Comtech EF Data CDM-550T, Manual

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EIA-232 Data Interface - Asynchronous Operation CDM-550T Satellite Modem
9–2 Rev. 1.3
9.3 Setup
The setup required for this mode is remarkably simple. Select RS232 as the interface
type, and then select the required baud rate for both transmit and receive data rates.
Detection of the asynchronous mode is fully automatic. If no clock is detected,
asynchronous mode is assumed, and the internal circuitry is switched accordingly. The
settings for both Tx and Rx clocking are 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.