unit X was either 1st or 5th, Y was either 2nd or 4th, and Z was 3rd in line. It is also possible to have multiple channels attached to a control unit from the same or multiple mainframes, thus providing a rich high-performance, multiple-access, and backup capability. Typically the total cable length of a channel is limited to 200 feet, less being preferred. Each control unit accounts for about 10 "feet" of the 200-foot limit. Block multiplexer channel IBM first introduced a new type of I/O channel on the Model 85 and Model 195, the 2880 block multiplexer channel, and then made them standard on the System/370. This channel allowed a device to suspend a channel program, pending the completion of an I/O operation and thus to free the channel for use by another device. A block multiplexer channel can support either standard 1.5 MB/second connections or, with the 2-byte interface feature, 3 MB/second; the latter use one tag cable and two bus cables. On the S/370 there is an option for a 3.0 MB/s data streaming channel with one bus cable and one tag cable. The initial use for this was the 2305 fixed-head disk, which has 8 "exposures" (alias addresses) and rotational position sensing (RPS). Block multiplexer channels can operate as a selector channel to allow compatible attachment of legacy subsystems. Basic hardware components Being uncertain of the reliability and availability of the then new monolithic integrated circuits, IBM chose instead to design and manufacture its own custom hybrid integrated circuits. These were built on 11 mm square ceramic substrates. Resistors were silk screened on and discrete glass encapsulated transistors and diodes were added. The substrate was then covered with a metal lid or encapsulated in plastic to create a "Solid Logic Technology" (SLT) module. A number of these SLT modules were then flip chip mounted onto a small multi-layer printed circuit "SLT card". Each card had one or two sockets on one edge that plugged onto pins on one of the computer's "SLT boards". This was the reverse of how most other company's cards were mounted, where the cards had pins or printed contact areas and plugged into sockets on the computer's boards. Up to twenty SLT boards could be assembled side-by-side (vertically and horizontally) to form a "logic gate". Several gates mounted together constituted a box-shaped "logic frame". The outer gates were generally hinged along one