Serial Port Design for Z80 - Questions

[Commander Dave]

Greetings!

I wanted to add a serial port to my 1981 Gorf arcade machine. Since it is
really just a specialized Z80 computer, I was just going to tap the signals
from the Z80 pins and create a small sub-board with a UART. I haven't really
got this working yet and was since I have virtually zero experience with any
hardware design, thought I might beg a question or two.

First, you can see the schematic and other info here:
http://tinyurl.com/yfe9utm (or: http://preview.tinyurl.com/yfe9utm )

First, was there a better UART that I could have chosen? The 16C550 seems to
have been around for quite awhile. I am looking for mainly availability and
simplicity rather than cost or other factors.

Second, I have to tap all the signals from the bottom of a card cage by
running individual wires to my board. I currently have this hard wired which
is a pain for debugging purposes. I need a connector system that takes these
individual wires and turns it into some kind of plug that could go into a
socket. I'm not too great with finding what is out there. Can someone make a
suggestion on what to use to do this? There are 20 signal connections that
need to be made on the current design.

Third, I know I am a total newb with design and prototyping. If you see
obvious errors with my design or implementation, please feel free to let me
know. Please be kind though, I bruise easily...

Cheers!
-Commander Dave

 

[Rich Webb]

Greetings!

I wanted to add a serial port to my 1981 Gorf arcade machine. Since it is
really just a specialized Z80 computer, I was just going to tap the signals
from the Z80 pins and create a small sub-board with a UART. I haven't really
got this working yet and was since I have virtually zero experience with any
hardware design, thought I might beg a question or two.

First, you can see the schematic and other info here:
http://tinyurl.com/yfe9utm (or: http://preview.tinyurl.com/yfe9utm )

First, was there a better UART that I could have chosen? The 16C550 seems to
have been around for quite awhile. I am looking for mainly availability and
simplicity rather than cost or other factors.

Second, I have to tap all the signals from the bottom of a card cage by
running individual wires to my board. I currently have this hard wired which
is a pain for debugging purposes. I need a connector system that takes these
individual wires and turns it into some kind of plug that could go into a
socket. I'm not too great with finding what is out there. Can someone make a
suggestion on what to use to do this? There are 20 signal connections that
need to be made on the current design.

Third, I know I am a total newb with design and prototyping. If you see
obvious errors with my design or implementation, please feel free to let me
know. Please be kind though, I bruise easily...

I haven't looked at the schematic (I'm allergic to tinyurl addresses)
but you might consider tossing in a microcontroller (with a built-in
buffered UART) instead of that 16550.

On the plus side for the 16550, it's of more or less the same generation
as the Z80 and they both are comfortable with "classic" address, data,
and control busses. Other than the number of connections required, that
might be the simplest approach.

On the plus side for the microcontroller, you can probably get by with
an address decoder to drive /enable, and the /WR plus one data bit for
the clock and data out, which would be three SPI-like lines to the uC.
Something will need to do the address decoding but if you're lucky then
all 16 bits aren't used and you can steal one for /enable.

WRT the connector. If you go with the 16550 then a default setup would
probably be to start looking at a ribbon cable and an IDC (insulation
displacement connector) socket on 0.1" centers. A 10x2 is pretty widely
available.

 

[Commander Dave]

Really, though, if you want to make an interface,
why not consider a USB slave port, using FTDI's
FT245 chip to map out a block of shared memory?

I didn't really look at USB because I am very limted in my knowledge of
electronics. Serial ports seemed easier to me because I have books that have
details on how to hook them up to the Z80 along with lots of schematics on
the web. I was also concerned about how to make/get a driver for the USB
working on the PC. The whole thing just seemed more complex than the serial
port.

The short answer is that I had a greater comfort level with the serial port
build. Perhaps on a later project I can implement a USB port for the
beastie...

Thanks for the suggestion!
-Dave

 

[Commander Dave]

I haven't looked at the schematic (I'm allergic to tinyurl addresses)
but you might consider tossing in a microcontroller (with a built-in
buffered UART) instead of that 16550.

I totally understand about the tinyurl allergy in this day and age. I did,
however, put in the link to the preview tinyurl version. Are those also
suspect in your eyes? In any case, I understand the safety issue and
sympatize. If you want to check out the schematic, just go to www.gorf.info
and click on the "Hardware Projects" and you will find it easily. My site is
very small at the moment.

