Mocomtech CIM-550 - Manual (Page 109)

CiM-550 IP Enabled Satellite Modem Rev. 2
Theory of Operation CD/CIM550.IOM
83
6.3 EIA-232 O PERATION
The CiM-550 supports asynchronous EIA-232 in addition to synchronous EIA-232
formats. This is useful, for example, in connecting two computers together, via their
serial ports, at opposite ends of a satellite link. There are, however, some restrictions and
limitations when using this mode, and the user may have to experiment a little in order to
get reliable communication.
6.3.1 ASYNC EIA-232 S
PECIFICATIONS
Maximum operating baud rate: 57.6 kbaud (QPSK and OQPSK)
28.8 kbaud (BPSK)
Additional data overhead required: Zero. A 9.6 kbaud connection uses a 9.6 kbps carrier.
Maximum allowable baud rate
tolerance: +/- 600 ppm
Character formats supported: Any, except those using 1.5 stop bits (see note below)
Flow control: None
Handshaking line supported: Data Carrier Detect (DCD)
Connectivity required: Rx data, Tx Data and Ground (DCD optional)
6.3.2 S ETUP
The setup required for this mode is remarkably simple. Select EIA-232 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.
6.3.2.1 B AUD R ATE A CCURACY
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.