Hardware
By Brendan Alford (brendan.alford@gmail.com)
Original version by Dylan Smith.
First things first: the Spectrum isn't hot pluggable. To avoid killing your machine even harder (or killing a working machine if you're making other uses of the board), make sure you do the following before plugging the board into the Spectrum. This may seem like old hat, but just some reminders:
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Make sure that the power really is turned off on the Spectrum. The best way to to this is to make sure the 9v supply is physically unplugged from the machine.
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Ensure that all the jumper blocks on the diagnostics board are actually jumpered, otherwise inputs they control will be left floating and odd things will happen.
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Observe anti static precautions, especially if the relative humidity is low - the risk is highest in the winter. Be especially cautious if you regularly experience static shocks! Both the Spectrum and the diagnostic board use chips with MOSFETs, whose gate oxide is easily destroyed by a shock you can't even feel. Handling the board by the edges is helpful, and avoid wearing clothing made from synthetics, because they turn you into a pretty good Whimshurst machine.
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Double check that the chips in the diagnostic board are all connected the right way around, especially ones you may remove frequently like the flash memory. While the flash chip can survive reverse orientation, it will short the Spectrum's 7805 voltage regulator but leave the switch mode power supply working - meaning the Spectrum's 4116 RAM will have some supply rails working and others not. This sort of thing kills 4116 RAM.
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If you've constructed the board yourself - don't forget to test the board for shorts and open circuits before using it for the first time, even if you got the board made by a PCB factory.
The board is very simple to use. For diagnostics work, check that:
- J1 is set to pins 1 and 2 (to page in the flash on power up)
- J2 is set to pins 1 and 2 (don't use the M1 signal)
The reason for jumpering out the M1 signal is that if you have a machine that has a Z80 with a poorly functioning M1 line, the board won't work properly. Normally, you should leave the M1 jumper in this position, and only move it to pins 2-3 when you specifically want to test the function of the Z80's M1 line. Many Spectrums were sold with Z80s that had bad M1 lines.
Once you power the machine up with the board plugged in, so long as the Z80 is working, all LED's should turn on then off briefly, signifying the tests are about to start. If the ULA is functioning, you should see the screen showing a blue, then green border and black paper on start up (assuming you're using the Diagnostics ROM from this site also).
The board is commanded by two Z80 I/O ports - port 31 to change the selected flash ROM memory page (and map in and out the Spectrum's ROM), and port 63 to light up the LEDs. To describe how this actually works on the hardware level, we'll start from the address decoding and work inwards from there.
When the Z80 CPU executes an IN or OUT instruction, the following things happen. Firstly, the Z80 places the I/O port address on the address bus - for the instruction OUT (63), A - the lower 7 bits of the address bus are set to 63 (3F in hex). Then the /IORQ line goes active (low voltage level), and the /WR line goes active (also logic low).
The board senses an OUT instruction with the chip U1 - a 74HC138. This is a 3-to-8 line decoder. Address lines A5, A6 and A7 are connected to the three address lines on the '138. You can count from 0 to 7 with three binary bits, and what the '138 does is activate output 0 to 7 depending on what 3 bit number arrives on its inputs. So really - what's important is what the Z80's address lines A5, A6 and A7 are outputting during an I/O cycle.
We're only using 2 outputs on this chip - output 0 for the memory bank selection, and output 1. So if the pattern on A5, A6, A7 from the Z80 is 000, then the memory is selected. If the pattern on these three address lines is 001, then the LED register is selected.
As well as the right pattern being on the address bus, we've got to be able to determine that the Z80 is making an I/O request, and not a memory request or an interrupt acknowledge. The '138 has three Enable pins for this purpose. Indeed, it doesn't matter what the state is on the three address pins on this chip if the enable pins aren't set to the right levels - all outputs will remain inactive.
To tell whether the Z80 is making an I/O request, two pins must be monitored - IORQ and M1. IORQ must be low and M1 must be high. (Why must M1 be monitored? This is because the Z80 acknowledges a maskable interrupt by a special M1 cycle, with IORQ low and M1 low.
Spurious address decodes can occur if only IORQ is used for an enable). Additionally, we also look at the state of WR to make sure we're getting an OUT instruction, not an IN instruction, since this board only requires I/O writes and not reads.
Of course, it would be simple if large numbers of Spectrums hadn't shipped with Z80s with faulty M1 lines. A faulty M1 line doesn't seem to affect a Spectrum at all if it's used stand alone, but it can stop some peripherals from working properly. Therefore, a jumper is provided on the board to ignore M1, so that the I/O decoding relies just on IORQ and WR being active together (which is also only the case during an OUT instruction, not an interrupt acknowledge).
If you change the jumper so that M1 is used, you can write a short BASIC program to test the functioning of M1:
10 OUT 63, 255
20 PAUSE 5
30 OUT 63, 0
40 PAUSE 5
50 GO TO 10
If the LEDs all rapidly blink on and off in a stable and consistent manner, M1 works. However, if the blinking is somewhat erratic, then M1 is faulty.
So all being well, the 74HC138 looks for an instruction for the board by monitoring the state of IORQ, WR and M1 to recognise an I/O write instruction, and uses the state of A5, A6 and A7 to select either the memory page register or the LED register. The output of the '138 goes low when it's selected.
The 74HC273 is not level triggered - rather, it latches the contents of the data bus on the rising edge of the signal on the CP pin. When the Z80 de-asserts IORQ, the active low output of the 74HC138 will return high. The Z80 keeps the contents of the address bus live for about half a clock cycle after it has de-asserted IORQ - this is to allow edge triggered devices to read the contents of the data bus without needing any additional glue logic. This timing means the rising edge of the 74HC138's output can be used to latch the value being sent by the OUT instruction that's running on the CPU.
The 74HC273 octal flip flop, U5, performs the function of being the memory page register. The board can take flash ROMs from 128K to 512K, which is far larger than the address space that a Z80 can access. Therefore, this chip controls the state of address lines A14 to A18 on the flash memory. This chip also has a reset pin, so on power up or when the reset button is pressed, A14 to A18 are all at logic low, so the first 16K page of ROM is selected. Additionally, bit 5 of this chip is used to control whether the Spectrum's ROM is paged in, or whether the flash is paged in.
The Spectrum provides a line called ROMCS at the edge connector. There is a resistor between the ULA's ROM chip select decoding logic and the ROM itself, and ROMCS is connected on the ROM's side of this resistor. Since the Spectrum's ROM is enabled when ROMCS is low, holding this line high will prevent it from being selected.
This is accomplished on the diagnostics board by bit 5 of the rom page register U5 being connected to the gate of a P-channel MOSFET. The MOSFET connects the +5v line to ROMCS when it is turned on. It is turned on by bit 5 of U5 being set to logic low with the jumper on J1 on pins 1 and 2.
If the jumper on J1 is on pins 2 and 3, the output of bit 5 is inverted before being sent to the gate of this transistor, which means bit 5 needs to be set to logic 1 for the flash to be paged in.
This allows the board also to be used as a general purpose ROM board, so that different ROMs can be paged in at will. Also, programs can be written to copy a program to RAM, for example, a snapshot taken on an emulator and written to the flash chip.
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