Update README.md

README.md

@@ -45,7 +45,7 @@ The main control logic is based on a 22V10 GAL. This is a device that needs prog
 
  ## Design Notes
 
- * U5 and U2 and surrounding circuitry are for the joystick interfaces - omit if not needed. The prototype board pictured does not yet have this circuitry.
+ * U3 and U4 and surrounding circuitry are for the joystick interfaces - omit if not needed. The prototype board pictured does not yet have this circuitry. U3 and U4 can be 74LS240 OR 74LS640 buffers. J5 and J6 (or wire jumpers) select which chip type is fitted.
  * The polyfuse (F1) is for overcurrent/fault protection for the ZX Spectrum. Replace with a wire link if deemed not necessary.
  * The prototype board used 10K resistors instead of 8K2.
  * D1: Some other online ROM board schematics show this diode (incorrectly) in the emitter leg of the transistor circuit.
@@ -66,5 +66,5 @@ Q1|2N3906 or any general purpose PNP transistor.
 D1|Any small signal silicon diode (eg: 1N4148). 
 SW2|BCD rotary switch. eg: APEM PT65-101 or equivalent. 4 x 2-pin jumpers or regular DIP switches could be used.
 U1|GAL22V10 programmed with the .jed code provided here. WinCupl source code is in the .pld file.
-U2, U5|74LS240 OR 74LS640 buffers. J5 and J6 (or wire jumpers) select which chip type is fitted.
-U4|A 28-way ZIF socket.
+U2|28-pin IC socket - use a quality turned pin one or (preferably) a ZIF socket.
+U3, U4|74LS240 OR 74LS640 buffers. See design notes above.