

Fibre Channel Physical Interface specification Pictures of parallel SCSI connectors Pictures of SSA connectors Pictures of FC-AL connectors Pictures of SAS connectors