Sequential Circuits Prophet-5 Service Manual - Audiofanzine

notice a slight difference in brightness between the BANK and PROGRAM numeric displays or^.2-11 CONTROL MATRICESThis paragraph describes how the computer scans the control panel to learn what keys or switches havebeen pressed, and to light the LEDs imbedded in switches that are on. As input devices, switches andkeys are scanned indenticaily. In both cases the microcomputer maintains tables in Scratchpad RAMwhich correspond to the status of the Switch/Keyboard Matrix (SW/KBD MIX). That is, each switch orkey has a corresponding memory bit which is set (1) or reset (0) if the switch or key is on or off. Althoughsimilar instructure the switch and keyboard tables are interpreted quite differently. For example theW-MOD FREQ A switch bit causes a digital output to close or open a solid-state switch, and light orextinguish its LED. But a key bit is converted to a key number (1 - 61) which becomes an OSC FREQ CVthrough the DAG. In addition, the fact of a key going on or off must be stored for voice assignment andgeneration of the CATEs.The SW and LED MTX are divided across PCBs 1 and 2; principal components being shown on schematicSD232. The keyboard is wired into the SW MTX through J201. The LED MTX includes all the elements ofDS224 dual BANK^PROGRAM display; all seven-segment decoding being done by software.The program scans the keyboard first. The basic procedure is to activate one matrix row of eightconsecutive keys, then check the intercepting columns for the presence of a bit. The resulting data sentto the GPU by the column bus drivers uniquely identifies a combination of switch closures in each row.Specifically, to scan the first eight keys the CPU sends the number 08(H) to the SW/KBD ROWDECODER by clocking -CSOL2. This selects 58 (U212-18), which holds the first matrix row high. If CO EOand GO happen to be held, the number 10010001 (91H) will be sent to the CPU when it clocks the bus'drivers with -CSIO. This number is then placed in Scratchpad; becoming the first byte In the keyboardtable. To read the next key row the CPU increments the driver to set S9, and reads the second key tablbyte,'Switches are scanned to fill a Scratchpad RAM table in the same way when the CPU sets the driver S0-S4The diodes wired throughout the SW/KBD MATRIX allow n-key rollover, which is the simultaneouspressing of any number of switches and keys. They prevent switched bits from returning through otherclosed switches on the same column, which would activate other rows.For troubleshooting it must be emphasized that most keyboard problems are caused by dirty bent orbroken J-wires. Dead notes not caused by J-wires usually occur in groups of eight, making it easy toisolate the problem row or column. If a switch does not function and its LED doesn't light, the problemmust be in the SW MTX. Check other switches to isolate malfunctions to a single row or column If theLED lights but the function is not enabled, the problem must be in the corresponding output latchsolid-state switch (4016), or analog circuitry.The LED matrix operates on the same line-by-line technique used to scan the switches and keys. In factthe LEDs are "mapped" similarly to the switches so the two tables will correspond. First a number fromthe Scratchpad LED table representing active LEDs in the first column is latched by the LED DRIVERSU204/5 as port -GSOLO. Then the first column is pulled low-causing current to flow through LEDswhose anodes are being pulled high—through U206-10 by latching data 01(H) to U207/08 LED SINKport -CSOL1. To output the next column, the CPU sends the next LED table byte to the LED DRIVERSand rotates the LED SINK bit to pull the second-row cathodes low, and so on. The LEDs are therefore notconstantly It they only seem so due to persistence effects accompanying our sight. Some owners maycomplain of them flickering while playing. Both effects are normal, resulting from different scan timesfor each display, and from the lengthening of the "loop" time with each new keystroke2-15