This is the standard configuration on how to Interface a uC like 8051 to PC RS232 with MAX232A. The UART or Serial port was not present in 8049/8749 chips which were the ancestors of 8051/8031. Pages of code were needed to make a software UART in MCS-48. The 8051 integrated the hardware UART and short commands made it tick. The most important innovation which made uC popular was the C in 80C31. C is for CMOS. This made the chip work cooler and work on even batteries and small power supplies.
80C51, SBC, Firmware and Circuits
The 8749 and 8751 are the UV EPROM type of uC. With limited erase/write cycles. The FLASH revolution changed every thing, you could update firmware over a phone line modem. Even without Ethernet, the firmware could be updated by making every embedded device a node on the EPABX. Now TCP/IP and Wi-Fi makes it all very easy.
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A PC based Analog Voltmeter. The Printer Port or Parallel Port Interface is used to acquire high resolution analog data using ICL7135. This is a method of getting analog data into your PC. This is only of Educational Value. Applications include Data Loggers and PC based Process Monitors, chart recorders. (USB, Wireless Interfaces and Tablet Computers are in vogue today
ICL7135 to Printer port Interface
The four Digit Drive outputs and four BCD outputs of the Multiplexed Display Driver of 7135, along with A-D status output which is also four, are routed to four input pins on printer port via three 74HCT373 Digital Switches. This is because we have less Input pins on Port and 4 are shared by making one 74HCT373 transparent and others High Impedance when PC takes a reading. So the three sets of four outputs of 7135 can be read sequentially.
74HCT139 U9A which is controlled by the PC program can select U3, U4 or U5 for sending data to 4 pins on printer port. Data sent on the eight output pins of printer port can be latched by U10B into U6. This eight outputs of U6 can drive relays or lamps.
The ICL7135 details can be seen here 41/2 A/D with BCD ICL7135 . This Mixed device was far ahead of its times, a product of Intersil.
Using this circuit on Printer Port, one could drive 256 Relays or 16K LEDs as Dot Matrix display. It can be used to drive a Large size multiplexed LED dot matrix display or Latched Relay-Solenoid-Motor-Lamp Array Drivers.
This circuit can be modified for a Static drive output or a fast changing output like a Waveform Generator. You can also make it a 16 Bit waveform generator. The frequency limited to the speed of the port or a fraction of it, depending on 8bit, 16bit or 32bit.
Now I have Some Explaining to do. Latch the U7 with a 8 Bit Data to address the device you want to talk to. So one among the 32 Output Devices can be Selected by a combination of G1-G2 of U5-U8 and U7 8 Bits, Split into Two Nibbles for Upper and Lower 16 Devices. That means 16 * 2 = 32 Devices of 1 Byte each,. 32 * 8 = 256 if my calculations are correct. Please verify.
One of the decoders U5 or U8 decode their respective nibble and output a Low on Selected device to Latch Data on the Chosen one (74HCT373). Why HCT ? Speed is good, low power and CMOS ! and works with TTL too. It Interfaced well for me on a Card with Both TTL and CMOS levels, with a Fast uC.
The 74HCT373 outputs are current amplified and isolated by darlingtons and optoisolators. Both source and sink examples shown. This circuit was not tested and documented properly. So there may be things missing. It is just a Concept design..
Printer Port 256 Relays 16K Dot Matrix – del20021