 Visual Display Unit
Interlace Board. 200.002
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 ACORN COMPUTERAcorn
Computers Limited, 4a Market Hill, Cambridge CB2 3NJ,
England, |
The Acorn Visual Display Unit Controller Board connects to the standard Acorn Computer Bus and contains a memory mapped character storage RAM, which is transparently written to or read from by the CPU.
An MC 6845 programmable controller IC provides all the synchronisation signals to drive a 625 line 50 fields per second VDU together with read addresses for the character RAM Characters are then fed to an SAA5050 character generator IC which produces the necessary dot patterns to create the characters to refresh the VDU.
The SAA5050 produces Teletext standard characters and has Red. Green and Blue drive outputs giving coloured characters or graphics.
The R.G.B. and sync outputs may be used to drive a colour encoder and modulator for a U.H.F. Television; also provided is a 1 volt / 75 ohm composite sync and video output which can directly drive a Monochrome Monitor on which the different colours will appear as different scales of grey.
The VDU controller PCB is supplied in kit form with a full set of IC sockets. It is easily assembled using a small soldering iron and useful hints on assembly may be found in the Acorn Microcomputer system 1 Technical Manual. The board operates from a single +5v supply from which it draws not more than 500 mA.
Also provided are listings for programs which set up the MC6845, display 25 instructions in hex on the VDU {with double or treble byte instructions on a single line) and allow the drawing or graphics or characters on the VDU These programs may be loaded and run using the Acorn system 1 Monitor. A new monitor ROM will shortly be available for linking the VDU and an ASC II keyboard to Acorns' 4K Fast BASIC.
PCB Acorn Computers Ltd. pt no 200,002
IC1 74LS139 decoder and 16 pin socket IC2 74LS02 NOR gate and 14 pin socket IC3 74LS13 schmitt trigger and 14 pin socket IC4 MC6845 CRT controller and 40 pin socket IC5,6&7 74LS157 data selector and 16 pin sockets IC8&9 2114-3 RAM and 18 pin sockets IC10 74LS174 hex flip-flop and 16 pin socket IC11 INS8208 buffer and 20 pin socket IC12 74LS86 exclusive-or gate and 14 pin socket IC13 SAA5050 character generator and 28 pin socket IC14 74LS00 NAND gate and 14 pin socket
Q1 BC107 transistor
D1 1N4148 diode
R1 1K resistor R2 2K7 resistor R3 470 R preset potentiometer R4 10K resistor R5 4K7 resistor not supplied with kit R6 22K resistor R7 4K7 resistor R8 10K resistor R9-12 4 off 10K resistors R13 4K7 resistor R14 10K resistor R15 470R resistor R16 82R resistor R17 18K resistor not supplied with kit R18 10K resistor R19 10K resistor not supplied with kit R20-22 3 off 10K resistors
C1 10 or 15 µF electrolytic capacitor C2-7 6 off 47 nF capacitors C8 270 pF capacitor C9 150 pF capacitor
NB For IC11 INS8208 is equivalent to DP8304
To use this board in the Acorn system a low for block-zero signal must be present on pin 31A of the Acorn bus. This is provided by connecting pin 8 of IC9 to pin 31A of the connector on the 6502 CPU board. The 5 volt regulator and the electrolytic capacitor must be removed from the CPU board and an external 5 volt supply at 3 amps is required on pin 1 A of all boards for a system using the CPU, VDU interface, 8K memory board and cassette interface.
A 1 volt at 75 ohm video monitor may be driven directly from connections 14 and 15 on the front of the board. Alternatively a UHF modulator may be driven which connects to the aerial input of a domestic television set using the circuit shown:
The component values given suit the Astec UM 1233E36 modulator although other types should work satisfactorily. IC3 on the VDU board forms a 6MHz dot clock which is adjusted using the preset resistor R3 to give characters of the right width on a steady picture after MC6845 has been initialised using the Applications programmes.
