Once the V20-MBC or Z80-MBC2 is plugged into a USB port of my Chromebox, starting a Minicom terminal emulator session makes the remote device boot up and display progress messages.

At some point, with the V20-MBC, starting Minicom did nothing and only the terminal printed its signon messages. The remote device wouldn't boot and there were no signs of life other than the lit LEDs.

The flat cable that runs from the V20-MBC ends in a serial-USB adapter with a USB connector that goes into the Chromebox. This is the adapter card with the USB connector at left and, at right, the pins the cable's connector plugs into:

To troubleshoot the issue I unplugged the USB connector from the Chromebox, which is when the cable's connector came out from the pins. That made clear the issue was a loose plug. Plugging the adapter back into the cable, reconnecting to the Chromebox, and starting Minicom made the V20-MBC automatically boot up as usual.

#v20mbc #sbc

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Just4Fun, the maker of the Z80-MBC2 and V20-MBC homebrew computers, clarfied to me the absence of some files is intentional.

Under CP/M-86, the TOD.CMD command for displaying and changing the date and time was left out because, on the V20-MBC, the date can be changed only from the boot and configuration menu. ASSIGN.CMD, a command for assigning physical to logical devices, is specific to the CP/M-86 version for the IBM PC and hence redundant on the V20-MBC.

Just4Fun doesn't recall exactly, but the absence of the Turbo Pascal sample .PAS files on the Z80-MBC2 is likely a consequence of the development process, possibly an early mass storage space constraint.

#v20mbc #z80mbc2 #sbc

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The V20-MBC homebrew computer I ordered finally arrived and I started checking it out.

The V20-MBC is a single-board computer by the same maker of the Z80-MBC2. Most of the design is common to both. However, unlike the Z80-MBC2, instead of a Z80 the V20-MBC has a Nec V20 chip that implements the instruction sets of the Intel 8080, which runs CP/M-80 2.2 on the V20-MBC, and the Intel 8088 with CP/M-86 1.1.

I got a V20-MBC Black Edition. It's a version of the V20-MBC with higher quality parts and a black PCB that, along with most of the other components also black, makes for a slick look:

I already love the device. These notes are my early impressions, but I'll continue sharing my experience with the V20-MBC.

Hardware

Both homebrew computers have similar size and layout and are accessed the same way, i.e. with a terminal emulator running on a desktop computer, a Chromebox in my case, connected via a USB serial line. Here is the setup, with the Chromebox at the top:

The performance of the 8088 of the Nec V20 under CP/M-86 is noticeably better than the Z80-based Z80-MBC2. I estimate CP/M-86 software on the V20-MBC is 2-3X as fast as CP/M-80 programs on the Z80-MBC2.

Software

The V20-MBC is operated the same way as the Z80-MBC2, except for a couple of differences.

To account for the two CPUs, the boot and configuration menu duplicates the entries that load executable code. This is the menu in a Minicom terminal emulator session under Crostini Linux on the Chromebox:

There are two options for uploading an executable in Intel HEX format to run on the bare hardware with no operating system, iLoad for 8086 code and iLoad-80 for Intel 8080 code. Similarly, Autoboot automatically boots a designated 8086 binary, Autoboot-80 an 8080 one.

Although I never used CP/M-86 before, thanks to my prior experience with CP/M-80 the 16-bit operating system looks familiar and I already feel productive. Aside from the additional transient commands of CP/M-86, the only major difference is executable program files have the .CMD extension instead of .COM as on CP/M-80. This screenshot of the A: drive directory shows the extensions:

Issues

A quick check reveals some CP/M-86 files are missing, such as the TOD.CMD and ASSIGN.CMD transient commands. This is not unexpected as the Z80-MBC2 is also missing some files, e.g. STAT.COM and the Turbo Pascal sample .PAS sources. No big deal, it's easy to find and download suitable replacements.

#v20mbc #sbc #retrocomputing

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I couldn't resist. I ordered an assembled V20-MBC Black Edition kit with 1 MB RAM, a homebrew computer by the same prolific maker of the Z80-MBC2. The devices share the same design, except instead of a Z80 like the Z80-MBC2 the V20-MBC features a Nec V20 bundling the 8088 and 8080 CPUs.

It's exactly what I needed, the perfect match for running my Assembly code on actual hardware and a complement to the Z80-MBC2.

Suite8080, my suite of Intel 8080 Assembly cross-development tools, includes the assembler I use for my 8080 programs executed on the Z80-MBC2 under CP/M-80, and now also on the V20-MBC as native 8080 code. I also set up an x86 Assembly cross-development environment to create 8086 code that will run on the V20-MBC under CP/M-86. And, of course, the Z80-MBC2 lets me run Z80-specific projects.

