On 4/22/01 at 9:43 PM Tony Firshman wrote:
>>It was confusing with some early control boards, when the electronics
>>were wired to be active when 'pulled low' to 0; rather than 'taken high'
>>to 1. Inverse logic.
>
>That is very common - called 'common collector'' - sorry about the
>unintentional pun. Nasta is the best one to expand on this - to do
>with the easiest way to set transistor based output. (less than a page
>plse Nasta (8-)# )
>
That's 'open collector' or 'open drain' in MOS logic.
Why have outputs that only pull one way? Simple: because you can tie them
together and have an external resistor (or if you want it a bit faster,
current source) that pulls 'the other way' to implement 'implicit logic' at
no extra cost. Depending on what you assign to a high or low voltage (cause
that's really arbitrary), you get so called 'wired or' or 'wired and'
logic.
Wired OR: common node is low if output 1 is low OR output 2 is low OR...
output n is low.
Wired AND: common node is high if output 1 is high AND output 2 is high
AND... output n is high.
Why outputs that pull low? Simply put, because they were there first.
Somewhere in the 'olden times' one of the very first logic chip families,
called RTL, due to technological reasons, used NPN transistors. With a
single supply, such circuits are in general much better at pulling an
output low than pulling it high. RTL actually had an internal resistor that
pulled high and a transistor that pulled low. Within certain limits the
user could then tie such outputs together creating additional logic without
additional cost. When newer chips, such as TTL became popular, their
ability to also pull high quite well, did not alow simply tying outputs
together. So, for compatibility, additional versions of TTL chips with only
a transistor that pulls low were introduced, such outputs were called 'open
collector'. SOmetimes this output transistor is specified for voltages much
higher than the power supply, this feature is used for logic level
conversion. When MOS logic rolled around, it became 'open drain'.
These conventions are often in use today, even though it's now equally easy
to make outputs pull low or high - simply because the 'one-way-pull' logic
is most cost effective for certain uses.
Incidentally, the QL, quite atypically, has one 'pull up' signal on it's
bus - DSMCL.
>
>I put in a couple of 7804s to invert the 4 L298 power driver enable
>lines, and disable output.
>
7404, surely :-)
Well, it's less than one page on my screen (it's set to 1600x1200 :-) ).
Nasta
