Or Return To On Line Magazine 1st April 2001
Updated 11/12/2004

There were errors in an earlier version of this article and download. They are now corrected.

For those that used the old diagram, the correct wiring is for both 5Ds on the two 74LS374s to be strapped to PB4, and both 6Ds to PB5

1 Introduction

This document describes an EPROM programmer I designed and built circa 1985, for
use with a BBC microcomputer.

Pictures of the finished programmer. Click on the thumbnail for a larger image




This one built by John Simpson:

2 Using the programmer

The EPROM programmer should be plugged into the BBC computer�s user port. The switch should always be set to the read position before starting the software. The software should be run from disc; this will show a menu allowing ROM images to be loaded, saved, programmed, and verified, and operating system commands to be issued.

3 Hardware

I "designed" the EPROM programmer by adapting an existing design, published in the Beebug magazine. My alterations were to add a second address latch and a state machine to select the read/write functions and latches. This allowed the programmer to be driven with just the user port, instead of using the printer port as well (as the Beebug design had done). My knowledge of hardware was just adequate for the task; the board works fine, but more experienced hardware designers will probably find flaws in the design.

The circuit diagram is shown in figure 1. Note that the 27128 EPROM is shown with the pins mirrored left for right, because the socket was mounted on the reverse (track) side of the board I built the programmer on. Also, for cleanliness in the layout, I have shown the +5v and 0v pins at the wrong ends of the edge connector; be careful when laying out a board not to copy this order onto the board. The circuitry at the bottom selects the programming functions. The CB2 line from the user VIA (6522 interface adapter) is used to control the programmer, with the CB1 line providing feedback. The low and high address latches are loaded first, and then the output enable or program enable are driven low to read or write the EPROM. Table 2 shows the state machine implemented by this hardware. The outputs from the circuit are CB1 (used for feedback to the BBC so it can detect which state the programmer is in), Enable (set low to make the EPROM read or write data from or to the data bus), CKLO (used to set the low order address latch) and CKHI (used to set the high order address latch). The CB2 line controls the state machine; it drives the clock line of the 74LS74 flip-flop, triggering a new state when it is set low and then high again. The 74LS374 latches are also edge-triggered, so the latches will only capture data

Figure 1: Circuit diagram of EPROM programmer
Click on the image for a larger picture

from the data bus when transitioning from low to high. There are only three states in the cycle; the remaining possible state (when Q1 and Q2 are both 1) will transition to the R/W state, and then the state cycle will repeat. When the software initialises the programmer, it repeatedly clocks the state until the CB1 line changes. I implemented the design using vero-board, managing to fit it into a 37 by 36 section of 0.1 inch pitch board. Photos may be found on my web site at http://knackered.org/angus/beeb/.

4 Software

ssd disc image here

The software to drive the EPROM programmer is a 6502 assembly program. It performs the functions of loading ROM images, saving ROM images, reading, writing, and verifying EPROMs. The 6502 assembler source is shown below, in the format for my own assembler. Some assembler directives (EQB, EQA, EQD) may be unfamiliar. Angus Duggan, 2004-12-10

Figure 2: State machine implemented by 74LS00, 74LS08 and 74LS74.

Their meanings are:

EQA Inserts an ASCII string at the current program counter position, with no termi-nation nor preceding length byte.

EQB Insert a byte at the current program counter position. A list of bytes may be specified, and constant arithmetic expressions may be used.

EQW Insert a two-byte word at the current program counter position, low byte first. A list of words may be specified, and constant arithmetic expressions may be used.

EQD Insert a four-byte double word at the current program counter position, least significant byte first. A list of double words may be specified, and constant arithmetic expressions may be used.

