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A project log for STEbus Z80 and FDC

STEbus Z80 and FDC, with 64K DRAM, and SCC. Renovation project, not my own design.

KeithKeith 11/26/2022 at 02:250 Comments

2022-11-26

Fitted copies of PAL chips from Steve's board.
Assembled code reverse engineered from Steve's board, and modified for an FDC with a non-inverted data bus.

Missing chips:

IC 9 = 74LS123 replaced with 74HC123.
IC 13 = 74LS273 which I don't have but could borrow from another board.
IC 16 = MC1488 is an RS232 buffer, I have some spare.
IC 17 = MC1489 is an RS232 buffer, I have some spare.

2022-11-27

Wondering how to tap the TTL-level serial signals to use an FTDI USB cable, I realised the parallel keyboard port has 16 pins and is fairly redundant because parallel keyboards are obsolete.

I had a look at the pinout and looked for a way to plug in the USB cables with the least effort.

By very good fortune there seems to be a way to route both serial channels to this header, with only the USB VCC (wire 3) needing to be disconnected and the USB !CTS (wire 2) moving to position 3:

USB
cable
fn (pin)
Serial
Channel
A (pin)
PL3 Serial
Channel
B (pin)
USB
cable
fn (pin)
!RTS (6) !CTS (18) kb d312 kb d4!CTS (22) !RTS (6)
RXD (5) TXD (15) kb d0 3 4kb d2TXD (25)RXD (5)
TXD (4)RXD (13)kb d156kb d7RXD (27)TXD (4)
!CTS (2 := 3)!RTS (17)kb d678kb d5!RTS (23)!CTS (2 := 3)
VCC (3) := ncdncGND910VCCdncVCC (3) := nc
GND (1)
GND1112GNDGNDGND (1)
DTRB1314DCDB
DCDA1516DTRA

This requires the RS232 buffers to be removed and four wires per channel to be routed to the keyboard data lines.

The keyboard data port is an input only, so the keyboard buffer chip IC30 does not need to be removed.

2022-11-28

I modified a USB cable as above, and added four wires from the SCC to the new cable header, and applied 5V power.
Nothing on the terminal. Logic probe showed the clock circuit working but CPU clock pin stuck high. Ditto the clock of the DRAM controller. Maybe a PCB track break that could be fixed with a wire?

I got Steve's board out to compare continuity. At this point I noticed the three 40-pin chips were the opposite way round! Close inspection of the pips on the PCB legend showed that these chips really are meant to have the polarity pips the opposite way round to all the other chips on the board.

Being powered up the wrong way round did not cause any to emit smoke, or get very hot, so I rotated them 180 degrees and hoped for the best. I was disappointed, no text on the terminal.

With all comms failures, experience has shown that the first question is to ask "have you got TX and RX wires the wrong way round?". This is an easy experiment so I swapped them over.

Success! I got a sign-on screen like this:

Press RETURN

Arcom ATLAS
Serial console running at 9600 baud

Insert system disk in drive A: and press RETURN

Unknown disk format
Insert system disk in drive A: and press RETURN


This proves the board is largely healthy and is working as expected for a board with no disk drive attached.

I have a drive and blank disks, but no system disk. So I can't try reading a known-good formatted disk.

All I can do for now is to go through the FDC setup procedure to check the critical timings are correct.

I have also swapped CTS/RTS over. 

2023-01-15

Unable to find 34-way FDD cables! I must have thrown them all out as very obsolete technology. Being impatient, I hacked down a 40-way cable.

Connected my board and my own FDD with the sawn-off cable. Placed board in case and switched on. Inserted disk from Steve, and got nothing. Puzzling.

I moved the board to my test rig, where it worked before, and it signed on. I then switched it off, attached the rack-mounted disk drive, then switched the board then the drive power on.

Press RETURN

Arcom ATLAS
Serial console running at 9600 baud

Insert system disk in drive A: and press RETURN

Error reading system disk:
Track-000, Side-0, Sector-001, Record not found, Retry (Y/N)

This is good in that it got as far as reading the floppy drive. The drive is pretty old, and the eject button is very stiff. 

I tried a different drive, which did not have a stiff eject button. I got this:

Press RETURN

Arcom ATLAS
Serial console running at 9600 baud

Insert system disk in drive A: and press RETURN

Unknown disk format
Insert system disk in drive A: and press RETURN

Not so good. 

This is now the same behaviour as Steve's board.

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