IBM 5500-01E, Руководство по эксплуатации, установке и обслуживанию

Страница: 102 / 192

Chapter 6: Twinax Client Cabling Requirements and Problem Solving
102 Planning, Installation, and Maintenance Guide
of your existing twinax cable runs will be affected. The IBM 5500 Control
Unit and newer AS/400s handle some twinax command sequences differently
than older twinax systems (such as System 36). Both the IBM 5500 Control
Unit and the AS/400 use a larger subset of the twinax command set than
previous systems and transmits commands closer together (less dead time
between commands). These changes in transmissions will not affect IBM
twinax devices; however any OEM devices not complying with the entire
twinax protocol can experience problems.
Cables that do not meet specifications can cause a higher error rate than on
previous twinax systems. Older twinax systems do not log temporary
workstation controller and workstation errors, but do log permanent errors,
which allows many marginal cable runs to operate sufficiently. Current
AS/400s log the first 5 temporary errors as well as permanent errors per
device address (per IPL or IOP reset). The IBM 5500 Control Unit will also
detect these errors.
Because the WSC nominal output level is less than of the S/36 or S/38, an out-
of-specification cable can operate on a S/36 or S/38, but cause a high error
rate on an 5500 Control Unit and AS/400 cable runs. When this occurs, bring
the cable up to the specifications.
Testing Cable Termination
A correctly terminated line will have two 55 ohm resistors, each connected
between a phase of the cable and the twinax shield at the last device in the
string. (Some machines, such as 5219, terminate the line with a transformer,
therefore the D.C. tests will not show expected resistance readings). The
termination resistance can be checked with an ohmmeter. An allowance must
be made for the length of the cable run. For a maximum cable run of 5000 feet
(1524 meters), the phase-to-shield resistance must be less than 125 ohms and
the phase-to-phase resistance must be less than 220 ohms.
Example:
If you are measuring a good and correctly terminated cable with a length of
1000 feet (305 meters) between phase and shield, you will see 55 ohms
termination resistance, + plus 11 ohms conductor resistance, + 3 ohms shield
resistance, giving a total measured resistance of 69 ohms
The two resistance values from each phase to shield should both be within
about 2% of each other.
Testing for Shorts
With both ends of the cable open, and not plugged into a machine, the phase-
to-phase resistance and the resistance from each phase to shield must be more
than 1 M Ohm.
Phase Connection Tests
To check for correct phase connections, vary off all the devices on that run,
and power the devices off. Disconnect the twinax cable from the machines.