ORG Sets the default origin (initial program counter) for the code. If an execution address is not also specified, the execution address is set to the origin, Use the low or high (second) byte of an expression evaluating to an address.
brkmsg=&FD
escape=&FF \ escape flag
brkvec=&202 \ BRK vector
irqvec=&206 \ interrupt vector 2
orb=&FE60 \ User VIA registers
irb=&FE60
ddrb=&FE62
t2l=&FE68
t2h=&FE69
acr=&FE6B
pcr=&FE6C
ifr=&FE6D
ier=&FE6E
osfile=&FFDD \ OS addresses
osnewl=&FFE7
osrdch=&FFE0
oswrch=&FFEE
osword=&FFF1
osbyte=&FFF4
oscli=&FFF7
zpwork=&70 \ work space for PRINT routine
length=&74 \ length of EPROM (high byte)
eaddr=&75 \ address in EPROM (high byte)
baddr=&76 \ address in buffer (page address)
middle=&78 \ address of loop routine
buffer=&3C00 \ buffer for ROMS &3C00-&7C00
delay=50000 \ 50ms delay in 1MHz clock cycles
menux=4 \ menu indentation
menuy=5 \ menu position
menu=7 \ number of menu options
filey=14 \ menuy+menu+2 = filename window position
selecty=13 \ filey-1 = cursor select position
commy=16 \ filey+2 = command window position
commx=0 \ command window indent
width=40 \ command window width
org &3000
>EPROG \ eprom programmer
jsr PRINT
eqw P2-P1
P1 eqb 22, 7, 134, 157, 129, 141, 31, 10, 0
eqa "AJCD Eprom Programmer"
eqb 31, 0, 1, 134, 157, 129, 141, 31, 10, 1
eqa "AJCD Eprom Programmer"
eqb 31, menux, menuy
eqa "1 - Load buffer from file"
eqb 13, 129, 32, 134, 31, menux, menuy+1
eqa "2 - Save buffer to file"
eqb 13, 129, 32, 134, 31, menux, menuy+2
eqa "3 - Copy EPROM to buffer"
eqb 13, 129, 32, 134, 31, menux, menuy+3
eqa "4 - Program EPROM from buffer"
eqb 13, 129, 32, 134, 31, menux, menuy+4
eqa "5 - Compare EPROM with buffer"
eqb 13, 129, 32, 134, 31, menux, menuy+5
eqa "6 - Check EPROM is blank"
eqb 13, 129, 32, 134, 31, menux, menuy+6
eqa "* - Issue MOS command"
eqb 13, 129, 32, 134, 31, menux-1, menuy+7
eqa "ESC - Exit from program"

