IBM 5500 Operational, Installation, And Maintenance Manual page 106

106
Chapter 6: Twinax Client Cabling Requirements and Problem Solving
The following procedure can be used on twinax cabling or the IBM Cabling
System (CS). Differences between the ohmmeter reading for twinax cabling
versus the IBM Cabling System are defined.
Step 1. Initial Questions
Answer the following questions to gain a clear understanding of the problem.
• Has the string ever worked?
• Have any devices or cables been added to the string?
• Have any devices moved recently?
• How many devices fail and how often?
• Indoor cable? Go to Step 2.
• Outdoor cable? Go to Step 10.
Step 2. Metering the Cable for Correct Termination
Note: Use an analog meter for the following steps. Do not use a digital meter.
The readings given are for a short twinax cable allowing for no resistance of
the cable itself. You will have to use some judgement based upon your
knowledge of the length of the cable. For twinax cabling, the minimum
reading for termination is 55 ohms and the maximum is 125 ohms. (55 ohms
termination + 14 ohms per 1000 feet of cable.)
For the IBM Cabling System, when measuring through the Impedance
Matching Device (IMD) at the system end, the minimum reading for
termination is 95 ohms and the maximum is 165 ohms. (95 ohms termination
+ 14 ohms per 1000 feet of cable.) For the IBM Cabling System, when not
measuring through the IMD at the system end, the minimum reading for
termination is 75 ohms and the maximum is 145 ohms. (75 ohms termination
+ 14 ohms per 1000 feet of cable.)
a. Disconnect the cable from the twinax workstation controller. (5500,
S/3X, AS/400, 5X94, etc.)
b. Meter from each phase conductor to the shield for proper
termination. The reading should be 55 ohms for twinax, 95 ohms
for IBM CS including the IMD at the system end, 75 ohms for
IBM CS not including the IMD at the system end. The readings
should be approximately the same for each phase conductor.
c. Reverse the meter leads and meter again. The readings should be
the same regardless of which meter lead is on the shield and which
lead is on the conductor.
d. Meter from phase to phase. The reading should be 110 ohms. (190
ohms for the IBM cabling system). Go to Step 3.
Step 3. Based upon the results of Step 2 above, branch as follows:
a. Readings correct - 55 ohms, go to Step 4.
b. Readings correct with meter leads one way but low the other way,
Planning, Installation, and Maintenance Guide