This is an old post, preserved for reference.
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All of the Commodore PET series of computers have an IEEE-488 port, used for disk drives and printers. (I checked all of my PETs, and none of them seem to be labelled, but imagine it says IEEE-488 over that port)
Reset buffered through 7417
POKE 59426,0will set all the outputs low. A word on states and levels here. Writing 0 to one of these outputs pulls the bus line low, this is the 'asserted' signal, and means it is active. When no device is asserted, the signal is pulled high by pull up resistors, 'released' to high. Nothing will ever drive the bus high, all they do is let it float or pull it low. To read it back, type
PRINT PEEK(59424)That should show 0. If it does not, there is a problem with PIA#1 or one of the MC3446 buffers. You can then try various other values up to 255 which should have all the output lines high. Testing other pins is a little more complicated, this table shows the values to poke and peek at.
Write 0 (Assert)
Write 1 (Release)
This can be useful to identify which pins are working, and any at fault, but it doesn't show if the problem is the output pin (or it's associated buffer) failing to drive the output low, or the input pin (or it's associated buffer) failing to read back the state correctly. You can check this with a multimeter on the pins of the port, but I have built a board to make that easier.