4 BBC Microcomputer EPROM Programmer
eqb 31, 3, filey, 131, 157, 132
eqa "Filename"
eqb 31, 35, filey, 156
P2 lda #234
ldx #0
ldy #255
jsr osbyte
cpx #0
beq NOTUBE
jsr PRINT
eqw P4-P3
P3 eqb 31, commx, commy
eqa "Please turn your TUBE off and re-run"
eqb 13, 10
P4 jmp ABORT
NOTUBE sei \ continue with setting up
lda irqvec
sta OLDIRQ
lda #<IRQ
sta irqvec
lda irqvec+1
sta OLDIRQ+1
lda #>IRQ
sta irqvec+1
lda #&B0
sta ier \ enable CB1 & t2 interrupts
lda #0
sta acr \ disable input latching etc
lda pcr
ora #&F0
sta pcr \ set CB2 to high output
cli
lda brkvec
sta OLDBRK
lda #<BRKERR
sta brkvec
lda brkvec+1
sta OLDBRK+1
lda #>BRKERR
sta brkvec+1
tsx
stx STACK
MAIN jsr ESCAPE \ ignore escape
jsr PRINT \ restore cursor position
eqw P8-P7
P7 eqb 26, 31, 1, menuy
eqb 32, 10, 8, 32, 10, 8, 32, 10, 8, 32, 10, 8
eqb 32, 10, 8, 32, 10, 8, 32, 10, 8
eqb 31, menux, selecty
P8 lda #21
ldx #0
jsr osbyte \ flush keyboard buffer
jsr osrdch \ main menu loop
bcc NOABORT \ error condition?
cmp #27 \ escape?
bne ABORT
lda #126
jsr osbyte
ABORT lda OLDIRQ+1
beq R1
sei
sta irqvec+1
lda OLDIRQ
sta irqvec
lda OLDBRK
sta brkvec
lda OLDBRK
sta brkvec+1
cli
R1 rts
NOABORT ldx #menu
CHKOPT dex
bmi MAIN
cmp OPTIONS, X
bne CHKOPT
lda #31
jsr oswrch \ indicate which option selected
lda #1
jsr oswrch
txa
clc
adc #menuy
jsr oswrch
lda #157
jsr oswrch
dex
bpl SAVE
\ Load file to buffer
jsr FILEWIND \ filename window
lda #0 \ filename input line
ldx #<FILINE
ldy #>FILINE
jsr osword \ read a line from input
bcs KILLOAD \ input error
jsr COMMWIND
jsr PRINT
eqw P12-P11
P11 eqb 31, width/2-7, 0
eqa "Loading..."
P12 ldx #15
LI lda LOADINFO, X
sta BLOCK+2, X \ clear out block
dex
bpl LI
ldx #<BLOCK
ldy #>BLOCK
lda #&FF
jsr osfile
jmp DONE2
KILLOAD jmp MAIN
SAVE dex
bpl COPY
\ Save file to buffer
jsr FILEWIND \ filename window
lda #0 \ filename input line
ldx #<FILINE
ldy #>FILINE
jsr osword \ read a line from input
bcs KILSAVE \ input error
jsr COMMWIND
jsr PRINT
eqw P14-P13
P13 eqb 31, width/2-7, 0
eqa "Saving..."
P14 ldx #15
SI lda SAVEINFO, X
sta BLOCK+2, X \ clear out block
dex
bpl SI
ldx #<BLOCK
ldy #>BLOCK
lda #0
jsr osfile
jmp DONE2
KILSAVE jmp MAIN
COPY dex
bpl PROGRAM
\ Copy Eprom to buffer
jsr FILEWIND \ Delete filename
jsr READY
bcs KILCOPY \ Error
jsr PRINT
eqw P16-P15
P15 eqb 31, width/2-8, 4
eqa "Copying"
P16 ldx #<LOOPCOPY
ldy #>LOOPCOPY
jsr LOOP
jmp DONE
KILCOPY jmp MAIN
LOOPCOPY jsr READ
sta (baddr), Y
clc
rts
PROGRAM dex
bpl VERIFY
\ Program Eprom from buffer
jsr WRITEY
bcs KILPROG
jsr PRINT
eqw P26-P25
P25 eqb 31, width/2-10, 4
eqa "Programming"
P26 ldx #<LOOPPROG
ldy #>LOOPPROG
jsr LOOP
jmp DONE
KILPROG jmp MAIN
LOOPPROG lda #&FF
sta ddrb
sta TIMER
lda (baddr), Y
sta orb
lda #&DF \ CB2 = 0
and pcr

