MECB Bus Pinout Update
Posted: Mon Oct 12, 2026 8:24 pm
Due to a couple of requests, which brought to my attention an oversight in not bringing out the 6809 / 6309 FIRQ input to the bus (Fast Interrupt Request), I’ve made an addition to the documented MECB Bus Pinout.
Firstly, note that the original pin-out was directly based on a minimalist version of the original Kontron ECB bus pin-out. As this bus was originaly intended for Z80 use, it provided for only a single /INT (and /NMI) interrupt line signals.
With the 6809 family, we now also have the benefit of a second faster interrupt (/FIRQ). With the 68008, we also have multiple priority interrupt signal potential.
After careful consideration, I’ve come to the conclusion that pin A23 (which is allocated /IQ0 on the currently published extended ECB pinout), would be the logical choice for allocating as a second standardised MECB interrupt line.
On my MECB Bus Pinout, this A23 pin had previously been flagged as *resv (reserved), so it does not conflict with any existing *user pin use.
This creates a generalised second interrupt pin, which also allows for its wider use (other than just for 6809 FIRQ).
e.g. We could also logically implement the second interrupt line as an additional priority interrupt level for the MC68008 CPU Card.
I now just need to progressively roll this change out through the documentation and via version updates to the relevant MECB Cards.
Firstly, there is the connection of the /IQ0 bus pin (A23), on relevant CPU Cards. Most notably the 6809 and 6309 CPUs /FIRQ input.
Secondly, implementing 3-pin headers for choosing between /INT or /IQ0 bus line connection on interrupt generating I/O Cards.
Noting that this might also present an available space challenge on cards like the Motorola I/O Card, which has no less than 4 device interrupt pins!
These changes will be reflected in new MECB Card versions. No change is necessary (of course), for the backplanes, as all pins are already connected.
In the meantime, I highlight this Bus Pinout Update so that anyone wanting to implement an additional interrupt line (e.g. /FIRQ on the 6809 or 6309), on their existing CPU Cards, can make the choice to jumper across to the previously reserved A23 pin, to ensure they maintain future peripheral Card compatibility.
The current (updated) MECB Bus Pinout can be found here: Minimalist Europe Card Bus (MECB) – Bus Pinout Documented!
Firstly, note that the original pin-out was directly based on a minimalist version of the original Kontron ECB bus pin-out. As this bus was originaly intended for Z80 use, it provided for only a single /INT (and /NMI) interrupt line signals.
With the 6809 family, we now also have the benefit of a second faster interrupt (/FIRQ). With the 68008, we also have multiple priority interrupt signal potential.
After careful consideration, I’ve come to the conclusion that pin A23 (which is allocated /IQ0 on the currently published extended ECB pinout), would be the logical choice for allocating as a second standardised MECB interrupt line.
On my MECB Bus Pinout, this A23 pin had previously been flagged as *resv (reserved), so it does not conflict with any existing *user pin use.
This creates a generalised second interrupt pin, which also allows for its wider use (other than just for 6809 FIRQ).
e.g. We could also logically implement the second interrupt line as an additional priority interrupt level for the MC68008 CPU Card.
I now just need to progressively roll this change out through the documentation and via version updates to the relevant MECB Cards.
Firstly, there is the connection of the /IQ0 bus pin (A23), on relevant CPU Cards. Most notably the 6809 and 6309 CPUs /FIRQ input.
Secondly, implementing 3-pin headers for choosing between /INT or /IQ0 bus line connection on interrupt generating I/O Cards.
Noting that this might also present an available space challenge on cards like the Motorola I/O Card, which has no less than 4 device interrupt pins!
These changes will be reflected in new MECB Card versions. No change is necessary (of course), for the backplanes, as all pins are already connected.
In the meantime, I highlight this Bus Pinout Update so that anyone wanting to implement an additional interrupt line (e.g. /FIRQ on the 6809 or 6309), on their existing CPU Cards, can make the choice to jumper across to the previously reserved A23 pin, to ensure they maintain future peripheral Card compatibility.
The current (updated) MECB Bus Pinout can be found here: Minimalist Europe Card Bus (MECB) – Bus Pinout Documented!