I considered a microcontroller, but as with many things, I don't have any
experience with them. I would have to do a great deal of research on
programming them and how they work. I have a greater comfort level with a
UART and serial ports, so that is why I went that route. I would like to get
into microcontrollers one of these days, however. Perhaps I will use one in
a future project.

On the plus side for the microcontroller, you can probably get by with
an address decoder to drive /enable, and the /WR plus one data bit for
the clock and data out, which would be three SPI-like lines to the uC.
Something will need to do the address decoding but if you're lucky then
all 16 bits aren't used and you can steal one for /enable.

Wow... this really shows my ignorance on microcontrollers. I didn't
understand hardly any of this paragraph. It's not that you wrote it badly, I
just have no knowlege in the microcontroller world. As I said above, I
should really get into microcontroller one of these days.

WRT the connector. If you go with the 16550 then a default setup would
probably be to start looking at a ribbon cable and an IDC (insulation
displacement connector) socket on 0.1" centers. A 10x2 is pretty widely
available.

Ah, now this is something I really needed... information on the connector
issue. I'll use the info you provided and see what I can find from Digikey.

Rich Webb Norfolk, VA

Thank you VERY much for all your suggestions and help. By the way, I used to
live in the Norfolk/Virginia Beach area. It seems like a lifetime ago... but
I remember it fondly.

-Commander Dave

 

[Rich Webb]

I totally understand about the tinyurl allergy in this day and age. I did,
however, put in the link to the preview tinyurl version. Are those also
suspect in your eyes? In any case, I understand the safety issue and
sympatize. If you want to check out the schematic, just go to www.gorf.info
and click on the "Hardware Projects" and you will find it easily. My site is
very small at the moment.

The layout looks reasonable at first cut. There is one spot where you
have a crossing but no junction dot (up by Vcc on the MAX232). Strongly
recommend sticking by the rules: ALL T-junctions are connections and NO
crossings are connections. Means that you may need to add an extra zig
or zag in a spot but it can remove any ambiguity.

WRT the MAX232: You'll probably want (or may only find) the ...232A
variant (or one of the many clones). The ...A chips use 0.1 uF caps
instead of 1 uF and you can usually do okay with the same caps used for
power rail bypassing (which you'll want to add to the schematic).

WRT whit3rd's suggestion on using USB. There are many difficult ways to
do that but there's also a (relatively) simple way by, more or less,
just replacing the MAX232 with an FTDI FT232R chip.
http://www.ftdichip.com/FTProducts.htm#FT232R. In effect, the Z80 and
16550 think that they are talking to an ordinary RS-232 serial line. On
the PC side, the FTDI driver installs a "virtual serial port." You may
already have a gizmo hanging off your PC that uses one. The principal
advantage is that PCs with serial ports are getting hard to find but
it's hard to find one nowadays without multiple USB ports.

I considered a microcontroller, but as with many things, I don't have any
experience with them. I would have to do a great deal of research on
programming them and how they work. I have a greater comfort level with a
UART and serial ports, so that is why I went that route. I would like to get
into microcontrollers one of these days, however. Perhaps I will use one in
a future project.


Wow... this really shows my ignorance on microcontrollers. I didn't
understand hardly any of this paragraph. It's not that you wrote it badly, I
just have no knowlege in the microcontroller world. As I said above, I
should really get into microcontroller one of these days.

The parallel bus arrangement that you've shown should work just fine. As
you do more projects, you may find that a lot of inter-device
communications have moved to serial busses instead; many fewer wires to
manage. One common bus is the SPI bus
http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus. Not
germane to the current project, though.

Ah, now this is something I really needed... information on the connector
issue. I'll use the info you provided and see what I can find from Digikey.

Ribbon cable and IDC connectors are often the easiest to use. You can
get them premade or get the connectors and squeeze 'm yourself.
http://dkc3.digikey.com/PDF/B092/P0043.pdf

Thank you VERY much for all your suggestions and help. By the way, I used to
live in the Norfolk/Virginia Beach area. It seems like a lifetime ago... but
I remember it fondly.

Former bubblehead turned contractor (they *pay* me to play with this
stuff ;-) The area hasn't changed much ... well, the traffic's probably
a little worse.

 

[Commander Dave]

The layout looks reasonable at first cut. There is one spot where you
have a crossing but no junction dot (up by Vcc on the MAX232). Strongly
recommend sticking by the rules: ALL T-junctions are connections and NO
crossings are connections. Means that you may need to add an extra zig
or zag in a spot but it can remove any ambiguity.

