What would be the least expensive N8VEM SBC possible?  Least Expensive But Still Functional N8VEM SBC V2 ECB Interface Parallel Interface Real Time Clock NVRAM Controller Other Suggestions Use flash memory instead of UV-erasible EPROM Use MAX232 analogs, instead of the original Maxim part Use CMOS 74HCT or 74AHCT ICs instead of the TTL 74LS Disk storage with bare SBC (without ECB backplane and Disk I/O board) Least Expensive But Still Functional N8VEM SBC V1

What would be the least expensive N8VEM SBC possible? 

To make this interesting and so that builders with enormous amounts of spare parts lying about can’t “cheat” by digging through the junk box let’s assume all parts must be purchased from a vendor.  The vendor should have a price that’s published.

Likewise, “fully functional” is a system that boots CP/M and communicates over the serial terminal but doesn’t have to do anything else.  Most of the on board peripherals are not really necessary and can be safely eliminated.  It makes for a less interesting SBC but certainly reduces cost significantly.

Also to make this interesting it does not have to include a serial cable but does have to come to a interface of some sort. In other words, if you hard wire the serial cable to the UART, you still need to include the whole cable including the DE-9 connector.  Alternatively, you could leave off the cable and DE-9 and just leave a dual row header instead.

Construction techniques are definitely a source of cost.  Using sockets of any kind can be eliminated.  Of course, I would never recommend it but I know Juha didn’t use sockets and his system worked great.  Desoldering that PCB must have been fun though (not!)

What else can be removed or substituted?

Actually this thought experiment may be useful for those interested in using the N8VEM SBC for educational purposes.

Thanks and have a nice day!

Andrew Lynch

Least Expensive But Still Functional N8VEM SBC V2

N8VEM SBC consists of several functional "units". Some of them are not required for minimal system - a system that can still boot CP/M, and run most applications, as long as they don't require additional hardware.

ECB Interface

ECB interface unit of the SBC allows connecting SBC to the ECB back plane, and through it to other ECB boards, such as ECB disk I/O adapter. This unit is not required if you're not planing to extend the system at this stage, and can be omitted.

Following components are parts of ECB interface: U8, U9, U11 (74LS245), U10 (74LS243), U17 (74LS244), U20 (DIN 41612), RR1 (1K array), C1, C5, C7, C10, C11, C13 (0.1 uF bypass capacitors).

Parallel Interface

There are several uses for the parallel interface on N8VEM SBC, most popular ones are connecting an IDE disk using PPIDE mini board and connecting DSKY board. It can also be used for controlling some external hardware (printer, EPROM programmer, watering sytem). If you are not planning using the parallel port, its components can be omitted.

Following components are parts of ECB interface: U19 (8255 PPI), P1 (13x2 pin header), K9 (3 pin header), C18 (0.1uF bypass capacitor).

Real Time Clock

N8VEM SBC board features Dallas DS1302 RTC. It can be used (as the name suggests) for the time keeping and also it has a few bytes non-volatile memory (NVRAM), that could be used for example, for storing system configuration. Currently (Jan 2012) BIOS doesn't support RTC functions, and RTC components could be omitted.

Following components are parts of RTC: U18 (74LS174), U21 (DS1302), X1 (32768 Hz crystal), C9 (bypass capacitor).

NVRAM Controller

Dallas DS1210 NVRAM controller was added in SBC V2. It is a very useful feature, that makes all the system RAM non-volatile, thus preserving RAM disk content when system is powered off. It is especially useful on minimal systems, without additional storage devices (hard disks, floppy drives). Still, it can be omitted (losing the battery backup functionality).

Part reference for NVRAM controller is U25 (DS1210). Note that if omitting this part, it is required to connect pin 5 to pin 6, and pin 1 to pin 8, so /CS and VCC go directly to SRAM.

Other Suggestions

Use flash memory instead of UV-erasible EPROM

Flash memory is generally cheaper, easier to obtain, and easier to work with. It could be instantly erased, without UV eraser lamp, it doesn't need special voltages for programming (and theoretically can be reprogrammed in system). The only drawback of the flash memory, is lower storage capacity - 512 KiB vs. 1024 KiB for 27C080 EPROM (this limitation is due to number of pins in 32 pin package). Recommended flash parts are SST 39SF040 (Mouser), and AMD 29F040 (Jameco). Possibly other flash ICs, with lower capacity (such as 29F020, which is frequently found on older motherboards) can be used instead.

Use MAX232 analogs, instead of the original Maxim part

The original MAX232 or MAX232A parts are quite pricy ($5 or so). Fortunately many other vendors make compatible parts, that are significantly cheaper. For example Texas Instruments TRS202, or Intersil HIN232. These parts can also work with 0.1 uF capacitors (smaller, cheaper, you'll be buying them for bypass capacitors anyway).

Use CMOS 74HCT or 74AHCT ICs instead of the TTL 74LS

CMOS 74HCT and 74AHCT logic ICs are usually cheaper than corresponding TTL ICs. They are also consume significantly less electricity. Note that some people reported problems with using these parts for ECB buffers / transmitters and also in SBC-188 (particularly using 74HCT373/74AHCT373 instead of 74LS373). But otherwise they should work just fine.

Disk storage with bare SBC (without ECB backplane and Disk I/O board)

If at some stage you decide you need an HDD, you can install the 8255 PPI chip, and use PPIDE mini board to connect an IDE drive or a compact flash card. An SD interface miniboard is currently under development, and when ready can be used in similar manner.

Least Expensive But Still Functional N8VEM SBC V1

Items marked with X are required. Instead of 74LS also ALL 74HCT can be used and saves power consumption.

Items marked with + are only required, if you need full RS232 levels. If you can live with TTL levels (ex: TTL to USB cable), than

leave these components out and bridge pin's 7-10 and 8-9 in the max232 socket. That gives you the serial connection on the

2x5 pin header - just with TTL levels.

Picture of minimum SBC with TTL level serial interface:

my_n8vem.jpg