Commercially available power-on self-test (POST) cards for PCs have onboard LED displays that show diagnostic codes during the PC's start-up. This arrangement is usually good enough. But if you were to try to use such a card to fix a crowded PC motherboard, you would appreciate a detachable, remote display.

The circuit in Fig 1 has two sections: an interface/transmitter board and a receiver/display board. On the interface/transmitter board, controller IC₁ contains an address decoder and a synchronous state machine. IC₂ and IC₃ are identical, registered seven-segment HEX transcoders. Listings 1 and 2 contain these PLDs' specifications. You can download the compressed ZIPfile attached to Edn's FTP Site which contains documentation, circuit diagrams, and listings.

When the PC's processor writes data to the diagnostic port (I/O location 80_HEX), the interface/transmitter board sends the appropriate bit patterns serially to the receiver/display board. Note that IC₂'s data output connects back to the serial input of IC₃. So, after 14 clock ticks, IC₃ gets the original data back. Thus, IC₃'s parallel-segment outputs can drive an onboard display if you desire to add one.

The receiver/display board comprises two low-cost serial-in parallel-out (SIPO) registers, IC₄ and IC₅. Because the design transfers data serially, the remote display needs only a four-wire cable. (DI #1626)