I see the spot you are talking about and will get that fixed. It should be a
junction there. I'm afraid that I didn't quite understand the meaning of
your rule. Are you saying not to make a junction out of a crossing? In the
case above, I would move the top wire (+5v) over a bit so it doesn't cross?
You may have to talk to me like a four year old on that one.

WRT the MAX232: You'll probably want (or may only find) the ...232A
variant (or one of the many clones). The ...A chips use 0.1 uF caps
instead of 1 uF and you can usually do okay with the same caps used for
power rail bypassing (which you'll want to add to the schematic).

This is another tough point for me. If I am not mistaken, "power rail
bypassing" is caps put between the power and ground rails to keep down
ripple? I was never clear on where to put the caps, especially on a wired
board, not to mention the value or type of caps to use (disc ceramic?). The
only thing I think I know is that they go near digital IC chips. I'm very
limited on my knowledge of electronics, but I keep on trying. I'll see if I
can find some reading material on the subject.

WRT whit3rd's suggestion on using USB. There are many difficult ways to
do that but there's also a (relatively) simple way by, more or less,
just replacing the MAX232 with an FTDI FT232R chip.
http://www.ftdichip.com/FTProducts.htm#FT232R. In effect, the Z80 and
16550 think that they are talking to an ordinary RS-232 serial line. On
the PC side, the FTDI driver installs a "virtual serial port." You may
already have a gizmo hanging off your PC that uses one. The principal
advantage is that PCs with serial ports are getting hard to find but
it's hard to find one nowadays without multiple USB ports.

Very true about disappearing serial ports and your solution would be the
simplist to implement given the design I have already. After I get the
serial port working, I may do a modification to change it to a USB using the
method you describe.

The parallel bus arrangement that you've shown should work just fine. As
you do more projects, you may find that a lot of inter-device
communications have moved to serial busses instead; many fewer wires to
manage. One common bus is the SPI bus
http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus. Not
germane to the current project, though.

Still nice to know. I have heard of SPI when looking at UART's. I recently
pulled a data sheet on a Maxim MAX3110E which I thought was something I
could use because it had a UART and line drivers built into the same chip. I
was all excited until I found that there were not any parallel data lines
going into the chip. It instead was SPI/QSPI/MICROWIRE compatable. I assume
that this is the mechanism that microcontrollers use today to pass data as
you have said. My problem would be that I wouldn't know how to get the
parallel data from the Z80 (D0-D7) into a serialized format that SPI uses.
I'm clueless on the subject, but I don't mind you talking about it. I do
love to learn, but just talk at a very low level or it will sail right over
my head.

Ribbon cable and IDC connectors are often the easiest to use. You can
get them premade or get the connectors and squeeze 'm yourself.
http://dkc3.digikey.com/PDF/B092/P0043.pdf

Excellent, excellent, excellent. Thank you so much for the link and the
help.

Former bubblehead turned contractor (they *pay* me to play with this
stuff ;-) The area hasn't changed much ... well, the traffic's probably
a little worse.

My gosh, man... the highway to the Norfolk naval base was practically at a
standstill in the mornings even back then. I would really hate to see it
now! I do wish I could come back and visit Virginia Beach, however. I have
some fond memories. I used to live about 1/2 mile from the beach and would
jog down to and run along the beach. In the summer the view was very nice
and in the cooler weather I had the beach to myself and found it very
peaceful.

Did they ever do away with that toll road that led down to the beach? There
was always talk about when it was paid for they would do away with it, but I
was skeptical. Do you happen to know?

Fondest Regards,
-Commander Dave

 

[Baron]

I totally understand about the tinyurl allergy in this day and age. I
did, however, put in the link to the preview tinyurl version. Are
those also suspect in your eyes? In any case, I understand the safety
issue and sympatize. If you want to check out the schematic, just go
to www.gorf.info and click on the "Hardware Projects" and you will
find it easily. My site is very small at the moment.

-Commander Dave

Curiously TalkTalk blocks your web site !
Among a number of others it blocks.

 

[Commander Dave]

Curiously TalkTalk blocks your web site !
Among a number of others it blocks.

Best Regards:
Baron.

How odd. I do use a free web hosting service (I can't afford anything else
yet), so perhaps it has something to do with that. I'm not familiar with
TalkTalk, but if they are blocking your access to other web sites too, you
may want to consider another service or at least write them and find out why
it is blocked. It sounds very inconvenient.

