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HW 30 - Sense Lines Page: 1 NOTE: This Technical Note has been retired. Please see the Technical Notes page for current documentation. CONTENTS Sense Lines The Original Scheme Extended Type-7 Sense Codes Ever wonder how your Mac knows what monitors are connected to it? Well, this Technical Note will tell you more than you probably wanted to know. [Sep 01 1992] Extended Type-6 Sense Line Protocol Multiple Scan Monitors Summary References Downloadables Sense Lines The sense line protocol was implemented when Apple recognized the need for a mechanism that would allow a display card to identify the monitor connected to it. For example, the built-in video display circuitry on CPUs such as the Macintosh Quadras and the PowerBook 160 and 165c can configure itself according to the monitor that is connected at boot time. The identification scheme works fine, but there is one problem. There are only three sense lines and this limits the number of different monitors to seven plus the no-connect case. To overcome this limitation, newer display cards and built-in video use extensions to the sense line scheme that allow for new codes. The first extension, type-7, is based on the following idea: When the display circuitry senses a configuration that in the original scheme signals "no display connected" (i.e. when none of the three sense lines are grounded), the card pulls down each sense line, one by one, and reads what the other lines return. To return a unique code, the only requirement is that the sense lines be interconnected, in the cable or the monitor itself, by wires or diodes. The beauty of this idea is that existing monitors are detected correctly and newer monitors can have their own encoding. The circuitry for detecting new monitors is relatively simple and has no active components. Adding the encoding to new or existing monitors involves only inexpensive diodes and wire. The type-6 extension has a similar approach. Back to top The Original Scheme In the original scheme, the display circuitry determines the type of monitor attached by reading three sense lines and comparing the signal value to ground. By convention, the sense lines are identified as 2, 1, and 0. Given three lines and two different states, on or off, there are a total of eight possible combinations. For a monitor to be recognized, a sense line on HW 30 - Sense Lines Page: 2 the monitor side is connected to ground to be read as a binary 0, or left unconnected to be read as a binary 1. The first seven monitors listed in Table S-1 show the required sense line states for them to be recognized using the original sense line scheme. It's worth noting that monitors using the original scheme need only have their sense lines read once to determine the monitor type. As new monitor types became available for the Macintosh, the extended sense line protocol was implemented. Back to top Extended Type-7 Sense Codes The extended sense line protocol is one of those oddities of software eng…