SCSI Internal (Single-ended)

50-pin IDC ribbon connector used for internal narrow SCSI-1/2 single-ended devices, with 8-bit data bus, active-low control signals, and 2.54 mm pitch.

Updated:

SCSI = Small Computer System Interface

SCSI Internal (Single-ended) – 50-pin IDC header 50 PIN IDC MALE at the controller & harddisk

SCSI Internal (Single-ended) – 50-pin IDC header 50 PIN IDC FEMALE at the cable

The internal single-ended SCSI connector is a 50-pin IDC (Insulation Displacement Contact) ribbon header with contacts on a 2.54 mm (0.1 inch) pitch, arranged in two rows of 25 pins. It carries an 8-bit narrow SCSI bus as defined in SCSI-2 (ANSI X3.131-1994) and its predecessors, and operates with Single-Ended (SE) 5 V logic signalling — each signal driven against a common ground reference. The connector was used in desktop and tower computers from the late 1980s through the early 2000s to attach internal hard disks, tape drives, and optical drives. All signal lines are active-low, as is standard throughout the SCSI-1 and SCSI-2 generations, and the bus requires a terminator at each of its two extreme ends.

PinNameDirDescription
2DB0IN/OUTData Bus 0
4DB1IN/OUTData Bus 1
6DB2IN/OUTData Bus 2
8DB3IN/OUTData Bus 3
10DB4IN/OUTData Bus 4
12DB5IN/OUTData Bus 5
14DB6IN/OUTData Bus 6
16DB7IN/OUTData Bus 7
18PARITYIN/OUTData Parity (odd Parity)
20GND-Ground
22GND-Ground
24GND-Ground
26TMPWRIN/OUTTermination Power
28GND-Ground
30GND-Ground
32/ATNINAttention
34GND-Ground
36/BSYIN/OUTBusy
38/ACKINAcknowledge
40/RSTIN/OUTReset
42/MSGOUTMessage
44/SELIN/OUTSelect
46/C/DOUTControl/Data
48/REQOUTRequest
50/I/OOUTInput/Output

Signals

  • DB0–DB7 — Eight bidirectional data bus lines forming the 8-bit narrow SCSI data path; active-low.
  • PARITY — Odd parity bit for the data byte; bidirectional, active-low.
  • GND — Ground return; all odd-numbered pins except pin 25 are grounded, providing shielding between signal pairs in the flat ribbon cable.
  • TMPWR — Termination Power; supplies voltage (typically 5 V) to the bus terminators at each end of the cable; bidirectional as any device may source it.
  • /ATN — Attention; asserted by the initiator to signal that it wishes to send a message to the target.
  • /BSY — Busy; asserted by the device currently controlling the bus to indicate the bus is in use.
  • /ACK — Acknowledge; asserted by the initiator to acknowledge receipt of a data or message byte during a REQ/ACK handshake.
  • /RST — Reset; resets all devices on the bus; bidirectional, may be asserted by any device.
  • /MSG — Message; asserted by the target during the Message phase to distinguish message bytes from command or data bytes.
  • /SEL — Select; used during Selection and Reselection phases to address a specific target or initiator.
  • /C/D — Control/Data; asserted by the target to indicate whether bytes on the bus are command/status/message (asserted) or data (negated).
  • /REQ — Request; asserted by the target to request a data or message transfer handshake cycle.
  • /I/O — Input/Output; indicates the direction of data transfer relative to the initiator — asserted means data flows to the initiator (input), negated means data flows to the target (output).

Notes

  • Direction is device-relative to the bus (other devices).
  • All odd-numbered pins, except pin 25, are connected to ground. Pin 25 is left open (no connection).
  • The connector body uses two rows of 25 pins at 2.54 mm (0.1 inch) centres; cables are standard 50-conductor flat ribbon, with a polarising key or notch to prevent reversed insertion.
  • Single-Ended (SE) signalling drives each line against a common ground; maximum cable length is 6 m for Slow/Fast SCSI and 3 m for Ultra SCSI (20 MT/sec).
  • Exactly two terminators are required per bus segment, fitted at the two physical ends of the cable. Passive termination (220/330 ohm resistor packs) is acceptable for Slow SCSI (up to 5 MB/sec); active termination is required for Fast SCSI (10 MB/sec) and above.
  • SE logic operates at 5 V; it is electrically incompatible with HVD (High Voltage Differential) equipment. Connecting an HVD device to an SE bus can damage both. LVD/MSE (multimode) devices include protection circuitry and can detect bus type via the DIFFSENSE pin, but will downgrade the entire bus to SE when mixed with SE devices.
  • The internal IDC 50-pin connector carries only the 8-bit narrow SCSI bus (no wide/16-bit support); 16-bit Wide SCSI requires the separate 68-pin high-density connector.
  • The SCSI-2 standard (ANSI X3.131-1994) designates this as an Alternative 2 non-shielded connector (low-density, 2.54 mm pitch), distinct from the Alternative 1 high-density 50-pin header at 1.27 mm pitch.

References

Category:Buses Connectors