Cheers!
-Commander Dave

 

[Rich Webb]

I see the spot you are talking about and will get that fixed. It should be a
junction there. I'm afraid that I didn't quite understand the meaning of
your rule. Are you saying not to make a junction out of a crossing? In the
case above, I would move the top wire (+5v) over a bit so it doesn't cross?
You may have to talk to me like a four year old on that one.

Exactly. The intent is to avoid depending on the "junction dot" to tell
whether or not there is a connection. That makes it much easier for
somebody who is unfamiliar with the schematic (which generally includes
the author, after a few months) to know what is intended.

This is another tough point for me. If I am not mistaken, "power rail
bypassing" is caps put between the power and ground rails to keep down
ripple? I was never clear on where to put the caps, especially on a wired
board, not to mention the value or type of caps to use (disc ceramic?). The
only thing I think I know is that they go near digital IC chips. I'm very
limited on my knowledge of electronics, but I keep on trying. I'll see if I
can find some reading material on the subject.

When devices change state, they momentarily "want" a lot of current in a
very short time. You'll want to have a local reservoir (the bypass cap)
between power and ground and located near to each of the chips you're
using. The inherent inductance of the interconnects on the board means
that you can't just, say, put a handful of caps off in one corner; they
need to by physically close to each switching device. Typically a 0.1 uF
monolithic ceramic.

Very true about disappearing serial ports and your solution would be the
simplist to implement given the design I have already. After I get the
serial port working, I may do a modification to change it to a USB using the
method you describe.


Still nice to know. I have heard of SPI when looking at UART's. I recently
pulled a data sheet on a Maxim MAX3110E which I thought was something I
could use because it had a UART and line drivers built into the same chip. I
was all excited until I found that there were not any parallel data lines
going into the chip. It instead was SPI/QSPI/MICROWIRE compatable.

Yes, wide parallel (address + data + control) busses are relatively rare
nowadays except for applications that need the highest available
bandwidth (e.g., the main memory interfaces in PCs). SPI (and the subset
Microwire (TM National Semiconductor)) and I2C (TM Philips/NXP) are two
that are commonly used for chip-to-chip communications. For example,
your PC almost certainly uses a subset of I2C, the SMBus, as its
internal low speed "system management" network for things like
temperature monitoring.

I assume
that this is the mechanism that microcontrollers use today to pass data as
you have said. My problem would be that I wouldn't know how to get the
parallel data from the Z80 (D0-D7) into a serialized format that SPI uses.
I'm clueless on the subject, but I don't mind you talking about it. I do
love to learn, but just talk at a very low level or it will sail right over
my head.

Stick with the parallel architecture for now. But if you look at the
signals on an SPI connection and compare them to the signals used for a
Z80 I/O operation, you may see that it could be possible, for example,
to use the write pulse (/WR) as the "clock" line in SPI.

Excellent, excellent, excellent. Thank you so much for the link and the
help.


My gosh, man... the highway to the Norfolk naval base was practically at a
standstill in the mornings even back then. I would really hate to see it
now! I do wish I could come back and visit Virginia Beach, however. I have
some fond memories. I used to live about 1/2 mile from the beach and would
jog down to and run along the beach. In the summer the view was very nice
and in the cooler weather I had the beach to myself and found it very
peaceful.

Did they ever do away with that toll road that led down to the beach? There
was always talk about when it was paid for they would do away with it, but I
was skeptical. Do you happen to know?

Yes, state route 44 dropped the tolls and became a segment of the
interstate. Naturally, Virginia Beach being Virginia Beach, it just
Would Not Do for them to get a mere spur designation (odd digit + main
interstate base number). So, instead of being I-364 like they should be,
they continued the I-264 designation from Norfolk and thus contributed
to the endless confusion of out-of-towners who justifiably expect that
an even-prefix auxiliary should reconnect to the main highway.
</rant>

 

[petrus bitbyter]

Greetings!

I wanted to add a serial port to my 1981 Gorf arcade machine. Since it is
really just a specialized Z80 computer, I was just going to tap the
signals from the Z80 pins and create a small sub-board with a UART. I
haven't really got this working yet and was since I have virtually zero
experience with any hardware design, thought I might beg a question or
two.

First, you can see the schematic and other info here:
http://tinyurl.com/yfe9utm (or: http://preview.tinyurl.com/yfe9utm )

First, was there a better UART that I could have chosen? The 16C550 seems
to have been around for quite awhile. I am looking for mainly availability
and simplicity rather than cost or other factors.

