Least Expensive But Still Functional ECB SBC V2

The ECB SBC V2 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 ECB 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 system). If you are not planning using the parallel port, its components can be omitted.

  • U19 (8255 PPI)
  • P1 (13×2 pin header)
  • K9 (3 pin header)
  • C18 (0.1uF bypass capacitor)

Real Time Clock

ECB SBC board features Dallas DS1302 Real Time Clock. It can be used for time keeping and also it has a few bytes non-volatile memory (NVRAM), that could be used for example, for storing system configuration.

  • 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). It can be omitted (losing the battery backup functionality).

  • U25 (DS1210)
  • Connect pin 5 to pin 6, and pin 1 to pin 8, so /CS and VCC go directly to SRAM.

RS232 Interface

If you can use a TTL to USB serial interface converter then the RS232 level converter can be omitted.

  • U22 (Max232)
  • C24-C28 (1uF capacitors)
  • Bridge pins 7-10 and 8-9 in the Max232 component position.
  • Bridge pins 14-11 and 13-12 in the Max232 component position if hardware flow control is required.

Examples of minimum connections required:

  • 9600 baud
  • 8 data bits
  • 2 stop bits
  • No flow control

Component Substitution

Use flash memory instead of UV-erasable EPROM

Flash memory is generally cheaper, easier to obtain, and easier to work with. It can be instantly erased, without UV eraser lamp, it doesn't need special voltages for programming and 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, 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.

Device - JumperSizeK1K6K8

In the situation that you do not have an EPROM programmer it is best to obtain a copy of the ROMWBW in a FLASH format so it can be reprogrammed in-system using the FLASH utility. A HOTFLASH technique can be used if you want to swap from an EPROM to a FLASH chip without an EPROM programmer:

Example starting with an AT27C080 EPROM and swapping to a AT49F040A FLASH chip.

  1. Transfer new ROM image from PC to SBC-V2 secondary storage i.e. SDCard
  2. Copy FLASH.COM from B: to secondary storage i.e SDCard
  3. Change to secondary storage directory.
  4. Remove EPROM
  5. Change jumpers to suit FLASH chip i.e. K1 unchanged, K6 from 1-2 to 2-3, K8 from 1-2 to 2-3.
  6. Insert FLASH chip. Insert pin 1 side first to ensure GND had contact first.
  8. Reboot

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.

Use MAX232 alternative.

The original MAX232 or MAX232A parts can be expensive. 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).

Disk storage without ECB backplane and Disk I/O board.

You can install the 8255 PPI chip, and use the PPIDE mini board to connect an IDE drive or a compact flash card.


Crystals and Oscillators.

The ECB SBC V2 has been been overclocked successfully, with speeds as high as 16Mhz reported by replacing the 4Mhz crystal.

An elegant solution for replacing the TTL oscillator with standard crystals was identified and described by trick-1, with the benefit of variable clock rate flexibility for overclocking.

Based on the circuit described here https://www.eleccircuit.com/crystal-oscillator-using-ttl/ a 4/8/10/12/16/20 MHz switchable clock circuit was implemented which demonstrated the maximum clock speed in his board was 10Mhz.


Untested but theoretically possible, you should be able to run the CPU off the 1.8432 UART clock by connecting pin 5 of P5 (CLK_UART) to P4 pin 5 (CLK_CPU).

boards/sbc/sbc_v2/minimum-build.txt · Last modified: 2019/06/10 11:10 by b1ackmai1er
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