Data Acquisition

Figure C.8Port C mode 1 output status informationThe 8255 may alternatively be used in mode 1 and the data written by polled (program)transfer. This is done as follows:• Write a byte to the control register to configure the 8255 for mode 1 and theappropriate group for data output• The program continually monitors the /OBF line by reading port C, waitingfor it to go high. A high indicates that the last data written to the port hasbeen read by the external device• Then the program can write new data to the port• The /OBF line goes low to indicate that there is data to be read by the externaldevice from the 8255• The external device pulses the /ACK input low and high to read the data.• This makes the /OBF line go high again and the cycle may be repeated untilall the required data has been writtenThe program could also enable the INTR line with the INTE flip-flop and then monitorINTR instead of the /OBF line. In this case, interrupts to the host computer are notenabled.C.9 Mode 2: strobed bi-directional bus I/OThis mode provides a means for communicating with an external device using an 8-bitbus for both transmitting and receiving data. Both input and output handshaking signalssimilar to mode 1 are provided to maintain proper bus flow discipline. Hardwareinterrupts signal the host computer that the port needs attention.The following characterizes mode 2:• Only group A operates in mode 2• One 8-bit bi-directional port, functions as both input and output• Five of the port C lines take on special functions• Both inputs and outputs are latched• One 3-bit simple I/O port• Data transfer by interrupts or polled I/O