Most signals used within the Spectrum are available from the edge connector at the back. A-Side connections, when referred to, represent connections on the component side of the board, and B-Side the reverse. The connections are listed below:

IA b15 of address bus
2A b13 of address bus
3A b7 of data bus
4A not connected
5A SLOT
6A b0 of data bus
7A b1 of data bus
8A b2 of data bus
9A b6 of data bus
10A b5 of data bus
11A b3 of data bus
12A b4 of data bus
13A INT Z80 interrupt line; connecting this to +5 will stop the interrupts generated by the ULA
14A NMI Z80 non-maskable interrupt line; this interrupt isn't used by the Spectrum.
A low pulse will cause BASIC to do a reset
15A HALT Z80 halt line which signals that a machine code halt instruction has been executed
16A MREQ standard Z80 control bus line
17A IORQ standard Z80 control bus line
18A RD standard Z80 control bus line
19A WR standard Z80 control bus line
20A -5V low current -5V supply
21A WAIT Z80 wait line which when held low will temporarily halt the Z80. A wait must not last for
longer than about 1ms otherwise the dynamic memory will forget!
22A + 12V smoothed 12V supply
23A + 12V unsmoothed 12V supply
24A M1 standard Z80 control bus line
25A RFSH Z80 memory refresh signal
26A b8 of address bus
27A b10 of address bus
28A not connected
IB b 14 of the address bus
2B b12 of the address bus
3B 5V supply
4B 9V supply
5B SLOT
12B b2 of the address bus
12B b3 of the address bus
13B IORQGE holding this line high (i.e.+SV) will stop the ULA responding to 11 0 requests.
With suitable circuitry it could be used to expand the number of 1/0 devices that the Spectrum can select

14B OV video ground
15B composite colour video signal
16B video Y signal
17B video V signal
18B video U signal
19B BUSRQ standard Z80 control bus line
20B RESET momentarily connecting this line to OV will reset the machine just as if the power had been switched off and on

21B b7 of address bus
22B b6 of address bus
23B b5 of address bus
24B b4 of address bus
25B ROMCS connecting this to +5V will remove the BASIC ROM from the Spectrum's memory map.
26B BUSACK standard ZSO control bus line
27B b9 of address bus
28B bl 1 of address bus

15B,16B, 17B, 18B can be used to drive a monitor. 15B is a composite video signal with a direct connection to the two output transistors (emitter follower) that drive the UHF modulator, which gives enough power to drive a monitor directly. However it is not 75 ohm impedance and so will not work well with some/most monitors. 16B,17B, 18B are all unbuffered outputs from the ULA and cannot drive a monitor directly - a buffer amplifier is required - and to derive a RGB signal requires a subtractor circuit. All in all, it is easier to connect via the composite signal!

Inside the Spectrum, near the video circuits tot he far left of the PCB, are four links marked U, V, Y and VID. If the pads are connected by a wire link, then the video signal will appear at the edge connector. If the pads are connected by a white line, you will have to solder a wire link to make the signals appear.