TRS-80 Model 4p

The TRS-80 (models 1, 3 and 4) are among the most popular z80 machines. They're very nicely designed and I got my hands on a 4p with two floppy disk drives and a RS-232 port. In this recipe, we're going to get Collapse OS running on it.

This is a work in progress. Collapse OS doesn't run on it yet.

Floppy or RS-232?

There are many ways to get Collapse OS to run on it. One would involve writing it to a floppy. I bought myself old floppy drives for that purpose, but I happen to not have any functional computer with a floppy port on it. I still have the motherboard of my old pentium, but I don't seem to have a video card for it any more.

Because my 4p has a RS-232 port and because I have equipment to do serial communication from modern machines (I didn't have a DB-9 to DB-25 adapter though, I had to buy one), I chose that route.

Gathering parts

A TRS-80 model 4p with a RS-232 port
A TRSDOS 6.x disk
A means to do serial communication. In my case, that meant:
- A USB-to-serial device
- A null modem cable
- A DB-9 gender changer
- A DB-9 to DB-25 adapter

Overview

We need to send sizeable binary programs through the RS-232 port and then run it. The big challenge here is ensuring data integrity. Sure, serial communication has parity check, but it has no helpful way of dealing with parity errors. When parity check is enabled and that a parity error occurs, the byte is simply dropped on the receiving side. Also, a double bit error could be missed by those checks.

What we'll do here is to ping back every received byte back and have the sender do the comparison and report mismatched data.

Another problem is ASCII control characters. When those are sent across serial communication channels, all hell breaks lose. When sending binary data, those characters have to be avoided. We use tools/ttysafe for that.

Does TRSDOS have a way to receive this binary inside these constraints? Not to my knowledge. As far as I know, the COMM program doesn't allow this.

What are we going to do? We're going to punch in a binary program to handle that kind of reception! You're gonna feel real badass about it too...

Testing serial communication

The first step here is ensuring that you have bi-directional serial communication. To do this, first prepare your TRS-80:

set *cl to com
setcomm (word=8,parity=no)

The first line loads the communication driver from the COM/DRV file on the TRSDOS disk and binds it to *cl, the name generally used for serial communication devices. The second line sets communication parameters in line with what is generally the default on modern machine. Note that I left the default of 300 bauds as-is.

Then, you can run COMM *cl to start a serial communication console.

Then, on the modern side, use your favorite serial communication program and set the tty to 300 baud with option "raw". Make sure you have -parenb.

If your line is good, then what you type on either side should echo on the other side. If it does not, something's wrong. Debug.

Punching in the goodie

As stated in the overview, we need a program on the TRS-80 that:

Listens to *cl
Echoes each character back to *cl
Adjusts ttysafe escapes
Stores received bytes in memory

That program has already been written, it's in recv.asm in this folder. You can get the binary with zasm < recv.asm | xxd.

It's designed to run from offset 0x4000 and write received data in 0x3000 and onwards.

How will you punch that in? The debug program! This very useful piece of software is supplied in TRSDOS. To invoke it, first run debug (on) and then press the BREAK key. You'll get the debug interface which allows you to punch in any data in any memory address. Let's use 0x4000 which is the offset it's designed for.

First, display the 0x4000-0x403f range with the d4000<space> command (I always press Enter by mistake, but it's space you need to press). Then, you can begin punching in with h4000<space>. This will bring up a visual indicator of the address being edited. Punch in the stuff with a space in between each byte and end the edit session with x.

But wait, it's not that easy! You see those 0xffff addresses? They're placeholders. You need to replace those values with your DCB handle for *cl. See below.

Getting your DCB address

In the previous step, you need to replace the 0xffff placeholders in recv.asm with your "DCB" address for *cl. That address is your driver "handle". To get it, first get the address where the driver is loaded in memory. You can get this by running device (b=y). That address you see next to *cl? that's it. But that's not our DCB.

To get your DBC, go explore that memory area. Right after the part where there's the *cl string, there's the DCB address (little endian). On my setup, the driver was loaded in 0x0ff4 and the DCB address was 8 bytes after that, with a value of 0x0238. Don't forget that z80 is little endian. 38 will come before 02.

Sending data through the RS-232 port

Once you're finished punching your program in memory, you can run it with g4000<enter> (not space). Because it's an infinite loop, your screen will freeze. You can start sending your data.

To that end, there's the tools/pingpong program. It takes a device and a filename to send. As a test, send anything, but make it go through tools/ttysafe first (which just takes input from stdin and spits tty-safe content to stdout).

On OpenBSD, the invocation can look like:

doas ./pingpong /dev/ttyU0 mystuff.ttysafe

You will be prompted for a key before the contents is sent. This is because on OpenBSD, TTY configuration is lost as soon as the TTY is closed, which means that you can't just run stty before running pingpong. So, what you'll do is, before you press your key, run doas stty -f /dev/ttyU0 300 raw and then press any key on the pingpong invocation.

If everything goes well, the program will send your contents, verifying every byte echoed back, and then send a null char to indicate to the receiving end that it's finished sending. This will end the infinite loop on the TRS-80 side and return. That should bring you back to a refreshed debug display and you should see your sent content in memory, at the specified address (0x3040 if you didn't change it).

Saving that program for later

If you want to save yourself typing for later sessions, why not save the program you've painfully typed to disk? TRSDOS enables that easily. Let's say that you typed your program at 0x4000 and that you want to save it to RECV/CMD on your second floppy drive, you'd do:

dump recv/cmd:1 (start=x'4000',end=x'4030',tra='4000')

A memory range dumped this way will be re-loaded at the same offset through load recv/cmd:1. Even better, TRA indicates when to jump after load when using the RUN command. Therefore, you can avoid all this work above in later sessions by simply typing recv in the DOS prompt.

WIP: that's where we are for now...

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