Archived - 25-Pin SCSI Terminator: How To Build

This article contains information for constructing a 25-pin terminator
This article has been archived and is no longer updated by Apple.
Pinouts for the SCSI port are as follows:
Pin No.
Signal Name
Description
1
REQ/
Request
2
MSG/
Message
3
I/O/
Input/Output
4
RESET/
Reset
5
ACK/
Acknowledge
6
BSY/
Busy
7
GROUND
Signal Ground
8
DB0/
Data Bit 0
9
GROUND
Signal Ground
10
DB3/
Data Bit 3
11
DB5/
Data Bit 5
12
DB6/
Data Bit 6
13
DB7/
Data Bit 7
14
GROUND
Signal Ground
15
C/D/
Control/Data
16
GROUND
Signal Ground
17
ATN/
Attention
18
GROUND
Signal Ground
19
SEL/
Select
20
DBP/
Data Parity
21
DB1/
Data Bit 1
22
DB2/
Data Bit 2
23
DB4/
Data Bit 4
24
GROUND
Signal Ground
25
TERMPWR
+5 volts

Inside the Apple SCSI Terminator each signal line (ends with "/") is connected to TERMPWR through a 220-ohm resistor and is also connected to GROUND through a 330-ohm resistor.

TERMPWR           DB1/ (example)         GROUND

|                    |                    |


|                    |                    |

|                    |                    |

|                    |                    |

|-----/\\/\\/\\/\\/------|-----/\\/\\/\\/\\/------|

|      220-ohm       |      330-ohm       |

|      resistor      |      resistor      |

|                    |                    |

|                    |                    |


The obvious tradeoff here is cost versus time to build this terminator. It would seem that size is also an issue; this is a lot of resistors and solder joints to deal with.

However, if someone needs it, and appearance is not an issue and energy is in abundance, this is a solution.
Last Modified: Feb 20, 2012
  • Last Modified: Feb 20, 2012
  • Article: TA25517
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