MEM 800

Posted on Feb 6, 2014

This web site provides schematics and instructions to construct a keypad circuit that can be interfaced to a PC using an 8255 PC interface board. The keypad is directly connected to a `client` computer that in turn transmits information from the keypad over a TCP/IP connection to a remote server. Some potential real world applications include remo

MEM 800
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te control of test equipment or implementing a process control system. ANSI C source code for a basic interface and Visual Basic 6. 0 code for a more advanced client/server application of the keypad are provided as well as schematics and construction tips. The keypad interface circuit is built using a small prototyping board and a combination of wire wrapping and soldering. The decoder is powered by +5VDC taken from the 8255 (Click here for a detailed description and instructions to build an 8255 PC interface card. ) via the 2-pin header connection. The rows (X1. X4) and columns (Y1. Y4) of the keypad are connected to the MM74C922 decoder through an 8-pin header as shown. The capacitors, C1 & C2, are connected to terminals KBM and OSC and control the key debounce period and the keyboard scan rate. From the MM74C922 data sheet, A 1uF capacitor connected to the KBM terminal gives a debounce period of 0. 01 seconds and the keypad data sheet shows that the typical debounce time is below 0. 01 sec. Similarly, a 1uF capacitor connected to the OSC terminal gives a key repeat scanning frequency of approximately 60Hz. Here we see some photos of the components of the keypad interface. Figure 1 shows the actual keypad used for this application. It is connected to the interface card, Figure 2, via the 8 pin header shown at left. The interface card is in turn connected to the 8255 Terminal Expansion card via the 8-pin (data connection)...

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