Second, I have to tap all the signals from the bottom of a card cage by
running individual wires to my board. I currently have this hard wired
which is a pain for debugging purposes. I need a connector system that
takes these individual wires and turns it into some kind of plug that
could go into a socket. I'm not too great with finding what is out there.
Can someone make a suggestion on what to use to do this? There are 20
signal connections that need to be made on the current design.

Third, I know I am a total newb with design and prototyping. If you see
obvious errors with my design or implementation, please feel free to let
me know. Please be kind though, I bruise easily...

Cheers!
-Commander Dave

The original Z80 UART (USART) is the Z80SIO. A powerfull device yet hard to
program correctly and it requires an external Baud-rate generator.

Instead of that Z80SIO an Intel 8251 was often used. Much easier to program
though it has some bugs like all Intel UARTS and their clones at the time.
It also requires a Baudrate generator.

The Intel 8250 was used in the first PCs. There were myriads of types
(including the clones) all with small differences and bugs. They were harder
to program then the 8251. Very hard to program if you had to anticipate for
all types and bugs. On the positive side they had a programmable Baudrate
generator build in.

When the PC-market boomed, Intel developped successors for the 8250: Among
them the 16450 and the 16550. FAIK the 16550A was the first bugfree type. I
guess your 16C550 to be a clone of that last one. As they were used in PCs
there's a lot of information about them on the net. Look for instance at:
http://www.beyondlogic.org/serial/serial.htm

If I had to deal with those old hardware I'd go for a 8251 using a 4060 for
the Baudrate generator as IMHO it is the most simple approach. As you have
that 16C550 already I guess you will stick at it.

Which brings me to the main question: What do you really want to do? Because
adding a serial interface to the existing hardware does not solve anything
on its own. You will need to program a kind of monitor in the Gorf that
communicates via that interface. Accepting commands and returning results.
Then, when you want the Gorf to perform its original function, you need to
get the firmware via the serial interface, store it in RAM and then pass
control to it. I do not know the hardware of that Gorf but I suppose you
will need to do some real hardware modifications. Not to mention the
requirements if you want to do some debugging.

petrus bitbyter

 

[Baron]

How odd. I do use a free web hosting service (I can't afford anything
else yet), so perhaps it has something to do with that. I'm not
familiar with TalkTalk, but if they are blocking your access to other
web sites too, you may want to consider another service or at least
write them and find out why it is blocked. It sounds very
inconvenient.

Cheers!
-Commander Dave

TalkTalk is a UK isp. They seem to block file sharing sites in
particular, so I suspect that your isp is probably within a group of
addresses that they block.

Of course they deny that they do anything of the kind and blame network
or bandwidth issues. They have already killed off Usenet and are
trying to kill of pop/smtp as well by forcing new subscribers to use
webmail.

 

[Commander Dave]

The original Z80 UART (USART) is the Z80SIO. A powerful device yet hard to
program correctly and it requires an external Baud-rate generator.

When I was considering UART's and based on my low level of experience, I
decided against the Z80SIO because it looked to be much more complex than
other solutions.

Instead of that Z80SIO an Intel 8251 was often used. Much easier to
program
though it has some bugs like all Intel UARTS and their clones at the time.
It also requires a Baudrate generator.

I considered the 8251 as there are tons of old Z80 books with instructions
on exactly how to interface and program the 8251 to the Z80. I decided
against it because the 8251 seemed to be an old chip and I wanted to try to
stay with something that would be easily obtainable from mainstream
companies like Digikey.

FAIK the 16550A was the first bugfree type. I
guess your 16C550 to be a clone of that last one. As they were used in PCs
there's a lot of information about them on the net. Look for instance at:
http://www.beyondlogic.org/serial/serial.htm

I think I have this site bookmarked already, but thanks for the link. Lots
of helpful information there.

If I had to deal with those old hardware I'd go for a 8251 using a 4060
for
the Baudrate generator as IMHO it is the most simple approach. As you have
that 16C550 already I guess you will stick at it.

Yeah, got to stick with it in this incarnation at least. As I said a few
lines up, I didn't go for the 8251 because I am thinking it is an obsolete
part and it might not be readily available from mainstream sources. I wanted
to stick with parts I could get from a mainstream company. I mainly was
wondering if there was anything better and newer than the 16C550 that I
should have used.

Which brings me to the main question: What do you really want to do?
Because
adding a serial interface to the existing hardware does not solve anything
on its own. You will need to program a kind of monitor in the Gorf that
communicates via that interface. Accepting commands and returning results.
Then, when you want the Gorf to perform its original function, you need to
get the firmware via the serial interface, store it in RAM and then pass
control to it. I do not know the hardware of that Gorf but I suppose you
will need to do some real hardware modifications. Not to mention the
requirements if you want to do some debugging.

