The surge in popularity for synchronized data communication can be traced,
in part, to the onslaught of new "wireless" network extension devices.
In the past, an asynchronous speed of 4800 baud was considered fast. Thus, a
19.2-kbit synchronized communication device could handle async data up to 4800
baud by oversampling without experiencing many problems.
But, with today's higher asynchronous rates and the requirement to operate
at equal data speeds, oversampling is no longer adequate to link two systems.
This simple converter (see the figure)
consists of two ICs and a voltage regulator. The Sipex MAS7838, which acts as
the converter, selects the conversion speed to that of the synchronized data
clock. It has internal switches and registers to perform the async to sync,
or sync to async, conversion. The Maxim MAX238 provides the RS232 drivers and
receivers for interfacing with the data bus. These chips require a 5-V dc power
supply; a generic 78L05 reduces the +12 V at the DB25 pin 9 to the +5 V needed.
A crystal frequency of 4.91 MHz is suitable for converting to 19.2 kbits or
a sub-multiple (9.6, 4.8, 2.4, etc.). Two 1N4001 diodes protect the external
RTS (ready to send) control circuitry if the RTS is enabled by S1. When JP1
is removed, the converter is transparent in the sync mode and no conversion
will take place.
The completed unit is mounted atop a universal breakout adapter, and the control
lines are jumpered according the chart in the figure. The physical size is approximately
1 by 2.25 by 2.5 in. and will easily plug into the DB25 socket on a synchronized
data communications equipment (DCE) communication device.
