Keyed Printed Circuit Boards

for a functional area located in one or more modules. In computers that use a chassis/assembly, cage/rack, or motherboard/backplane design, the functional areas are located on a single pcb or a group of pcb’s located in a single area. The pcb’s generally face in one direction whether they are used in a modular, chassis/assembly, cage/rack, or motherboard/backplane design. Some equipment provides card guides or brackets and locking or tiedown bars, so pcb’s will not suffer intermittent problems as a result of shock and vibrations. KEYED PRINTED CIRCUIT BOARDS.— Pcb’s are keyed to ensure that a different card type is not inserted into a slot or the correct pcb is not inserted backwards. The manufacturer will either cut a slot into the plug-in side of the pcb or put plastic sleeving on one or more of the connector pins (fig. 2-14, frame A). With the pin/plastic sleeving method, the connector receptacle must match the pin(s) with sleeving to accommodate the pcb’s connector pin(s) (plug [s])(fig. 2-14, frame B). COLOR-CODED PRINTED CIRCUIT BOARDS. —Pcb’s are identified by numbers. Some pcb’s in computers show the number(s) with color bands using the standard color code (also shown on fig. 2-14, frame B). With the color-code technique, you can check the card number. The color code is also very convenient when you are working with groups of cards that have the same card number. Refer to NEETS, Module 19, The Technician's Handbook, for the standard color code. MAINTENANCE FEATURES OF PRINTED CIRCUIT BOARDS. —Some pcb’s have indicators and test points that are very helpful when you perform maintenance. Figure 2-14.—Keyed pcb’s: A. Cut slots on a pcb; B. Plastic sleeving on pins. 2-14