A fellow named Don posted the same question yesterday. I like to modify the
code on the Gorf machine and it has been a hassle in the past to burn a new
EPROM every time I wanted to test on real hardware. I want to have a serial
port attached so I can download code into memory.... kind of a dump and
execute type thing. I have modified the original game code so it looks for
an extra EPROM in a vacant socket if the debug switch is on. If it is there,
it jumps to my code, if not, it just resumes the normal game diagnostics. I
would have a monitor/debug program in the extra EPROM to control the serial
port. In case you are curious, I have ruled out a Z80 ICE and a EPROM
emulator for various reasons, not because they aren't a good idea, but for
other personal reasons (which I think I covered in previous postings if you
are curious).

I do not know the hardware of that Gorf but I suppose you
will need to do some real hardware modifications. Not to mention the
requirements if you want to do some debugging.

Actually, since the Gorf machine already has static RAM (and some dynamic
also) on board, the only hardware mods is the connections to the bus from
the bottom of the cardcage (about 20 signal wires), power connections (again
from the card cage power connector) and the custom EPROM's which are all in
original sockets. One of my goals was to make as few changes to the original
hardware as possible. As for the debugging, most would be done with the MAME
emulator and only code that worked on the emulator would be tested on real
hardware which usually requires minimal debugging (just a tweak or two). A
simple monitor/debug program would be enough to do the job for me.

BTW. Next time you want to post the same question in several newsgroups,
address them all at once instead of one at a time.

Yeah, that's my bad. When I first started looking for an electronics
newsgroup to post in, I went to rec.electronics. Then I saw that it was
pretty dead there, but then found another I thought might be better and
posted it there. Then I found another and another and posted in those. I was
a real cluster-flub on my part, but next time I will know which groups are
active and will post to all at once as you have suggested. I apologize for
the inconvenience.

(Note: I copied and pasted this reply from the sci.electronics.design group
because of this same problem.)

petrus bitbyter

Thank you for your help and suggestions!
-Commander Dave

 

[Jasen Betts]

Greetings!

I wanted to add a serial port to my 1981 Gorf arcade machine. Since it is
really just a specialized Z80 computer, I was just going to tap the signals
from the Z80 pins and create a small sub-board with a UART. I haven't really
got this working yet and was since I have virtually zero experience with any
hardware design, thought I might beg a question or two.

First, you can see the schematic and other info here:
http://tinyurl.com/yfe9utm (or: http://preview.tinyurl.com/yfe9utm )

First, was there a better UART that I could have chosen? The 16C550 seems to
have been around for quite awhile. I am looking for mainly availability and
simplicity rather than cost or other factors.

8250 ? but the 16550 is basically just an improved version.

Second, I have to tap all the signals from the bottom of a card cage by
running individual wires to my board. I currently have this hard wired which
is a pain for debugging purposes. I need a connector system that takes these
individual wires and turns it into some kind of plug that could go into a
socket.

I'm not too great with finding what is out there. Can someone make a

suggestion on what to use to do this? There are 20 signal connections that
need to be made on the current design.

use a DB25 if you can't find something better, probably easier to
prototype with a DB25 than with an edge connector.

Third, I know I am a total newb with design and prototyping. If you see
obvious errors with my design or implementation, please feel free to let me
know. Please be kind though, I bruise easily...

I see no capacitors between VCC and ground, add atleast one 0.1uf (or
larger)

unused inverter inputs should be connected to +5 or ground.
unused outputs can be left unconnected.

unused UART inputs should probably be similarly treated, (or you could
connecte /CTS and /DTD to /RTS) , connecting /RI to +5 is probably
apropriate, and appears to be convenient.

I note that you're not connecting the interrupt line,
the hassle you save by omitting interrupt hardware may not be worth it.

 

[Jasen Betts]

This is another tough point for me. If I am not mistaken, "power rail
bypassing" is caps put between the power and ground rails to keep down
ripple? I was never clear on where to put the caps, especially on a wired
board, not to mention the value or type of caps to use (disc ceramic?).

Disc ceramic is suitably old-school to match your Z80, but these days it's
monolithic ceramic that's used instead, basically it's a 2mmx1mm surface
mount part, if you want leaded parts it's the same surface mount chip with
wires bonded on, a coating of resin and some apropriate numbers written on
it.