The 1024 byte character memory appears in block zero of the Acorn address map filling pages 04, 05 06 and 07. Two registers, which set up the 6845, are at addresses 0800 and 0801 and these repeat throughout page 08. The register at 0800 is a 5 bit write only register used to determine which of the data registers is accessed at 0801. The data registers, their addresses and their functions are tabulated here.
| Register Address |
Function | Program Unit |
Type | No of Bits Used |
| 00 | Horizontal total | char | Write only | 8 |
| 01 | Horizontal displayed | " | " | 8 |
| 02 | H Sync Position | " | " | 8 |
| 03 | H Sync Width | " | " | 4 |
| 04 | Vertical total | char row | " | 7 |
| 05 | V total adjust | scan line | " | 5 |
| 06 | Vertical displayed | char row | " | 7 |
| 07 | V sync position | char row | " | 7 |
| 08 | Interlace mode | - | " | 2 |
| 09 | Max scan line address | scan line | " | 5 |
| 0A | Cursor start | scan line | " | 5+2 |
| 0B | Cursor end | scan line | " | 5 |
| 0C | Start address H | - | " | 6 |
| 0D | Start address L | - | " | 8 |
| 0E | Cursor H | - | Read/Write | 6 |
| 0F | Cursor L | - | Read/Write | 8 |
| 10 | Light pen H | - | Read only | 6 |
| 11 | Light pen L | - | " | 8 |
Using the programs provided all these registers are initialised for 625 line, 50 fields per second, non-interlaced operation with 25 rows each of 40 characters.
Hardware scrolling is used in the system which, with transparent access, gives a clean display and uses only a very small proportion of CPU's time. The character memory may be read from or written to at any time like any other piece of RAM
The light-pen input to the board may be used after breaking the link to 0v and fitting a 4K7 resistor for R5. When the light-pen input goes high, the contents of the character refresh address counter are strobed into registers 10 and 11 i.e. high byte and low byte. Thus the screen co-ordinates of the pen position may be determined.
The SAA 5050 character generator produces the following characters represented by the hex bytes shown.
20 space 30 0 40 @ 50 P 60 - 70 p 21 ! 31 1 41 A 51 Q 61 a 71 q 22 " 32 2 42 B 52 R 62 b 72 r 23 £ 33 3 43 C 53 S 63 c 73 s 24 $ 34 4 44 D 54 T 64 d 74 t 25 % 35 5 45 E 55 U 65 e 75 u 26 & 36 6 46 F 56 V 66 f 76 v 27 ' 37 7 47 G 57 W 67 g 77 w 28 ( 38 8 48 H 58 X 68 h 78 x 29 ) 39 9 49 I 59 Y 69 i 79 y 2A * 3A : 4A J 5A Z 6A j 7A z 2B , 3B ; 4B K 5B ¬ 6B k 7B ¼ 2C + 3C < 4C L 5C ½ 6C l 7C || 2D _ 3D = 4D M 5D ® 6D m 7D ¾ 2E · 3E > 4E N 5E 6E n 7E ÷ 2F / 3F ? 4F O 5F # 6F o 7F delete
Note that there are a few exceptions to the usual ASCII, the most noticeable of which is at 23 where £ replaces # which is moved from 23 to 5F.
Also available are the following teletext control characters with their hexadecimal codes shown:
01 Alpha Red 11 Graphics Red 02 " Green 12 " Green 03 " Yellow 13 " Yellow 04 " Blue 14 " Blue 05 " Magenta 15 " Magenta 06 " Cyan 16 " Cyan 07 " White 17 " White
08 Flash 18 Conceal Display 09 Steady 19 Contiguous Graphics 0A End Box 1A Separated Graphics 0B Start Box 0C Normal Height 1C Black Background 0D Double Height 1D New Background 1E Hold Graphics 1F Release Graphics
These characters affect the characters displayed to the right of them on a line, and provide the following options:
ALPHA (COLOUR) causes following characters on the line to be in the colour specified.
GRAPHICS (COLOUR) In Graphics mode each character space displayed is divided into 6 cells. Each cell is illuminated, in the specified colour, if a corresponding bit in the ASCII code stored at that location is set.
