CANOGA PERKINS 2270 Fiber Optic Modem - Manual (Page 20)

20
Canoga Perkins
2.7.3 Asynchronous Operation
The 2270 can operate in the data-only asynchronous mode by setting the Clock Mode to INTER-
NAL and the Clock Rate to the highest frequency setting (switch positions 1-3 all closed).
This allows for a maximum asynchronous rate of one-quarter of the internal clock frequency. For
example, if a 2200-C-08 oscillator is used, the maximum frequency is 20 Mbps, and the highest
asynchronous data rate would be 5 Mbps. This maximum is based on the assumption that a
maximum of 25% peak-to-peak jitter is allowable in the received data signal.
This particular configuration is called "Sampled Asynchronous Transmission." It requires configur-
ing the modem for internal clock and connecting data signals to the normal data input/output pins.
2.7.4 Data Rates
The maximum data rate permissible depends on the type of interface used. Only the special high-
speed interfaces can be operated above 10 Mbps.
Data rate selection for the 2270 Modem is accomplished by using the front panel switches (see
Figure 1-1). Table 2-B provides a data rate table for use in selecting a specific rate in relation to the
crystal oscillator factory-installed in the modem. The oscillator frequency is marked on the case of
the crystal. Special oscillator frequencies may be requested, if needed.
2.7.5 Data Rate vs. Module Frequency
If a high-speed interface is exchanged for a low-speed version, or vice versa, it may be necessary
to make an oscillator change on the main modem circuit board. Table 2-B lists the standard
oscillators, or you may contact the factory for application assistance.
2.7.6 Slave Clock Operation
The slave clock mode may be selected by closing switch position 5 on the front panel eight-
section switch bank. This will use the 2270's optical received clock as its transmit clock output.
The setting of switch position 6 has no effect. Refer to Section 2.7.7.2 regarding switch position 4.
The System Test feature will function with a modem configured for slave clock operation. However,
the user must be aware of the following limitation: If the System Test is enabled, a modem
configured for slave clock will provide the System Test timing to the user as receive and transmit
clock. This could cause a potential problem, since the System Test timing may not be at the
correct data rate for the user device.
A solution to this problem is selecting an oscillator module which allows selecting Internal/Test
clock equal to the data rate of the user devices.
NOTE: The 2270 Modem will not pass data with an external optical loopback when con-
figured for slave clock operation.
2.7.7 Consideration of Propagation Delays
Whenever the modem is sending a transmit clock to the DTE, it is important to understand the
effect of the time required for that clock to propagate from the modem to the DTE.
Clock-to-Data phasing is particularly important in any synchronous data link. The modem ex-
pects the data to be valid (unchanging) at the point in time when the clock transitions to "clock"
the data.