Gentner VRC2000 - Command Channel Outputs; DTMF Encoder;Decoder; Real-Time Clock

Page 106 VRC2000 Installation and Operations Manual
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until the closest value is found. The AD573 has a high-byte enable (HBE)
and a low-byte enable (LBE), which allows the processor to read 10 bits of
data for each metering channel, rather than the normal eight bits.
Just like the status channels, the VRC2000 stores the data of all metering-
channel inputs in memory for quick processing and easy access.
Command Channel Outputs
The VRC2000 has 32 outputs configured into 16 command channels with
two commands each. Each of these outputs are open collector drivers rated
to 48Vdc at 250mA (noninductive). Each switch can be programmed as
latching or momentary. When a command channel is programmed to be
momentary, the output can be enabled for .125–32 seconds via user
programming.
U48, U50, U52 and U54 (all ULN2BO3A Darlington pair drivers) are
devices providing the 32 command-channel switches.
Real-Time Clock
The MC146818 time-of-day clock (U25) pulses the microprocessor at the
critical 256Hz rate to allow time-of-day functions, and report the operator
the time and date an alarm was detected. Y3 (a 32.768KHz crystal) controls
the clock’s accuracy. By adjusting C46 (a trimmer capacitor) this clock is
set at the factory to 32.768KHz +/- .3Hz. No further adjustment should ever
be needed. This keeps the time of day accurate to within two seconds per
month.
DTMF Encoder/Decoder
To make the VRC2000 operate on the toll grade dial-up telephone network
with the capability of both sending and receiving data using DTMF tones, a
SSI202 DTMF decoder and a MK5089 DTMF generator are used. The
SSI202 (U18) and MK5089 (U35) are both clocked by the 3.58MHz crystal
(Y2).
The SSI202 decoder has a receive-level window of 40dB, centered at
-20dBm. This allows for both local and long-distance access. Typical local
loop signals arrive at the SSI202’s analog input at -4–-6dBm. Long-distance
calls typically have a level of -l5dBm.
The MK5089 is used in conjunction with U13 (a NE5900) to generate and
monitor outbound alarm reporting telephone calls and for auto-logging
functions. The MK5089 DTMF generator initiates the call while the
NE5900 is a call-progress decoder.
The NE5900 is a narrow bandpass filter which monitors the power in the
telephone band to detect the following conditions:
• Ringing
• Reorder
• Fast Busy
• Slow Busy
• Dial Tone
Appendix D:
Continued