The bit assignments are:
b0 b1 b2 b3 b4 b5
Bit 5 in the byte is always set for a graphics character, if bit 5 is clear then the upper case, alphabet and characters ¬ , ½, ® , , #, @ are available in the same colour.
| FLASH | causes the following characters on the line to flash. |
| STEADY | terminates the flash option on a line. |
| END BOX & START BOX | Are options for using the circuit to super- impose text onto a normal TV picture. |
| DOUBLE HEIGHT | Must appear on two consecutive lines followed by the same characters in which case the characters fill the two lines specified. |
| NORMAL HEIGHT | Clears the double height option on a line. |
| CONCEAL DISPLAY | Switches off the character on a line these can only be revealed by applying a signal to the character generator chip (or deleting the CONCEAL character). |
| CONTIGUOUS GRAPHICS | In this mode the graphics cells in a character are joined up. |
| SEPARATED GRAPHICS | In this mode the graphics cells are separated by one character bit width. |
| NEW BACKGROUND | Sets the background to the colour of the last colour specifying character. |
| BLACK BACKGROUND | Resets the background colour to black. |
| HOLD GRAPHICS | Causes the last graphics character to be displayed when a control character is entered. |
| RELEASE GRAPHICS | Causes a space (Background colour) to be displayed when a control character is entered. |
The character generator assumes an initial setting at the start of each line of: -
ALPHA WHITE, STEADY, END BOX, NORMAL HEIGHT.
CONTIGUOUS GRAPHICS, BLACK BACKGROUND, RELEASE GRAPHICS.
The program listed below may be loaded using the Acorn system 1 monitor starting at address 0300. This program is intended to be used as a subroutine by other programs and will take the contents of the accumulator and display the corresponding ASCII character on the screen at the current cursor position on being jumped to (JSR) at 0300. Before returning this subroutine will advance the cursor position and if the current line is full it will scroll the screen. Four control codes in the accumulator are intercepted by the program and produce the following action.
0A ... Line feed
0C ... Form feed i.e. clear screen and home cursor
0D ... carriage return
7F ... back space cursor and delete character
The code 0C must always be used after switch on to initialise the 6845 controller IC, clear the character RAM and home the cursor.
If the SAA5050 characters generated by the codes 0A, 0C, 0D and 7F are required on the screen the same characters may be produced using the codes 8A, 8C, 8D and FF which the subroutine does not intercept.
VDU ACORN 6502 Assembler
0010: 0300 VDU ORG $0300 0020: 0300 SCAP ORG $0020 0030: 0300 LINE ORG SCAP +01 0040: 0300 WORK ORG LINE +02 0050: 0300 SCRA ORG $0400 memory addresses for the screen 0060: 0300 SCRB ORG $0500 0070: 0300 SCRC ORG $0600 0000: 0300 SCRD ORG $0700 0090: 0300 CRTA ORG $0800 6845/6545 CRT controller 0100: 0300 CRTB ORG $0801
