SCSI External Centronics 50 (Single-ended)

Pinout of the 50-pin Centronics external SCSI connector (CN50) used for narrow single-ended SCSI-1 and SCSI-2 devices at up to 10 MB/sec.

Updated:

The 50-pin Centronics (CN50) connector is the standard external narrow SCSI connector defined in SCSI-2 (ANSI X3.131-1994) for single-ended (SE) 8-bit bus configurations. It carries 8 data lines, odd parity, and the nine SCSI control signals over a 50-pin shielded housing with slot-type contacts at 2.16 mm pitch, and supports data rates up to 10 MB/sec (Fast SCSI). The connector appears as the female socket on host adapters and peripheral devices, with the male plug on the cable end. Governed by the T10 Technical Committee under INCITS, this connector type is sometimes called the SCSI-1 external connector and was superseded by the high-density 50-pin (DH50) and 68-pin formats for faster SCSI-2 and SCSI-3 applications.

50 PIN CENTRONICS FEMALE 50 PIN CENTRONICS FEMALE at the controller & devices

50 PIN CENTRONICS MALE 50 PIN CENTRONICS MALE at the cable.

PinNameDirDescription
1GND-Ground
2GND-Ground
3GND-Ground
4GND-Ground
5GND-Ground
6GND-Ground
7GND-Ground
8GND-Ground
9GND-Ground
10GND-Ground
11GND-Ground
12RES-Reserved
13OPEN-
14RES-Reserved
15GND-Ground
16GND-Ground
17GND-Ground
18GND-Ground
19GND-Ground
20GND-Ground
21GND-Ground
22GND-Ground
23GND-Ground
24GND-Ground
25GND-Ground
26DB0IN/OUTData Bus 0
27DB1IN/OUTData Bus 1
28DB2IN/OUTData Bus 2
29DB3IN/OUTData Bus 3
30DB4IN/OUTData Bus 4
31DB5IN/OUTData Bus 5
32DB6IN/OUTData Bus 6
33DB7IN/OUTData Bus 7
34PARITYIN/OUTData Parity (odd Parity)
35GND-Ground
36GND-Ground
37GND-Ground
38TMPWRIN/OUTTermination Power
39GND-Ground
40GND-Ground
41/ATNINAttention
42n/c-Not connected
43/BSYIN/OUTBusy
44/ACKINAcknowledge
45/RSTIN/OUTReset
46/MSGOUTMessage
47/SELIN/OUTSelect
48/C/DOUTControl/Data
49/REQOUTRequest
50/I/OOUTInput/Output

Signals

  • GND — Signal ground; pins 1–25 (except 12, 13, 14) are all grounds on this connector, providing return current paths paired with each signal line.
  • RES — Reserved; pins 12 and 14 are reserved and must not be connected.
  • OPEN — Pin 13 is left open (unconnected) and serves no signal function.
  • DB0–DB7 — Eight bidirectional data bus lines (pins 26–33), active-low; together they form the 8-bit narrow SCSI data bus.
  • PARITY — Bidirectional odd parity bit (pin 34) for the data bus; the initiator or target asserts parity so that the total number of asserted data + parity lines is always odd.
  • TMPWR — Termination power (pin 38); supplies +5 V to bus terminators; typically sourced by the host adapter and sometimes by drives, via a diode to prevent back-feed.
  • /ATN — Attention (pin 41); driven by the initiator to signal that a message is pending; active-low.
  • n/c — Pin 42 is not connected.
  • /BSY — Busy (pin 43); asserted by whichever device currently controls the bus; active-low.
  • /ACK — Acknowledge (pin 44); driven by the initiator during each data or message byte transfer to handshake with /REQ; active-low.
  • /RST — Reset (pin 45); resets all devices on the bus when asserted; active-low.
  • /MSG — Message (pin 46); driven by the target to distinguish message phase from command/data phases; active-low.
  • /SEL — Select (pin 47); used during the selection and reselection phases to address a target or re-connect an initiator; active-low.
  • /C/D — Control/Data (pin 48); driven by the target to indicate whether the current bus phase is a command/status/message phase (asserted) or a data phase (negated); active-low.
  • /REQ — Request (pin 49); driven by the target to request a data, command, status, or message transfer byte; active-low.
  • /I/O — Input/Output (pin 50); driven by the target to define data transfer direction relative to the initiator: asserted = data flows to initiator (input), negated = data flows from initiator (output); active-low.

Notes

  • Direction shown is relative to the device on the bus (device perspective).
  • The CN50 connector uses slot-type contacts at 2.16 mm pitch in a shielded metal housing with clip or bail locks; it is larger than the high-density 50-pin (DH50) SCSI-2 connector and is not physically compatible with it.
  • Single-ended (SE) signalling drives each line against a common ground using 5 V logic; it is electrically incompatible with High Voltage Differential (HVD) devices and must not be mixed with them on the same bus.
  • Maximum cable length for SE: 6 m for Slow SCSI (5 MB/sec); 3 m for Ultra SCSI (20 MB/sec). The CN50 connector is generally limited to Slow or Fast SCSI (up to 10 MB/sec).
  • Exactly two terminators are required per bus segment, one at each extreme end. Passive termination (220/330 Ω resistor networks) is acceptable only at Slow SCSI speeds; active termination is recommended for Fast SCSI.
  • TERMPWR (pin 38) must be present on the bus; it is typically provided by the host adapter. Verify that at least one device on the segment supplies TERMPWR.
  • All SCSI control signals are active-low as indicated by the leading slash (e.g., /BSY, /REQ).

References

Category:Buses Connectors