Wiring the new assembler panel

A project log for YGREC8

A byte-wide stripped-down version of the YGREC16 architecture

Yann Guidon / YGDESYann Guidon / YGDES 07/20/2019 at 20:280 Comments

As mentioned in A new assembler panel, I'll soon (hopefully) get a bunch of 4PDT interlocked buttons !

I have already assigned their function but how will I make them work ? Electrically, I only want to have switches, eventually some diodes, and I would love to have the panel use the least power possible, which means ideally no relay. I hope that the panel can be reused for other technologies with the least amount of changes.

The critical information is contained in these 2 diagrams :

Already we can see 2 fields that are (almost) fixed :

There is no interlocked switch with 19 buttons. The most I have found is 12 so I have chosen to use a pair of 10-buttons rows.

How will this work, electrically ?

The panel uses "positive logic" where you implement a bit set to 1 with an electrical contact to an extra signal (usually a bus/rail, such as a positive voltage source). The switches will steer each instruction signal to the common voltage, depending on its encoding.

We can already write some truth equations for the most significant bits of the instruction word :

As usual the OR operator "|" is implemented by parallel switches, while the AND operator "&" connects switches in series.

The terms contain XT  which means the Normally Open side of the XT switch. So XT is a MUX for two sub-buses (b14,b13,b12).

Due to the interlock mechanism, /XT is redundant because it is always off when SH/SA/RO/RC/LDCL/LDCH/IN/OUT/INV opcodes are selected. However the 10 other opcodes are not interlocked and XT must disconnect the affected bits to prevent the other row from interfering.

Updated equations:

This is translated into the following diagrams :

The good news is : there is apparently no need for a complex switch for XT because its signal can be shared with several subsignals, thus saving switches...

The bad news is : to prevent certain cases of feedback/bypass (despite the use of independent switches) the output of XT must be guarded with diodes.

However, there are multiple switches per button so the number of diodes is kept low. Here is the census of the usage of SPDT switches per opcode (so far) :

AND: 0
OR: 1
XOR: 1
: 2
SUB: 2
ADD: 3
SET: 1
SH: 1
SA: 2
RO: 2
RC: 3
IN: 2
OUT: 2
INV: 3

OTOH the XT signal must be split into 5 diode-protected paths.

IN and OUT each use only 2 SPDT switches and 2 are left so each can feed (through a diode) the rotary encoders for IMM8.

XT also can enable the IMM8 encoders through the I8/R switch and 2 diodes. There are 2 encoders :

IMM8_hi = IN | OUT | ( XT & I8/R )

IMM8_lo = IN | OUT | ( XT & I8/R ) | I4/R

Due to the nature of the Hex switches, each output bit must be guarded by a diode as well...

Another interlocked switch selects the format : I4/R, I8/R and REG (they are mutually exclusive).

I8/R is only available in XT mode and otherwise defaults to I4/R mode.

The SRI field (as well as b6) is enabled with

SRI_enREG  & /INV & /IN & /OUT

This uses inverted logic and probably one of the spare/free SPDT of INV/IN/OUT.

Note : I have ordered 8PDT switches for the format selector because I thought it would be useful for actually MUXing the signals but it is not interlocked with XT and others and IN/OUT/INV would not reset the format to IMM8... Diodes seem to be necessary to enable the REG, COND and IMM fields.

The SND field is decoded with the following circuit :

The SRI and CND fieds are identical with some differences :