0110: 0300 A4 20 CHATS LDY SCAP character to screen 0120: 0302 C9 20 CMPIM $20 0130: 0304 90 37 BCC CTL all control characters 0140: 0306 C9 7F CMPIM $7F 0150: 0308 F0 27 BEQ DELETE 0160: 030A 20 CD 03 TOSCRN JSR WRCH 0170: 030D C8 INY 0180: 030E C0 28 CPYIM $28 0190: 0310 90 05 BCC VDUB automatic scroll when line filled 0200: 0312 20 80 03 FILLED JSR SCROLL 0210: 0315 A0 00 VDUA LDYIM $00 0220: 0317 20 B5 03 VDUB JSR CALCN 0230: 031A 84 20 STY SCAP 0240: 031C A0 0F LDYIM $0F rewrite cursor position 0250: 031E 8C 00 08 STY CRTA 0260: 0321 A4 23 LDY WORK 0270: 0323 8C 01 08 STY CRTB 0280: 0326 A0 0E LDYIM $0E 0290: 0328 8C 00 08 STY CRTA 0300: 032B A4 25 LDY WORK+02 0310: 032D 8C 01 08 STY CRTB 0320: 0330 60 VDUC RTS
0330: 0331 88 DELETE DEY 0340: 0332 30 FC BMI VDUC refuse to delete before line start 0350: 0334 A9 20 LDAIM ' ' write in a blank 0360: 0336 20 CD 03 JSR WRCH 0370: 0339 A9 7F LDAIM $7F 03B0: 033B D0 DA BNE VDUB
0390: 033D C9 0D CTL CMPIM $0D carriage return? 0400: 033F F0 D4 BEQ VDUA 0410: 0341 C9 0A CMPIM $0A line feed? 0420: 0343 F0 06 BEQ SCROL 0430: 0345 C9 0C CMPIM $0C form feed? 0440: 0347 F0 09 BEQ CLEARS 0450: 0349 D0 BF BNE TOSCRN
0460: 034B 20 80 03 SCROL JSR SCROLL scroll screen and rewrite cursor 0470: 034E A4 20 LDY SCAP 0480: 0350 B0 C5 BCS VDUB
0490: 0352 48 CLEARS PHA clear entire buffer 0500: 0353 A0 00 LDYIM $00 0510: 0355 A9 20 LDAIM ' ' 0520: 0357 99 00 04 CLEAR STAAY SCRA 0530: 035A 99 00 05 STAAY SCRB 0540: 035D 99 00 06 STAAY SCRC 0550: 0360 99 00 07 STAAY SCRD 0560: 0363 C8 INY 0570: 0364 D0 F1 BNE CLEAR 0580: 0366 84 20 STY SCAP 0590: 0368 A0 0F LDYIM $0F
0600: 036A 8C 00 08 SETCRT STY CRTA set up all the CRT parameters 0610: 036D B9 D9 03 LDAAY CRTTAB 0620: 0370 8D 01 08 STA CRTB 0630: 0373 88 DEY 0640: 0374 10 F4 BPL SETCRT 0650: 0376 A9 C0 LDAIM $C0 0660: 0378 85 21 STA LINE 0670. 037A A9 07 LDAIM $07 0680: 037C 85 22 STA LINE+01 0690: 037E 68 PLA 0700: 037F 60 RTS
0710: 0380 08 SCROLL PHP scroll subroutine 0720: 0381 48 PHA 0730: 0382 D8 CLD 0740: 0383 A0 28 LDYIM $28 0750: 0385 20 B5 03 JSR CALCN 0760: 0388 A5 23 LDA WORK 0770: 038A 85 21 STA LINE 0780: 038C A5 25 LDA WORK+02 0790: 038E 85 22 STA LINE+01 0800: 0390 A0 0D LDYIM $0D 0810: 0392 8C 00 08 STY CRTA 0820: 0395 A5 21 LDA LINE 0830: 0397 38 SEC 0840: 0398 E9 C0 SBCIM $C0 0850: 039A 8D 01 08 STA CRTB 0860: 039D 88 DEY 0870: 039E 8C 00 08 STY CRTA 0880: 03A1 A5 25 LDA WORK+02 0890: 03A3 E9 03 SBCIM $03 0900: 03A5 8D 01 08 STA CRTB 0910: 03A8 A0 27 LDYIM $27 0920: 03AA A9 20 LDAIM ' '
0930: 03AC 20 CD 03 CLEARL JSR WRCH 0940: 03AF 88 DEY 0950: 03B0 10 FA BPL CLEARL 0960: 03B2 68 PLA 0970: 03B3 28 PLP 0980: 03B4 60 RTS
0990: 03B5 08 CALCN PHP do calculation to make sure that the 1000: 03B6 48 PHA processor and CRT controller agree on 1010: 03B7 D8 CLD position of screen r : 1020: 03B8 18 CLC 1030: 03B9 98 TYA 1040: 03BA 65 21 ADC LINE 1050: 03BC 85 23 STA WORK 1060: 03BE A5 22 LDA LINE+01 1070: 03C0 69 00 ADCIM $00 1080: 03C2 85 25 STA WORK+02 1090: 03C4 29 07 ANDIM $07 1100: 03C6 09 04 ORAIM $04 1110: 03C8 85 24 STA WORK+01 1120: 03CA 68 PLA 1130: 03CB 28 PLP 1140: 03CC 60 RTS
1150: 03CD 20 B5 03 WRCH JSR CALCN 1160: 03D0 84 25 STY WORK+02 1170: 03D2 A0 00 LDYIM $00 1180: 03D4 91 23 STAIY WORK 1190: 03D6 A4 25 LDY WORK+02 1200: 03D8 60 RTS
