CANOGA PERKINS 2270 Fiber Optic Modem - Manual (Page 57)

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2270 Fiber Optic Modem
5.2.3 System Loopback Test
This section describes the "System Test" Remote Loopback test feature in detail. See Figure 5-1
for an illustration of System Test.
The System Test provides a simple way to verify most of the components in a 2270 link without
any special test equipment:
1. Both optical transmitters
2. Both optical receivers
3. Both transmit and receive optical fibers
4. The far-end electrical interface and user device (via the far-end local loopback)
5. The near-end electrical interface and user device (via the near-end local loopback)
Three loopback points are set in the system during the System Loopback test. One is set at the
far-end modem electrical interface, another at the far-end modem optical interface and another at
the near-end electrical interface (see Figure 5-1).
To activate the System Loopback test, set the Loop switch to REM or drive the electrical interface
Remote Loopback control lead in the electrical interface to the ACTIVE (positive voltage ) state.
Refer to Section 3, "Data Interfaces," for pin numbers.
When activated, the near-end modem will begin transmitting a test pattern to the far-end modem
and the Loop On indicator will light.
When the far-end modem detects the test pattern presence, the Loop On indicator will light and
the Rx Data indicator will blink. This means that the test pattern was properly received and that the
System Test optical loopback has activated.
The far-end modem will then return the test pattern back to the near-end modem. Also, all data
transmission from the far-end user device will be looped back toward the receive of that same
device.
When the near-end modem detects the presence of the returned test pattern, the Rx Data indica-
tor will blink. This indicates that the test pattern was properly received at both ends of the link.
NOTE: When a System Test is successful, both Rx indicators should be blinking approxi-
mately twice per second and both Loop On indicators should be solidly lit.
If the Rx indicators do not blink at a steady rate, or if the Loop On indicators do not solidly light, the
System Test is failing (or intermittent) and the link requires repair.
The data rate of the System Test pattern generator is set by selecting the internal clock rate as
given in Table 2-B.
NOTE: The pattern used is a 2 20 -1 pseudorandom pattern. Do not use this pattern for
checking through the interfaces unless the data checker is disabled by the TST OFF
jumper or by selecting a remote test option from Table 5-A which excludes the "System
Test" feature.