Yet another computer museum
The BEM-3X/BertMon5.3 system

The B.E.M 3X is a Eurocard controller board intended for the micro-controller/PLC market. It has three sockets for RAM or ROM, a MC6809 CPU, three 6522 VIA's, 2651 USART and an MM58274 RTC.

In a second lease on life, it is the hardware foundation of the BertMon system. This firmware is created by Robert Henneke and consists of a 4 kByte monitor, and a 4 kByte Disassembler/NMI-Debugger. The remainder of the 16 kByte ROM is available for more extensions or a programming language.

The BertMon code is alas not Open Source.

BEM-3X
The board is used in a standard configuration with an exception for the NMI-debugger. The /IRQ pin of one of the VIA's is directly connected to the CPU /NMI pin, bypassing the normal interrupt circuit. Four I/O pins of this VIA are used for status LEDs by the monitor.

The B.E.M-SBC3X features:

Two jumper selectable memory map configurations Maximum ROM: 32 kByte (and 16 kByte RAM) and Maximum RAM: 40 kByte (and 16 kByte EPROM)

Three 28-pin JEDEC JC-42 sockets (27XXX EPROMs)

Single +5V power supply (with CMOS components, less than 250 mA)

MM58274 Real Time Clock

Lithium Battery for RTC and optionally (CMOS) RAM

Flexible Watchdog Timer

Separate clocks for CPU, USART and RTC

3 VIAs (65C22) for 60 I/O pins, 6 timer/Counters, and 3 shift registers

1 USART (2651 or 2661) for synchronous or asynchronous serial I/O communication

In the context of the BertMon monitor, the Maximum RAM configuration is used and parts of one VIA is used by the system (1 Timer/Counter and 4 I/O pins)

BEM-3X
solder-side
The board has long (~0.5") pins protruding on the solder side, allowing to use the board to be piggy-backed on a larger board and use all the I/O. The gray wire is to make the /WR signal available for the Memory replace board.

Extra
serial port
The B.E.M-3X has three VIA's but just one serial port (USART). This board replaces one of the VIA's bu a ACIA complete with BAUD rate generator. The VIA I/O connector is used to select the BAUD rate. The board was designed for a 74HC4060 and a crystal, but this modified version uses a 74HC4040 and crystal oscillator. This allows for a larger range of BAUD rates.
The modification is intended for a serial FlexNet connection, which replaces floppy disks with a remote server, which works on floppy images.

Memory
replace
The original memory configuration has a 16 kByte EPROM from $C000 to $FFFF, but to run Flex 09, one needs RAM at $C000-$DFFF. This board does exactly that. The orange wire is to have a /WR signal available for he RAM. The board works, but parts of the RAM is written to by the monitor (it seems).

Documentation:

Scanned documentation, datasheets and program listings

Local links:

The MicroKit09, a recreation of a MC6809 SBC in two parts.
The datasheet of the Brutech Electronics B.E.M-SBC[23]X 6502/6809 SBC boards.
ACIA transplant schematic.
Memory transplant.
Test routine for the MM58274 RTC.

Generic links:

FlexNet source

Latest update: 2026-09-10

e-mail

BEM-3X		The board is used in a standard configuration with an exception for the NMI-debugger. The /IRQ pin of one of the VIA's is directly connected to the CPU /NMI pin, bypassing the normal interrupt circuit. Four I/O pins of this VIA are used for status LEDs by the monitor. The B.E.M-SBC3X features: Two jumper selectable memory map configurations Maximum ROM: 32 kByte (and 16 kByte RAM) and Maximum RAM: 40 kByte (and 16 kByte EPROM) Three 28-pin JEDEC JC-42 sockets (27XXX EPROMs) Single +5V power supply (with CMOS components, less than 250 mA) MM58274 Real Time Clock Lithium Battery for RTC and optionally (CMOS) RAM Flexible Watchdog Timer Separate clocks for CPU, USART and RTC 3 VIAs (65C22) for 60 I/O pins, 6 timer/Counters, and 3 shift registers 1 USART (2651 or 2661) for synchronous or asynchronous serial I/O communication In the context of the BertMon monitor, the Maximum RAM configuration is used and parts of one VIA is used by the system (1 Timer/Counter and 4 I/O pins)
BEM-3X solder-side		The board has long (~0.5") pins protruding on the solder side, allowing to use the board to be piggy-backed on a larger board and use all the I/O. The gray wire is to make the /WR signal available for the Memory replace board.
Extra serial port		The B.E.M-3X has three VIA's but just one serial port (USART). This board replaces one of the VIA's bu a ACIA complete with BAUD rate generator. The VIA I/O connector is used to select the BAUD rate. The board was designed for a 74HC4060 and a crystal, but this modified version uses a 74HC4040 and crystal oscillator. This allows for a larger range of BAUD rates. The modification is intended for a serial FlexNet connection, which replaces floppy disks with a remote server, which works on floppy images.
Memory replace		The original memory configuration has a 16 kByte EPROM from $C000 to $FFFF, but to run Flex 09, one needs RAM at $C000-$DFFF. This board does exactly that. The orange wire is to have a /WR signal available for he RAM. The board works, but parts of the RAM is written to by the monitor (it seems).

Yet another computer museum The BEM-3X/BertMon5.3 system

Yet another computer museum
The BEM-3X/BertMon5.3 system