The SBC V2 is a Zilog Z80 processor board. It's a 100x160mm board that is capable of functioning both as a standalone SBC or as attached to the ECB bus.

Previously known as the N8VEM SBC, development began in 2006 wth V1 and is currently still in development.

For most people, the SBC-V2-004 offers the most convenience and functionality.

• 4Mhz+ Zilog Z80 CPU.
• Up to 512Kb paged SRAM.
• Up to 1Mb EPROM or 512Kb Flash ROM.
• Serial Interface.
• IDE interface.
• Real Time Clock.
• Onboard sound.
• Battery backup for RTC and SRAM via super capacitor.
• Standard ECB bus interface.
• +5V power connector for Single Board Computer operation.
• Reset button.
• Status LED.

ROMWBW BIOS featuring:

• CP/M / ZSDOS
• RAM & ROM disk support.
• ROM based BASIC, FORTH and Monitor.
• Prebuild disk images

The following table outlines the correct jumper settings for the SBC V2 board:

* = default setting

The SBC V2 doesn't have any video capabilities in its solitary form. Therefore, in order to test it, you will need to connect it to a serial terminal (which will provide the keyboard input and monitor output). The easiest way to do this is to construct a cable which will provide a serial port (with a D-sub connector or DE-9 plug) which will plug into a “host” computer running a serial terminal emulation program. This program will allow you to interface with the SBC V2 board using the host computer’s keyboard and monitor.

First, you must build a cable with an IDC-10 plug on one end (plastic rectangular connector with 2 rows of 5 pins) and a female DE-9 plug (ie. a serial port plug) on the other end.

The female DE-9 plug is what you will plug into the serial port of your host computer. Serial ports (on the back of the computer) are male ports (ie. they have pins), so the plug at the end of this cable must be a female plug (they have holes). It's not recommended to use accessory serial cables to make this connection (such as null modem cables, etc.). This is because many such cables are wired for specific applications, and may not work with your SBC V2 board. For instance, a serial data transfer cable is wired very differently from a “straight-through” serial cable (pin 1 to 1, 2 to 2, etc.) which is different from a null modem cable. Because of this cable inconsistency, it's preferred to build the complete custom cable from the beginning knowing exactly what pin is connected where and why, and not to use any serial extension cables. The exception to this is using a serial cable that you know is wired as a “straight through” cable (with each pin on one plug connected to each pin on the other plug: 1 to 1, 2 to 2, etc).

The following cable layout shows what is being connected where in this cable:

Pin 1 on the IDC plug is marked with an embossed triangle on the plug, and this pin corresponds to pin 1 on the pcb which is marked with a square solder hole (at the lower-right most position of the plug on the SBC V2 pcb). The pins on the DE-9 plug are usually marked right on the plug itself in tiny numbers. Strip some wires and solder away. Instead of stripping wires and soldering manually, you can also use ribbon cable and special “direct connect” plugs that connect directly to the ribbon cable without soldering (they have rows of metal “pins” that press down and penetrate the ribbon cable to make the connections). Because of the non-standard pin connections that are needed in this cable (per the above table), however, you will likely need to do some soldering or “custom connecting”.

Always double and triple check where you’re soldering something before you solder it. When you’ve built your cable, use your multimeter to check connectivity between each pin on the IDC-10 side and the DE-9 side according to the arrangement above to make sure you got it right.

In order to test connections in a plug you cannot stick the multimeter lead into the hole (because it will not fit). Instead, take a spare piece of wire, stick it into the hole for the pin you want to test, and then touch the multimeter lead to that wire.

ROMWBW will automatically identify the type of serial chip installed. Depending on the capability of the serial chip, flow control may or may not be available. Chip data sheets cannot be relied upon to accurately identify the chips capability.

In the absence of flow control, guidelines for baud rate settings are:

4Mhz CPU Crystal - 38400 baud

8Mhz CPU Crystal - 57600 baud

Baud rate can be changed using the CP/M MODE program i.e. MODE COM0:57600,8,N,1 /P

If garbled characters are seen on the display or serial file transfers fail, a lower baud rate should be tried.

There is extensive firmware support for the SBC V2 and associated ECB peripheral cards. Currently there are two firmware builds still being actively developed - UNA and ROMWBW.

See here for current software builds and information on historical projects.

A debug boot ROM can be installed for testing - try James Moxham's ROMIMAGE.BIN from here.

The DS1302 real time clock can be set under CP/M using the rtc utility program.

Loading CPM...
CP/M-80 Version 2.2C for the N8VEM, October 2008
Run XM from A drive, this downloads file to B drive
A>rtc
Start RTC Program
RTC: Version 1.0
RTC: Commands: E)xit T)ime st(A)rt S)et R)aw L)oop C)harge N)ocharge D)elay I)nit G)et P)ut H)elp
RTC: trickle charger disabled.
RTC>

Set the time and date with the Init command first and then use the Set command to program it.

RTC.COM can also be used to set up trickle charging for the backup battery or super capacitor if connected. More Information on charging Super Capacitors can be found here: https://www.maximintegrated.com/en/design/tools/calculators/product-design/supercap.cfm

The utility can also set and read the RTC memory.

