CRTCs 1/2 have a fixed VSYNC width value of 16. VSYNC width can be configured with R3 on CRTCs 0/3/4. If 0 is programmed this gives 16 lines of VSYNC.
On CRTCs 0/3/4, even Even if the number of lines of VSYNC is set to 1 in R3, the Gate Array will output black for 26 lines (including 4 lines of C-VSYNC to the monitor). This is also true if the duration of the CRTC VSYNC is reduced to 2 µseconds.
Bit0 of port B of the PPI is directly connected to the VSYNC pin of the CRTC.