Angus Duggan, 2004-12-10 5
sta pcr
lda #<delay \ 50 ms delay
sta t2l
lda #>delay
sta t2h
WAIT lda TIMER
bne WAIT
lda #&F0 \ CB2 = 1
ora pcr
sta pcr
clc
rts
VERIFY dex
bpl CHECK
\ Verify eprom against buffer
jsr READY
bcs KILPROG
jsr PRINT
eqw P28-P27
P27 eqb 31, width/2-9, 4
eqa "Verifying"
P28 ldx #<LOOPVFY
ldy #>LOOPVFY
jsr LOOP
jmp DONE
KILVFY jmp MAIN
LOOPVFY jsr READ
cmp (baddr), Y
clc
beq R4
sec
R4 rts
CHECK dex
bpl MOSCALL
\ Check blank eprom
jsr READY
bcs KILPROG
jsr PRINT
eqw P30-P29
P29 eqb 31, width/2-8, 4
eqa "Checking"
P30 ldx #<LOOPCHK
ldy #>LOOPCHK
jsr LOOP
jmp DONE
KILCHK jmp MAIN
LOOPCHK jsr READ
cmp #&FF
clc
beq R6
sec
R6 rts
MOSCALL \ Operating system call
jsr COMMWIND \ set up command window
lda #’*’
jsr oswrch \ indicate input required
lda #0 \ OS input line
ldx #<OSLINE
ldy #>OSLINE
jsr osword \ read a line from input
bcs KILLOSC \ input error
lda #14
jsr oswrch \ page mode on
ldx #<INPUT
ldy #>INPUT
jsr oscli
lda #15
jsr oswrch
KILLOSC jmp MAIN
LOOP stx middle \ Common loop for prog etc.
sty middle+1
jsr PRINT
eqw P32-P31
P31 eqa "...& "
P32 ldy #0
RECHECK jsr ESCAPE \ check escape key
bcs QUITLOOP
lda #8 \ get to right place
jsr oswrch
jsr oswrch
lda eaddr
jsr HEX \ print high address
REPEAT tya \ print low address
jsr HEX
lda #8 \ move back into position
jsr oswrch
jsr oswrch
lda #&FF
sta ddrb
sty orb \ load low address
jsr TOGGLE
lda eaddr \ load high address
sta orb
jsr TOGGLE
ldx CB1
jsr MIDDLE \ do centre routine
bcs NOTEQUAL
cpx CB1 \ check CB1 interrupt
bne PROGOK
jmp PROGERROR
PROGOK iny \ increment address
bne REPEAT
inc eaddr
inc baddr+1
dec length
bne RECHECK
clc
QUITLOOP rts
NOTEQUAL jsr PRINT
eqw P34-P33
P33 eqb 7, 31, width/2-8, 5
eqa "Comparison error"
P34 sec
rts
MIDDLE jmp (middle) \ indirect
DONE bcs P37
jsr PRINT \ Print Message
eqw DONE2-P35
P35 eqb 31, width/2-2, 5
DONE2 jsr PRINT
eqw P37-P36
P36 eqa "Done"
P37 jmp MAIN
HEX pha \ print two hex digits
lsr A \ get left digit
lsr A
lsr A
lsr A
jsr DIGIT
pla
and #&F \ get right digit
DIGIT cmp #10
bcc NUMBER
adc #6
NUMBER adc #48 \ add digit base
jmp oswrch
READY jsr COMMWIND \ get ready to READ
jsr PRINT
eqw P18-P17
P17 eqa " Set the programmer switch to"
eqb 130
eqa "READ,"
P18 jmp PREPARE
READ lda #0 \ read byte from programmer
sta ddrb
lda #&DF \ CB2 = 0
and pcr
sta pcr
lda irb
pha
lda #&F0 \ CB2 = 1
ora pcr
sta pcr
pla
rts
WRITEY jsr COMMWIND \ get ready to WRITE
jsr PRINT
eqw P20-P19
P19 eqa " Set the programmer switch to"
eqb 129
eqa "WRITE,"
P20 \ jmp PREPARE
PREPARE jsr PRINT
eqw P22-P21
P21 eqb 31, 1, 1
eqa "then select the EPROM type -"
eqb 13, 10, 132, 31, menux-1, 2, 135
eqa "1 - 2764"
eqb 13, 10, 132, 31, menux-1, 3, 135

