Comtech EF Data CDM-550T - Manual (Page 94)

CDM-550T Satellite Modem Revision 3
RS-232 Data Interface - Asynchronous Operation MN/CDM550T.IOM
9–2
irrelevant - the selected clocking options are ignored, and the internal clock circuitry is
reconfigured for this ‘special’ mode.
9.4 Other Considerations
9.4.1 Baud Rate Accuracy
When connecting devices where the baud rate is not within the tolerance of
± 600ppm, the Tx
FIFO and Rx Buffer will occasionally underflow or overflow, causing characters to be lost. To
avoid this, while the external device is connected and transmitting data, connect a frequency
counter to pin 15 of the rear panel Data Connector (the 25 pin ‘D’ type female). Measure the
frequency, then set the transmit frequency to be equal to this value on the local modem, and set
the receive frequency to this value at the other end of the link. If a frequency counter is not
available, 10 minutes of trial and error will be an adequate substitute. Set the modem to an IF
loop, and then try changing the baud rate by approximately 500 ppm at a time. At 9600 baud, this
equates to either 9605 or 9595 baud, with further increments of 5 bps. If changing the baud rate in
a positive direction results in more frequent buffer and/or FIFO slips, then move it in the opposite
direction. Do this until the slips are eliminated.
9.4.2 Async Character Formats Using 1.5 Stop Bits
For situations where an async character format using 1.5 stop bits cannot be avoided, set the
modem data rate to exactly twice the incoming baud rate. This uses twice as much bandwidth, but
will permit this mode of operation.