DM5210 analog input module 1-8 RTD Embedded Technologies, Inc.
Pull-up/Pull-down Resistors on Digital I/O Lines
The 8255 programmable peripheral interface provides 20 TTL/CMOS compatible digital I/O lines which can be
interfaced with external devices. These lines are divided into four groups: eight Port A lines, four upper Port B lines
(the four lower lines are used to control board functions), four Port C Lower lines, and four Port C Upper lines. The
16 lines of Ports A and C are available at the P2 I/O connector. You can install and connect pull-up or pull-down
resistors for these 16 lines as described below. For example, you may want to pull lines up for connection to
switches. This will pull the line high when the switch is disconnected. Or, you may want to pull lines down for
connection to relays which control turning motors on and off. These motors turn on when the digital lines controlling
them are high. The Port A lines of the 8255 automatically power up as inputs - which can float high - during the few
moments before the board is first initialized. This can cause the external devices connected to these lines to operate
erratically. By pulling these lines down, when the data acquisition system is first turned on, the motors will not switch
on before the 8255 is initialized.
To use the pull-up/pull-down feature, you must first install single in-line resistor packs in any or all of the three
locations around the 8255, labeled PA (Port A), PCL (Port C lower), and PCH (Port C upper). The four Port B lines
cannot be pulled up or down by installing resistor packs. PA takes a 10-pin pack, and CL and CH take 6-pin packs.
Figure 1-10 shows this circuitry.
After the resistor packs are installed, you must connect them into the circuit as pull-ups or pull-downs. Locate
the three-hole pads on the module near the resistor packs. They are labeled G (for ground) on one end and V (for
+5V) on the other end. The middle hole is common. PA is for Port A, CL is for Port C Lower, and CH is for Port C
Upper. To operate as pull-ups, solder a jumper wire between the common pin (middle pin of the three) and the V pin.
For pull-downs, solder a jumper wire between the common pin (middle pin) and the G pin. Figure 1-11 shows Port A
lines with pull-ups, Port C Lower with pull-downs, and Port C Upper with no resistors.
Fig. 1-10—Pull-up/Pull-down Resistors for the 8255
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