RTC.COM is part of Wayne Warthen's ROMWBW package.

Bit 6 of RTC port $70 is connect to an on board jumper JP2 which can be read by software. By default it is tied high. Removing jumper JP2 will result in the being tied low (=0).

Effectively this creates a configurable jumper setting which the ROMWBW bootrom can use for determining the primary communications console.

This facility is not enabled by default in ROMWBW and the setting is ignored. To enable, rebuild the ROMWBW package with the CRTACT setting set to true. The CRT type can also be configured at this time:

SBC_std_cust.asm

CRTACT        .SET    TRUE           ; CRT ACTIVATION AT STARTUP
VDAEMU        .SET    EMUTYP_ANSI    ; DEFAULT VDA EMULATION (EMUTYP_TTY, EMUTYP_ANSI, ...)

This will cause ROMWBW to check the status of BIT 6 at startup. If “0” (jumper removed) it will change the primary communications console from the Serial UART to the first CRT console found. If a CRT console is not found it will default back to the Serial UART. If BIT 6 = “1” (jumper in place) it will boot as normal with the primary console using the Serial UART.

There is no inbuilt sound on the SBC V2-003 board. Complex sound output can be achieved with the ECB-SCG (Sprite-Colour-Graphics) board.

Rudimentary sound support can be add the SBC V2 by utilizing one of the unused outputs of the RTC interfacing latch and connecting a simple amplifier to this output.

On U18 74LS174 connect a ground wire from pin 8 and a signal wire from pin 2 to an amplifier. Generic LM386 amplifiers are available from many sources and work well.

With this addition and the SPKENABLE setting set to TRUE in ROMWBW, the SBC V2 will issue a beep sound on bootup.

A transistor speaker driver has been incorporated in the SBC-V2-004 so a speaker can be connected directly to the board.

Sound support has been added to NASCOM BASIC in ROMWBW 2.9.2-pre.17:

PLAY O,N,D

Where:
 O = OCTAVE 0-8
 N = NOTE 0-11
 D = DURATION 1-8 (8ths of a second)

Example:
 Middle C for one second = PLAY 4,0,8

The following modification allows the cpu clock speed to be changed from full to half speed under program control. The purpose of this modification is to provide a level of compatibility with MSX applications while retaining the high speed capability. Specifically a 7.159Mhz crystal can be installed for normal operation including high density floppy disk support but be switched to 3.5795Mhz to drive the Sprite Colour Graphics board when required.

This modification requires a 74LS74 to be modified and mounted on top of U4. Five leads are required to connect to other parts of the circuit. Suggested installation method:

Prepare connection points for new clock source:

1. Remove Z80 CPU U4 and fold out pin 6 and reinsert.
2. Remove 74LS244 U17 and fold out pin 6 and reinsert.

Prepare a 74LS74 for mounting on 74LS10 U4

1. Fold up pins 1-6 and 8-13 and snip off the ends leaving the stubs. Pin 7 and 14 remain for connecting to 74LS10 U4.
2. Solder a wire from pin 14 to pin 1 and then to pin 11 (brown wire).
3. Solder a wire from pin 4 to pin 9 (blue wire)
4. Solder wire from pin 13 to pin 5 (green wire)
5. Solder wire from pin 12 to pin 7 (yellow wire)

Mounting 74LS74 and connecting flying leads on component side of board:

1. Place the 74LS74 over 74LS10 U4 so only pin 7 and 14 are touching.
2. Solder pin 14's together and pin 7's together.
3. Solder a wire from pin 2 of the 74LS74 to pin 5 of 74LS174 U18 (purple wire)
4. Solder a wire from pin 11 of the 74LS74 to pin 12 of 74LS06 U26 (orange wire)
5. Solder a wire from pin 3 of 74LS74 to pin 13 74LS06 U26 (red wire).
6. Solder a wire from 7 of the 74LS74 to lifted pin 6 of 74LS244 U17 and then to lifted pin 6 of Z80 CPU U24 (yellow wire)

Solder side connections:

1. Solder wire from pin 5 of CPU clock P4 to pin 13 74LS06 U26 (red wire).

Software control

Bit 4 of port 70h is used to select the clock speed. The normal condition is for Bit 4 to be set to 0 (low) and this will select the full clock speed of the installed crystal. Writing a 1 (high) to Bit 4 of port 70h will activate the 74LS74 clock divider. The speed switch is currently not supported in ROMWBW. Use of the RTC or speaker will reset Bit 4 to 0 i.e. full speed operation. Further, precalculated software delays or delays based on the initial speed detected at boot time do not take into account the possibility of the speed changing.

Demonstration code:

1 OUT 112,0
2 PRINT "NORMAL SPEED"
3 GOSUB 10
4 OUT 112,8
5 PRINT "SLOW SPEED"
6 GOSUB 10
7 END
10 FOR X=1 TO 64
20 PRINT ".";
30 FOR Y=1 TO 256
40 NEXT Y
50 NEXT X
60 PRINT
70 RETURN

Demonstration video