The shipping is on its way to me, I'll share my experience with the V20-MBC.

#v20mbc #retrocomputing #sbc #x86

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The Z80-MBC2 homebrew Z80 computer is a living retrocomputing museum that runs early microcomputer operating systems such as CP/M, QP/M, and UCSD p-System. Since the latter is the one I know least, I explore it with the Z80-MBC2.

On UCSD p-System the user runs programs and manages the system through a series of hierarchical menus, unlike the command interpreters of other operating systems. Navigating the menus down the hierarchy is easy, but sometimes I'm not sure how to get up one level. Pressing the Q key works most of the times, but occasionally nothing happens and I get stuck.

Running UCSD p-System gives a sense of how slow the output is on serial terminals. For example, a demo program to plot an ASCII sine wave takes almost 20 seconds over a 115200 bps serial line to the 8 MHz Z80-MBC2.

It's best demonstrated by a screencast I recorded, which shows the output as well as the menus. In the video the Z80-MBC2 runs in a Minicom terminal emulator session under Crostini Linux on my Chromebox.

#z80mbc2 #retrocomputing #sbc

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One of the Z80-MBC2 boot menu options, iLoad, allows uploading an executable program in Intel HEX format. This is cool because it enables running code on the bare metal with no operating system, such as a resident monitor or software that directly accesses the hardware.

To test iLoad I uploaded and run a demo to the Z80-MBC2, a Z80 Assembly program that prints a message to the console and blinks the board's User LED.

The source file, Blink demo Z80-MBC2.asm, is included in the sources of the Z80-MBC2 firmware in directory src. I assembled the program with the zasm Z80 assembler and converted it to HEX format with the bin2hex tool that comes with the z80pack Z80 emulator.

The screenshot shows the steps for uploading the demo to the Z80-MBC2 and running it. It's a Minicom session under Crostini Linux on my Chromebox.

First off, I brought up the Z80-MBC2 boot menu. After selecting the iLoad option, the Z80-MBC2 waited for the file transfer to start.

Next, in Minicom I executed the Paste file command (activated by the Ctrl+A Y keystroke), which reads an ASCII file and sends its contents as if it were typed in, and selected the program file blink.hex. A few seconds later the message Z80-MBC2 User led blink demo running... appeared on the console and the board's User led started blinking.

I completed the process successfully only after tweaking the transmission parameters.

The Z80-MBC2 developer notes the board's serial port can't handle high-speed data transfers, and advises to set a 50-90 ms newline transmission delay in the settings of the terminal emulator used for the transfer.

In Minicon I set the delay with the Newline Tx delay (Ctrl+A T D) option. But with my setup iLoad transfers abort even with delays up to 150 ms, causing the Z80-MBC2 to display a checksum error and halt.

To overcome the issue I set a character transmission delay of at least 5 ms with the Character Tx delay option (Ctrl+a T F).

#z80mbc2 #z80 #sbc

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ChromeOS 104 delivered the ability to access USB serial devices from Android, an option for controlling the Z80-MBC2 computer from an Android terminal emulator app.

I didn't realize chromeOS 104 improved the support for accessing USB serial devices also from web apps implementing the WebUSB API. On my Chromebox, version 104 is the first that enables controlling the Z80-MBC2 from the web. Here's a CP/M 3.0 session in a serial terminal emulator web app:

Up to chromeOS 104, web terminals failed to connect to the Z80-MBC2 as they didn't detect the USB device. With version 104 I tested the following terminal apps, most of which work:

• serial terminal
• SerialTerminal.com: works but outputs double-spaced text lines
• Tibbo WebTerminal: there's apparently no option for connecting to USB devices
• USB Serial Debug Log: doesn't detect the Z80-MBC2
• web-serial-terminal

These apps operate the same way. A connection button brings up a system dialog listing the serial devices, like the Z80-MBC2's CP2102 chip. Once connected, the apps behave like other terminal emulators.

Although useful as an additional option for controlling the Z80-MBC2 on chromeOS, these web terminals are experimental or basic apps, have limited functionality, and miss major features like XMODEM file transfer.

#z80mbc2 #sbc #Android #chromeOS

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ChromeOS 104 landed on my Chromebox delivering a pleasant surprise, the ability to access serial USB devices from Android apps.

