Building ZX Spectrum Clone - Harlequin - Part 2

Post date: Mar 02, 2016 5:26:40 PM

Read part 1 here

Board Bring-Up

Yesterday evening I received the missing IC socket, soldered it to the board, plugged that 74HC245 IC, connected the keyboard, and quickly moved to test the board. For testing purposes I used this Diag ROM. During the initial power on no magic smoke escaped, and it all looked pretty good. My oscilloscope was showing the right CPU clock frequency, it appeared that CPU is fetching the instructions, and it was some kind of signal on the video output. So I decided to connect the board to the TV... Well, the board appeared to be dead: I didn't get any picture, and it didn't generate any sounds either. It turned to be a few simple mistakes - I plugged it into an Audio input instead of the Composite, and I forgot to install jumpers one connecting the speaker, another one connecting A14/PGM signal to the EPROM :-). That was soon corrected, and I proceeded with the testing.

Upper RAM Issue

This time Diag ROM started properly, and I've got the picture on the TV, and some beeps sounded through the speaker. While performing the upper RAM test Diag ROM reported errors at address $FFDC, I repeated the test, this time the error was elsewhere but still within the last 256 bytes of memory space (addresses $FFxx). I tried swapping out and replacing the SRAM chip, which didn't help. So I am still puzzled, perhaps there is something wrong with the address decode / chip select logic?! (I have to go back and check the schematic) Any ideas?

Update: I have tested my board using System Test ROM and ZX Spectrum Diagnostics and these programs found no problems with upper RAM whatsoever... So it must be a bug in the Diag ROM.

Defective Video Input?

Other than the issue above it appeared that the Diag ROM was working OK, except that the picture on the TV was black and white.

I worried that I've done something wrong with setting up the board to work in NTSC mode. I checked the signals on the relevant pins on AD724JR video encoder, and they all looked good. I hooked up my oscilloscope, and it showed perfect (as far as I can tell) composite color signal. I checked the video timing - it looked OK too. After several minutes of confusion, searching the Internets, etc., I've decided to try another video input on the TV. It worked just fine this time:

ZX Spectrum ROM

I did a few more tests and finally I replaced the Diag ROM with the ZX Spectrum ROM. It booted up happily. And I even ran a small BASIC program (oh boy, now I need to recall how to program this thing):

To be continued...