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PDP-8 TTY Output from Switch Register


This program repetetively writes the bit pattern from the switch registers 4 through 11 to the TTY output. It's useful for determining baud rate problems as well as general diagnostics. To use it to determine a baud rate problem (my card was outputting at 90 baud instead of 110 baud), selectively raise individual bits and look at the data being transmitted (I just hex dump the output of the serial port). If there is more than one bit on in the serial data stream when only one bit should be on (as indicated by your selection of the SR values) then the baud rate on the receiver is too high -- a single bit is being stretched out to several bit positions.


0000 7200 cla / clear accumulator 0001 6046 tls / dummy write (to clear flag) loop, 0002 6041 tsf / skip if flag set 0003 5002 jmp loop / loop waiting for flag to clear 0004 7604 las / get character from SR into AC 0005 6046 tls / write character to TTY 0006 5002 jmp loop / loop for next character
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