When I plugged the Z80-MBC2 Z80 homebrew computer into the Chromebox under chromeOS 104, the system popped up this notification allowing me to connect the device to Android or Linux, not just Linux as before:

The notifcation confirms the detection of the Z80-MBC2's CP2102 chipset and says:

USB device detected

Open Settings to connect CP2102 USB to UART Bridge Controller to Linux or Android apps

Connect to Linux Connect to Android

I had long been looking forward to accessing the Z80-MBC2 from Android. I researched a great terminal emulator app, Serial USB Terminal, which can connect to serial USB devices, features basic ANSI support, and can transfer files via XMODEM. Although the app runs fine on the Chromebox, I never figured how to connect to the Z80-MBC2. It turns out it wasn't possible, until chromeOS 104.

Selecting the notification's option to connect to Android prompts to run Serial USB Terminal, optionally setting it as the default app for Android connections.

I interacted a bit with the Z80-MBC2 from the Android terminal emulator and it's usable. Here's what a CP/M 3.0 session looks like in the app in landscape tablet mode, the window layout that works best with a terminal:

Input goes in a text field separate from the terminal output. It feels awkward on the desktop but natural on mobile devices with touch interfaces.

Serial USB Terminal's ANSI support seems limited or incomplete, but I haven't checked extensively.

I tried transferring a file via XMODEM from the terminal emulator to the Z80-MBC2 under CP/M 3.0. But, as with Crostini Linux, nothing happens and the XMODEM transfer doesn't work. More experimentation may provide clues on the XMODEM issue.

So far I haven't played with the Z80-MBC2 much from Android, but it's great to have another option for controlling the device.

#z80mbc2 #sbc #Android #chromeOS

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On my Chromebox, XMODEM file transfer from Linux to the Z80-MBC2 is broken, but I've been trying with every new chromeOS version just in case an update delivers a fix. So far the transfer still doesn't work with chromeOS up to 104.

#z80mbc2 #sbc #chromeOS

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Exploring the Z80-MBC2 is a required learning step for the adventurous hobbyists and electronic enthusiasts who use this Z80 homebrew computer, to whom it's aimed.

The device comes only with schematics, source code, and a few brochures. However, despite the lack of an official manual, there is a lot of information on the Z80-MBC2 scattered across a number of websites. Put together, this content makes for a solid starting point for further exploration.

In my own journey I'm collecting these resources, which I share here in the hope they will help you get started. I'll update the post with more as I discover them.

Features and operation

The closest thing to a manual is the Z80-MBC2 guide by DarS007, a user who compiled an unofficual user guide. It consists of original notes and snippets from other sources organized into an architectural overview, software information, and tips.

The Z80-MBC2 developer Just4Fun published most of the available information on the project's Hackaday page. All its sections are well worth reading in full, particularly Details and Logs. Section Discussion contains Z80-MBC2 user posts with interesting links and tips.

I also recommend the project page of the Z80-MBC, the Z80-MBC2 predecessor with which it shares most hardware and software features. These pages provide complementary content, i.e. the Z80-MBC page has useful information missing from the Z80-MBC2 page.

Just4Fun published much of the same content on his personal website and has an Instructables guide.

The Z80-MBC2 user Coopzone posted some notes on managing virtual disks and files. Another user blogged his experience with assembling and using the Z80-MBC2.

Operating systems

The Z80-MBC2 can boot a number of operating systems such as CP/M 2.2 and 3.0, as well as UCSD p-System.

Nearly all the existing documentation on these historical systems applies to the versions that ship with the Z80-MBC2. The same goes with the application software for these operating systems that's preinstalled on the Z80-MBC2, for example WordStar and Turbo Pascal.

Some of these operating systems and programs are less known. Refer to the official QP/M and Collapse OS official websites for documentation on Q/PM 2.71 and Collapse OS, bootable from the Z80-MBC2.

As for lesser known application software, see the documentation of the VDE text editor on which the CP/M ZDE16 editor that comes with the Z80-MBC2 is based.

Programming languages

Along with the operating systems, the Z80-MBC2 firmware can boot a couple of programming language interpreters like the BASIC and Forth systems in the ROMs of typical 1980s microcomputers.

The Z80-MBC2 ships with a Nascom BASIC implementation modified by Grant Searles. See the Nascom BASIC documentation.

Development tools

In addition to using the supplied software, it's fun to program the Z80-MBC2 in C or Assembly. The project sites of the major compilers and development tools usable for the Z80-MBC2 usually provide documentation:

• SDCC: a C compiler and assembler
• SDCC – Interfacing with Z80 assembler code: information on calling Z80 Assembly from C with SDCC
• z80-mbc2: the Assembly source of this memory test tool and a resident monitor has details on the Z80-MBC2 internals and I/O

#z80mbc2 #sbc #retrocomputing

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