Lexicon 200, Service Manual

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Lexicon
6-11
(ROW_REG_[2:0]/), a reset line (FP_RESET/), two front panel switch monitoring signals
(SWITCH_ROW[1:0]/) and an eight bit data bus (FRNT_D[7:0]).
Octal Registers
The purpose of these registers (U2-U4) is to latch the data value presented by the Main Board depending
upon which register is selected for activation by the decoded strobes. All LEDs are electrically arranged in a
21 row by 3-column matrix. These three octal registers access the rows and columns of that matrix. U3 and
U4 perform the sole function of accessing sixteen of the matrix rows, while U2 accesses the remaining five
rows and the three columns. The rows are connected to the LED cathodes while the columns are
connected to the LED anodes. R4-R6 provide current limiting to the bases of Q1-Q3, while R1-R3 ensure
that the transistors will turn off when they become inactive.
The outputs of U4 are series terminated with resistors R20-R27. These provide limiting of the forward
current through each LED. The values were chosen to limit the current sinking demands of U4. The strobe
for U4 is decoded from address 0x4C02. The following table illustrates the relationship between Data Bus
and LED. Column division is readily implied by the table layout as well.
Address: 0x4C02
Write Only
Active LED Cathodes
Active Row Data Bus Bit COLUMN_0 COLUMN_1 COLUMN_2
LED_ROW0/ 0 D23 (Dual Stereo) D21 (Chorus) DISP1 (Segment F)
LED_ROW1/ 1 D32 (Threshold:EDIT) D24 (Cascade) DISP1 (Segment G)
LED_ROW2/ 2 D25 (Mono Split) D33 (Attack) DISP1 (Segment E)
LED_ROW3/ 3 D26 (Dual Mono) D17 (Flange) DISP1 (Segment D)
LED_ROW4/ 4 D15 (Plate) D27 (Mix) DISP1 (Segment C)
LED_ROW5/ 5 D12 (Chamber) DISP2 (Segment A) DISP1 (Segment B)
LED_ROW6/ 6 D29 (EQ) D5 (R_MID) DISP1 (Segment A)
LED_ROW7/ 7 D35 (Load Button LED) DISP2 (Segment F) D28 (Adjust)
U3 performs in exactly the same manner as U4. The series termination resistors are designated as R12-
R19. The strobe for this register is decoded from address 0x4C03. As before, the column division may be
implied by the table layout.
Address: 0x4C03
Write Only
Active LED Cathodes
Active Row Data Bus Bit COLUMN_0 COLUMN_1 COLUMN_2
LED_ROW8/ 0
D11 (Compressor:EDIT) DISP2 (Segment G) D19 (Hall)
LED_ROW9/ 1
D20 (Ambience) DISP2 (Segment E) D30 (Lvl/Bal)
LED_ROW10/ 2
D38 (Store Button LED) DISP2 (Segment D) D13 (Gate)
LED_ROW11/ 3
D14 (Echo/Delay) DISP2 (Segment C) D22 (Pitch/Detune)
LED_ROW12/ 4
D2 (L_MID) DISP2 (Segment B) DISP1 (Decimal
LED_ROW13/ 5
D4 (R_CLIP) DISP2 (Decimal Pt) D16 (Room)
LED_ROW14/ 6
DISP3 (Segment F) D7 (Threshold: D31 (Ratio)
LED_ROW15/ 7
DISP3 (Segment G) D18 (Rotary/Tremolo) D34 (Release)
U2 follows the same model as U3 and U4 for FRNT_D[4:0]. The series limiting resistors are designated as
R7-R11. The address decode for this strobe is 0x4C04.
Address: 0x4C04
Write Only
Active LED
Active Row Data Bus COLUMN_0 COLUMN_1 COLUMN_2
LED_ROW16/ 0 DISP3 (Segment E) D1(L Clip) D37 (Compressor Button
LED_ROW17/ 1 DISP3 (Segment D) D36 (Edit Button LED) D3 (L_Signal)