Not all 82C55’s modes are permitted. Since the board is not able to furnish an IRQ management,the only 82C55’s mode is the “0” one. It means that the control word has four not configurable bits: The mode “0” provide two 8-bits ports and two 8-bits, each one may be individually configured asinput or output writing a particular byte at the control word.The board’s software configuration basically depends by his hardware configuration. Below aresummarized these particularity. E497-A (48 TTL I/O channels) All the ports, A, B, C_low and C_high, can be configured independently as input or output asbefore mentioned. E497-B (16 optoinsulated Inputs + 16 high current Outputs) In this case, due by the presence of input/output supplementary circuitry, the board’s softwareconfiguration is imposed. More exactly: PPIPortDirection 82C55_1C_highInput or Output AOutput BOutput C_low(*) 82C55_2C_highNone AInput BInput C_lowNone
(*)The CN4’s pins of C_low port, are used to provide the external powersupply (EXT_V) for the output drivers. 11