1210: 03D9 3F CRTTAB = $3F total number of characters per line, 1220: 03DA 28 = $28 40 characters displayed 1230: 03DB 33 = $33 position of horizontal sync 1240: 03DC 05 = $05 width in m S of horizontal sync pulse 1250: 03DD 1E = $1E total number of character row 1260: 03DE 02 = $02 additional no. of lines for 312 total 1270: 03DF 19 = $19 25 displayed character rows 1280: 03E0 1B = $1B position of vertical sync pulse 1290. 03E1 00 = $00 set non-interlace mode 1300: 03E2 09 = $09 set 10 lines per character row 1310: 03E3 68 = $68 slow blink cursor from line 9 1320: 03E4 09 = $09 to line 10 1330: 03E5 04 = $04 high address of VDU ram 1340: 03E6 00 = $00 low address of VDU ram 1350: 03E7 07 = $07 high address of initial cursor position 1360. 03E8 C0 = $C0 low address of initial cursor position
Two further programs may be loaded which empty the VDU control subroutine.
The keywrite program is entered at MAIN (0280). When this program is first used after switching on a form feed can be sent to the screen to program the 6845 by pressing the 'r' key.
The program then accepts pairs of Hex characters before sending them to the screen, by-passing the control interpreter to allow any code to be sent. The control keys provide the following functions
l Line feed g space ^ delete p space V carriage return s space r form feed m return to monitor
KEYWRT ACORN 6502 Assembler
0010: 0280 KEYWRT ORG $0280 0020: 0280 TEMP ORG $0026 0030: 0280 RESTRT ORG $FF04 0040: 0280 DISPLA ORG $FE0C 0050: 0280 VDU ORG $0300 0060: 0280 20 0C FE MAIN JSR DISPLA 0070: 0283 90 0E BCC SEND hex key ? 0080: 0285 29 07 CONTRL ANDIM $07 0090: 0287 F0 1B BEQ RETURN 0100: 0289 A8 TAY look up control keys in table 0110: 028A B9 A6 02 LDAAY TABLE-01 0120: 028D 20 00 03 SENDER JSR VDU 0130: 0290 4C 80 02 JMP MAIN
0140: 0293 0A SEND ASLA 0150: 0294 0A ASLA 0160: 0295 0A ASLA 0170: 0296 0A ASLA 0180: 0297 85 26 STA TEMP 0190: 0299 20 0C FE JSR DISPLA 0200: 029C B0 E7 BCS CONTRL 0210: 029E 05 26 ORA TEMP mix in low digit 0220: 02A0 09 80 ORAIM $80 fool control character check 0230: 02A2 30 E9 BMI SENDER forced branch to sender 0240: 02A4 4C 04 FF RETURN JMP RESTRT M key returns to monitor 0250: 02A7 20 TABLE = $20 G key gives space bar 0260: 02AB 20 = $20 P key gives space bar 0270: 02A9 20 = $20 S key gives space bar 0280: 02AA 0A = $0A L key gives line-feed 0290: 02AB 0C = $0C R key gives form-feed 0300: 02AC 7F = $7F ^ key gives delete 0310: 02AD 0D = $0D ^ I key gives carriage return
The mini-Disassembler provides a formatted listing of 25 lines of 6502 program instructions in machine code presented in Hexadecimal.
The start address of the program to be disassembled is entered into the m address of the Acorn Monitor using the m key and the disassembler program is then executed from 0200 using the g key in the normal way. A formatted 25 lines of instructions will be displayed on the screen and the program ends in a jump back to the monitor. The m address is stepped forwards so re-running the program will display the next 25 program lines.
