8255 Datasheet Download 4 0 ~UPD~

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The PCM-3724 provides 48 fully buffered digital input/output lines. It features Intel 8255 Mode 0 software compatibility for use with all popular software drivers. It also provides the reliability of fully buffered inputs and outputs and high drive capacity at a cost comparable with non-buffered boards.

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Please note that measuring the current draw at the power supply will generally not provide an accurate measure of the coil current. Since the input voltage to the driver can be significantly higher than the coil voltage, the measured current on the power supply can be quite a bit lower than the coil current (the driver and coil basically act like a switching step-down power supply). Also, if the supply voltage is very high compared to what the motor needs to achieve the set current, the duty cycle will be very low, which also leads to significant differences between average and RMS currents. Additionally, please note that the coil current is a function of the set current limit, but it does not necessarily equal the current limit setting. The actual current through each coil changes with each microstep. See the DRV8825 datasheet for more information.

This 3-stablebi-directional 8-bit buffer is used to interface the 8255A to thesystems data bus. Data is transmitted or received by the buffer uponexecution of input or output instructions by the CPU. Control words andstatus information are also transferred through the data bus buffer.

The functionalconfiguration of each port is programmed by the systems software. Inessence, theCPU ¡§output¡¨ a control word to the 8255A. Thecontrol word contains information such as¡§mode¡¨,bit set¡¨, bit reset¡¨, etc. that Initializes thefunctional configuration of the 8255A.

The 8255A containsthree 8-bit ports (A , B, and C). All can be configured in a widevarietyof functional characteristics by the system software but each has itsown special features or personally to further enhance the power andflexibility of the 8255A.

When the reset Inputgoes ¡§high¡¨ all ports will be set to the Inputmode (i.e., all 24 lines will be in the high Impedance state). Afterthereset is removed the 8255A can remain in the input mode with noadditional Initialization required. During the execution of the systemsprogram any of the other modes may be selected using a single outputInstruction. This allows a single 8255A to service a variety ofperipheral devices with a simple software maintenance routine.

The Mode definitionsand possible mode combinations may seem confusing at first but after acursory review of the complete device operation a simple , logicalI/O approach will surface. The design of the 8255A has taken intoaccountthings such as efficient PC board layout, control signal definitionvs PC layout and complete functional flexibility to support almostany peripheral device with no use of the available pints.

When the 8255A isprogrammed to operate in mode 1 or mode 2, control signals are providedthat can used as interrupt request input to the CPU. The interruptrequest signal generated from port C, can be inhibited or enabled bysetting or resetting the associated INTE flip-flop, using the bitset/reset function ofport C.

ACK(Acknowledge Input). A ¡§low¡¨ on this inputinforms the 8255Athat the data from port A or port B has been accepted. In essence,a response from the peripheral device indicating that it has receivedthe data output by CPU.

Any set of eightoutput buffers, selected randomly from Ports B and Ports C can source1mAat 1.5volts. This feature allows the 8255A to directly drive Darlingtontype drivers and high-voltage displays that require such sourcecurrent.

In Mode O, Port Ctransfers data to or from the peripheral device. When the 8255 isprogrammedto function in Modes 1 or 2, Port C generates or accepts¡§hand shaking¡¨ signals with the peripheraldevice. Reading thecontents of Port C allows the programmer to test or verify the¡§status¡¨ of each peripheral device and changethe program flow accordingly. 2b1af7f3a8