Pioneer ER-101

I added some I/O port info from that manual, which is described on page 16. Can someone double check this? Preferably someone who can read Japanese

4.6 VDP (Video Display Processor)

Both the internal VDP and the external VDP on expanded processor (ER101) are accessed from CPU at the same time on any data-write. On the other hand, data bus conflict would occur if CPU received data from both VDP. Therefore CSR terminal of VDP on ER101 is pulled up, due to avoid reading from CPU. That means, data-write is available to both VDP, but data-read is able against only computer internal VDP.

Image output is assumed by external VDP, because internal VDP does not support external synchronization. However, in the case of using image from only computer, both VDP are available. Besides, INT output from only internal VDP is available.

what does it say about that bit 4 of I/O port AA? What exactly is it used for?

4.7.2 Sound control circuit

It does ON or OFF of muting against external stereo sound input.
By writing 7th bit of VCON register and output of 4th bit of MTON register, it generates muting signal for L/R channel(LMUTE, RMUTE).
On muting R channel, L->H operation against LMUTE is required because leading edge of LMUTE is used for latch.