Black Box ME444A-530-R3, ME444AE-R3, ME445C-X.21, ME445C-35, ME444A-R3, ME445C-530 Modulation Timing

256-KBPS LINE DRIVER (V.35, RS-530, OR X.21)

NOTE

In multipoint applications, do not use the 0-msec option for the RTS–CTS delay.

2.2.2MODULATION TIMING

This circuit supplies the transmit clock to the encoder. Three clock sources are available on the XMT CLK jumper (J3):

•INT—Internal clock. From the modem’s internal crystal oscillator.

•EXT—External clock. From DTE.

•RCV—Receive clock. Recovered from the receive signal.

There is also an ASY (asynchronous) setting, but this has no function and should not be selected.

2.2.3SIGNAL LEVELS

Two options are available for the transmit and receive signal levels: 0 and -6 dBm. You can control the transmit level with the XMT LVL jumper. You can control the receive level with the RCV LVL jumper.

2.2.4RECEIVER

The receiver comprises several circuits, as shown in the block diagram (see Figure 2-1):

•The RECEIVE FILTER, which removes all the out-band frequencies.

•The EQUALIZER, which comprises several equalizers activated according to data rate.

•The digital Automatic Gain Control (AGC), which automatically compensates for the attenuation of the line.

12