MINDIS ACORN 6502 Assembler
0010: 0200 MINDIS ORG $0200 0020: 0200 MOD * $0000 0030: 0200 COUNT * $000E 0040: 0200 VDU * $0300 0050: 0200 RESTRT * $FF04
0060: 0200 A9 18 START LDAIM $18 disassemble 25 lines 0070: 0202 85 0E STA COUNT 0080: 0204 D8 CLD 0090: 0205 A9 0C LDAIM $0C start with a form-feed 0100: 0207 20 00 03 JSR VDU
0110: 020A A9 0D MAIN LDAIM $0D carriage return/line feed for each line 0120: 020C 20 00 03 JSR VDU 0130: 020F A9 0A LDAIM $0A 0140: 0211 20 00 03 JSR VDU 0150: 0214 A5 01 LDA MOD+01 display current address 0160: 0216 20 58 02 JSR SPBYTE 0170: 0219 A5 00 LDA MOD 0180: 021B 20 5F 02 JSR BYTOUT 0190: 021E A0 00 LDYIM $00 0200: 0220 A2 01 LDXIM $01 x will be the byte count of the opcode 0210: 0222 B1 00 LDAIY MOD fetch opcode, find it's no. of bytes 0220: 0224 C9 20 CMPIM $20 'jsr' is an anomaly and is done first 0230: 0226 F0 17 BEQ CBYTE 0240: 0228 29 9F ANDIM $9F 0250: 022A F0 15 BEQ ABYTE binary 0XX00000 is 1 byte 0260: 022C 29 1D ANDIM $1D 0270: 022E C9 19 CMPIM $19 0280: 0230 F0 0D BEQ CBYTE binary XXX110X1 is 3 bytes 0290: 0232 29 0D ANDIM $0D 0300: 0234 C9 08 CMPIM $08 0310: 0236 F0 09 BEQ ABYTE binary XXXXX0X0 (now) is 1 byte 0320: 0238 29 0C ANDIM $0C 0330: 023A C9 0C CMPIM $0C 0340: 023C F0 01 BEQ CBYTE binary XXXX11 XX is 3 bytes 0350: 023E CA DEX all others are 2 byte
0360: 023F E8 CBYTE INX 0370: 0240 E8 INX 0380: 0241 A0 00 ABYTE LDYIM $00 0390: 0243 B1 00 LDAIY MOD 0400: 0245 20 58 02 JSR SPBYTE 0410: 0248 E6 00 INC MOD increment 0420: 024A D0 02 BNE NOINC 0430: 024C E6 01 INC MOD +01
0440: 024E CA NOINC DEX print all bytes required 0450: 024F D0 F0 BNE ABYTE 0460: 0251 C6 0E DEC COUNT 0470: 0253 10 B5 BPL MAIN finished the 25 lines
0480: 0255 4C 04 FF GETOUT JMP RESTRT
0490: 0258 48 SPBYTE PHA print a space and then the byte 0500: 0259 A9 20 LDAIM ' ' 0510: 025B 20 00 03 JSR VDU 0520: 025E 68 PLA
0530: 025F 48 BYTOUT PHA print a byte 0540: 0260 4A LSRA 0560: 0261 4A LSRA 0660: 0262 4A LSRA 0570: 0263 4A LSRA 0580: 0264 20 68 02 JSR DIGOUT 0590: 0267 68 PLA
0600: 0268 29 0F DIGOUT ANDIM $0F print the bottom Hex digit in A 0610: 026A 09 30 ORAIM $30 0620: 026C C9 3A CMPIM $3A 0630: 026E 90 02 BCC PUT 0640: 0270 69 06 ADCIM $06 0660: 0272 4C 00 03 PUT JMP VDU
The VDU controller may reside in memory with the Keywriter, the Mini-Disassembler or both and subsequently they may be stored on cassette tape in the usual way. User program space is then reduced to the range 0020 through 00FF with the page 0100 to 01FF used for stack space as before. The RAM in the INS 8154 IC's is also free to the user .
The text above was checked by editing it into the ca65 assembler format, and using a diff program to compare the listings. Several errors were spotted an corrected. The ca65 source files are available to save others the chore of editing them again.
