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Overclocking RS232C Card for Fujitsu Micro 7

It turned out that Intel P82C51A seems to be able to run at 38400bps.  I confirmed by modifying my RS232C card for Fujitsu Micro 7.

The Fujitsu-original RS232C card for FM-7 supports only up to 19200bps.  The Fujitsu-original card uses Intel 8251A, which was manufactured around 1982.  The maximum performance of 1982 Intel 8251A may really be 19200bps.

There are some 8251-compatible chips.  OKI 82C51A officially supports up to 38400bps.  I have a bunch of Intel P82C51A, which was manufactured in 1990, 8 years after the original Intel 8251A.  The new Intel chip may be able to go up to 38400bps.  I just did experiments.

RS232C card for Fujitsu Micro 7 feeds 2.5MHz clock to a counter (M8 74LS393), and then takes one of the output bits of 74LS393 to give to the clock input (TXC, RXC) to 8251A via M10 74LS04.  Lower bits flips more frequently than higher bits.  Bit 2 flips every 4 ticks of 2.5MHz clock.  Bit 6 flips every 64 ticks.  The Fujitsu-original RS232C card uses dip switches to select which bit to be given as TXC and RXC.  My RS232C card configures by jumper pins.

The bit 2 of the counter 74LS393 corresponds to 19200bps.  If I want to run it at 38400bps, I need to take the next lower bit, that is bit 1.

Bit 1 output of the counter is pin 4 of 74LS393.  It is received by pin 3 of M10 74LS04, but it is directly connected to an array of jumper pins.  By directly connecting these pins, 82C51A should run at 38400bps.  But, I want to leave a possibility of supporting 19200bps.  So, I just added a jumper pin.

Actually when I made 20190107 version, I cut unused pins not to mistake.  I should have kept those pins for this expansion, but it was too late.

Working on 20181223 version, that I use with my FM77AV2.

A jumper pin added to 20190107.

I covered roots of the pin connected to the wire with BONDIC EVO.

No visible change on the front side.

The new jumper pin is floating on the soldering side of the board.  I used BONDIC EVO for covering the root of the pins where wires are connected.  When I saw BONDIC EVO, I thought it should be perfect to cover those pin-wire joints, and it worked as I expected.  Perfect sealing.  It is amazing that we can buy such UV-hardening plastic at $20 or so.

The experiment was very successful.  Both OKI 82C51A and Intel P82C51A were able to communicate at 38400bps.  I tested 3 different Intel P82C51A and all of them had no problem handling 38400bps.  I think it is safe to assume Intel P82C51A manufactured in 1990 can support 38400bps.

It is unknown if Intel 8251A originally was capable of supporting 38400bps but Intel just didn't want to guarantee, or if Intel increased the speed when they made a CMOS version ('C' of '82C51A' means CMOS version).

My Tape BIOS Redirector more than doubled the speed.

Dragon Slayer(Falcom)
Buffered mode 19200bps            86 sec
Buffered mode 38400bps            37 sec

Plazma Line (Techno Soft)
Buffered mode 19200bps            40 sec
Buffered mode 38400bps            16 sec

Disk BIOS Redirector also benefit from 38400bps:

Thexder boot to the opening BGM
19200bps                          63 sec
38400bps                          33 sec

I think it is comparable to the actual floppy-disk drive. 

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