The original Central Data board uses 3624 PROMs. On the P1-board those are replaced by Philips/Signetics 82S115 PROMs. The chips are similar, but have different chip select signals.
I have five 82S115 PROMs (see the description of Popmon). One of them is missing, and one of the remaining chips developed bad bits: some 0's have spontaneously flipped to 1's. Apparently PROMs degrade over time!
The nicest solution from a restoration perspective would be to source new chips. I do not have a suitable programmer, and instead I looked at an alternative that was both more flexible and cheaper.
The ATMEL AT28C64B (full documentation) is an 8 Kbyte EEPROM. During normal read operations it behaves just like a static ROM chip, but it can also be programmed simply by writing to an address location. There are some limitations to writing: at most 64 bytes can be written in a single operation, and after that operation the chip must be given a few milliseconds to program the bytes. It cannot be used as a non-volatile static RAM chip, but nearly so. For example, changing a single byte can simply be done using the Alter Memory command in the monitor program. And it uses only a single 5 Volt supply voltage. My local electronics shop sells the AT28C64B for less than €12.
On the daughterboard I added the following simple circuit. The connectors on the left refer to the P1 bus. Note that the P1-board has room for eight PROMs, for a total of 4 KByte. The AT28C64B is twice that size, and therefore the top address line is unused and connected to ground. Signals marked with a bullet • are active-low; normally active-low is indicated by an overline, but that is cumbersome in sPlan.
The first jumper allows you enable/disable the write function. During normal operations it is safer to disable the WE• signal.
The second jumper allows you to position the memory at address 0000 or 4000. During normal operations the chip replaces the PROMs on the board: the location will be set to 0000. But for initial programming it is useful to use the original monitor program to boot the computer, load a small utility program, and flash the chip to its desired contents.
Note that PromReq is a combination of A12, A13, A14 (all low) and OpReq (high). PromReq is active high.
The photo below show the circuit as built on the daughterboard. The ribbon cables connect to the main computer board. The AT28C64B and 74LS00 in their sockets in the middle. On the right a black reset button and a frame for audio and video output. Audio connects to my Macbook, that acts as a cassette recorder.
The following utility program (source in memcopy.2650) can be used to flash the AT28C64B chip. Note that one way to check that the chip has completed its write operation is to re-read the last byte written. As long as re-reading yields a different value than was written, the chip is still busy.
dflt 16 org 1510 1510 MEMPROG: 1510: 1B 06 bctr,un Mprog 1512 FROM: 1512: 00 00 ACON 0000 1514 TO: 1514: 0C 00 ACON 0C00 1516 FLASH: 1516: 40 00 ACON 4000 1518 Mprog: 1518: 08 78 lodr,r0 FROM 151A: CC 15 D7 stra,r0 PFROM 151D: 08 74 lodr,r0 FROM+1 151F: CC 15 D8 stra,r0 PFROM+1 1522: 08 72 lodr,r0 FLASH 1524: CC 15 D9 stra,r0 PTO 1527: 08 6E lodr,r0 FLASH+1 1529: CC 15 DA stra,r0 PTO+1 152C NextBlock: ; Calculate the #bytes to program: TO-PFROM 152C: 77 09 PPSL WC+CAR 152E: 08 65 lodr,r0 TO+1 1530: AC 15 D8 suba,r0 PFROM+1 1533: C2 strz,r2 1534: 08 5E lodr,r0 TO 1536: AC 15 D7 suba,r0 PFROM 1539: C1 strz,r1 153A: 75 08 CPSL WC ; R1,R2 now contain the number of bytes to program 153C: 01 lodz,r1 153D: 98 09 bcfr,z Maxblock 153F: E6 00 comi,r2 0 1541: 1C 00 83 bcta,eq RETU 1544: E6 40 comi,r2 d'64' 1546: 99 02 bcfr,gt GoProgram 1548 MaxBlock: 1548: 06 40 lodi,r2 d'64' 154A GoProgram: 154A: CA 36 strr,r2 PLen 154C: 3F 15 B8 bsta,un PROGRAM 154F: E7 FF comi,r3 h'ff' 1551: 98 06 bcfr,eq IncAddrs 1553: 3F 15 96 bsta,un ProgError 1556: 1F 00 83 bcta,un RETU 1559 IncAddrs: 1559: 77 08 PPSL WC 155B: 75 01 CPSL CAR 155D: 08 23 lodr,r0 PLen 155F: 8C 15 D8 adda,r0 PFrom+1 1562: CC 15 D8 stra,r0 PFrom+1 1565: 20 eorz,r0 1566: 8C 15 D7 adda,r0 PFrom 1569: CC 15 D7 stra,r0 PFrom 156C: 75 01 CPSL CAR 156E: 08 12 lodr,r0 PLen 1570: 8C 15 DA adda,r0 PTo+1 1573: CC 15 DA stra,r0 PTo+1 1576: 20 eorz,r0 1577: 8C 15 D9 adda,r0 PTo 157A: CC 15 D9 stra,r0 PTo 157D: 75 08 CPSL WC 157F: 1F 15 2C bcta,un NextBlock 1582 Plen: 1582: 00 DB 0 1583 Msg: 1583: 45 72 72 6F 72 20 DB "Error at block \00" 1589: 61 74 20 62 6C 6F 158F: 63 6B 20 5C 30 30 1595 MSgC: 1595: 00 DB 0 1596 ProgError: 1596: 05 00 lodi,r1 0 1598 Ploop: 1598: 0D 75 83 loda,r0 Msg,r1 159B: 18 0B bctr,z Paddr 159D: C9 76 strr,r1 MsgC 159F: C3 strz,r3 15A0: 3F 03 96 bsta,un WCHR 15A3: 09 70 lodr,r1 MsgC 15A5: D9 71 birr,r1 PLoop 15A7: 17 retc,un 15A8 Paddr: 15A8: 0E 15 D7 loda,r2 PFROM 15AB: 3F 00 6A bsta,un HXOT 15AE: 0E 15 D8 loda,r2 PFROM+1 15B1: 3F 00 6A bsta,un HXOT 15B4: 3F 00 24 bsta,un LFCR 15B7: 17 retc,un ; Subroutine PROGRAM: Copy a block of bytes into Flash ; In: ; R2: number of bytes to copy (max 64) ; FROM: address containing the data ; TO: address of the Flash memory ; Return: ; r3: contains FF on error, any other number on success ; 15B8 PROGRAM: 15B8: A6 01 subi,r2 1 15BA: 05 FF lodi,r1 h'ff' 15BC: 20 eorz,r0 15BD: C3 strz,r3 Counter 15BE LOOP: 15BE: 0D B5 D7 loda,r0 *PFROM,r1,+ 15C1: CD F5 D9 stra,r0 *PTO,r1 15C4: 01 lodz,r1 15C5: E2 comz,r2 15C6: 98 76 bcfr,eq LOOP ; All written. Now compare the last byte written 15C8 wait: 15C8: 0D F5 D7 loda,r0 *PFROM,r1 15CB: ED F5 D9 coma,r0 *PTO,r1 15CE: 18 06 bctr,eq Done ; Not yet ready, increment the counter 15D0: 87 01 addi,r3 1 15D2: E7 FF comi,r3 h'ff' 15D4: 98 72 bcfr,eq wait 15D6 Done: 15D6: 17 retc,un 15D7 PFROM: 15D7: 00 00 dw 0 15D9 PTO: 15D9: 00 00 dw 0 HXOT EQU 006a LFCR EQU 0024 RETU EQU 0083 WCHR EQU 0396
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