6 BBC Microcomputer EPROM Programmer
eqa "2 - 27128"
eqb 13, 10
P22 lda #&80
sta eaddr \ EPROM start at &8000
lda #&40
sta length \ default length = &4000
lda #&3C
sta baddr+1 \ Buffer start at &3C00
lda #0
sta baddr
GETLEN jsr osrdch
bcs R5
cmp #’2’
beq LENOK
cmp #’1’
bne GETLEN
lsr length
lda #11
jsr oswrch
LENOK lda #11
jsr oswrch
lda #9
jsr oswrch
lda #157
jsr oswrch
lda #&FF \ make outputs safe
sta ddrb
sta orb
ldx CB1 \ take old count
jsr TOGGLE
cpx CB1 \ once
bne QUITPREP
jsr TOGGLE
cpx CB1 \ twice
bne QUITPREP
jsr TOGGLE
cpx CB1 \ three times...
beq PROGERROR
QUITPREP clc
R5 rts
PROGERROR jsr PRINT \ No response from programmer
eqw P24-P23
P23 eqb 7, 31, width/2-12, 5, 136
eqa "EPROM Programmer Error"
eqb 13, 10
P24 sec
rts
TOGGLE lda #&DF \ make CB2 go low then high
and pcr
sta pcr \ low
ora #&F0
sta pcr \ high
rts
ESCAPE lda escape \ test & reset escape condition
clc
bpl NOESC
lda #126
jsr osbyte
sec \ carry set if escape detected
NOESC rts
PRINT pla \ print in-line codes
sta zpwork
pla \ two byte size
sta zpwork+1
jsr GETIND
sta zpwork+2
jsr GETIND
sta zpwork+3
PRLOOP jsr GETIND
jsr oswrch
lda zpwork+2
bne DECLOW
dec zpwork+3
DECLOW dec zpwork+2
bne PRLOOP
lda zpwork+3
bne PRLOOP
jsr INCADR
jmp (zpwork)
GETIND ldy #1 \ get data from indirect address
lda (zpwork), Y
INCADR inc zpwork
bne R2
inc zpwork+1
R2 rts
BRKERR ldx STACK \ action taken on BRK
txs
jsr PRINT
eqw P39-P38
P38 eqb 15, 13, 10, 10, 7
eqa " OS Error : "
P39 ldy #1
BRKMSG lda (brkmsg), Y
beq BRKQUIT
jsr oswrch
iny
bne BRKMSG
BRKQUIT jsr osnewl
jmp MAIN
IRQ pha \ interrupt routine
lda ifr \ test interrupt condition
and #32 \ timeout ?
beq AGAIN
lda t2l \ clear interrupt condition
inc TIMER
AGAIN lda ifr
and #16 \ CB1 ?
beq CHAIN
lda orb \ clear interrupt condition
inc CB1
CHAIN pla
jmp (OLDIRQ) \ goto next interrupt handler
COMMWIND jsr PRINT \ setup command window
eqw P6-P5
P5 eqb 28, commx, 24, commx+width-1, commy, 12
P6 rts
FILEWIND jsr PRINT \ setup filename window
eqw P10-P9
P9 eqb 28, 15, filey, 34, filey, 12
P10 rts
\ Data area follows...
OPTIONS eqb ’1’, ’2’, ’3’, ’4’, ’5’, ’6’, ’*’
OSLINE eqw INPUT \ OSWORD 0 block for commands
eqb 255, 32, 127
FILINE eqw INPUT \ OSWORD 0 block for filenames
eqb 19, 32, 127
CB1 eqb 0 \ counter for CB1 interrupts
TIMER eqb 0 \ counter for t2 timeouts
OLDIRQ eqw 0 \ Old IRQV2
STACK eqb 0 \ stack pointer
OLDBRK eqw 0 \ Old brkvec
BLOCK eqw INPUT \ Load file parameter block
eqd 0
eqd 0
eqd 0
eqd 0
LOADINFO eqd buffer
eqd 0
eqd 0
eqd 0
SAVEINFO eqd &FFFF8000
eqd &FFFF8000
eqd buffer
eqd buffer+&4000
INPUT \ input buffer

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