Issue
141 April 2002
Replacing
Relays with Ladder Logic
Part
2: The T100MD-1616+
by
Fred Eady
The
T100MD-1616+
In
Part 1 of this series, I talked about the T100MD-888+,
a programmable, Internet-capable PLC with eight physical
inputs and outputs. As you might have grasped from its
moniker, the T100MD-1616+ has 16 physical I/O ports. The
basic operations I described last time also can be applied
to the more robust T100MD-1616+ PLC you see in Photo 1.
|
(Click
here to enlarge)
|
Photo
1—What you see here is a fully loaded T100MD-1616+.
The analog area (unlabeled blue screw terminals) is
directly left of the red-tagged CPU module. The SRAM
battery/clock module is sandwiched between the SRAM
module and IC socket. |
The
major differences between the T100MD-888+ and T100MD-1616+
can be found in the analog area. Unlike the T100MD-888+,
the T100MD-1616+ demands a separate power supply for the
analog components. Instead of the maximum of eight 10-bit
A/D channels, the number of ’1616+ A/D channels has been
reduced to four. The first two 1-V full-scale analog channels
are buffered with LM324 op-amps set for a gain of five.
The remaining two channels are not buffered and accept
0- to 5-V inputs. All of the analog inputs are protected
with a Zener diode just in case you pull a "Fred."
The
two T100MD-888+ DACs are unbuffered outputs that you must
condition. The first analog output on the T100MD-1616+
is configured at the factory to provide a 20-mA current
loop signal. Current loop output allows a longer distance
between nodes, as wire resistance does not inhibit its
signal. The current loop output is converted to a voltage
using a resistor across the output terminals. Ohm’s Law
prevails here and to achieve a 0- to 5-V output, a 250-W
resistor would be used. The current loop DAC on the T100MD-1616+
is calibrated using an onboard potentiometer and a single
line of TBASIC code, SETDAC 1,2048.
The
second analog output of the T100MD-1616+ is an unbuffered,
unprotected, high-impedance output that is directly connected
to the T100MD-1616+ CPU. This analog output is included
to allow you to build a custom D/A interface for applications
that requires more than one DAC or cannot be adapted to
20-mA current loop operation.
My
T100MD-1616+ came with the 4 × 20 backlit LCD and a battery-backed
62256 SRAM module. The advantages of the LCD are obvious.
Having the battery-backed SRAM enables the T100MD-1616+
I/Os, timers, internal variables, and counters to retain
their values at PLC power down. Only PWM data isn’t retained
in the battery-backed SRAM. Another feature of the battery-backed
SRAM is the inclusion of a PLC real-time clock that’s
nonvolatile.
