If the use of
the links is the obvious one (that is, to select outputs of
the '4040 and feed it into some more circuitry), you could make an
external PCB with an osiccilator and divider chip on it and connect the
output to the appropriate pin on the jumper block.
Yes, that would work. One whole side of the 4x2 header used to jumper
baud rates is directly connected to the UART, so it just needs a CMOS
clock of the right frequency applied there, and the shorting block
removed altogether.
I'm assuming the other row of that 4*2 block goes to outputs on the HC4040.
I have 2 more suggestions, neither of which makes a permanent
modification to the board.
The first is to make a little PCB containing a freqeuncy divider that
will, say, divide down the 38400 baud clock to 110 buud. The input and
output go to a 2 pin socket that plugs onto the 4*2 header in the 38400
baud position, you get power and ground from <wherever>. Either plug that
socket in to get 110 board, or a jumper link to get one of the original 4
baud rates.
The seconmd is to remove that HC440 chip and replace it by a 16 pin
(turned pin) DIL socket. You can then plug an HC44040 in if you want to go
back to the original design. But you could also make a module that plugs
in there (it could easily be made on stripboard with pin headers to fit
into the HC4040 socket, for example) containing an osvillator (if the one
on the original PCB is not a suitable frequncy) and divider chips to give
whatever 4 baud rates you want, and feed them to the appropriate pins on
the HC4040 socket -- the onse that are wired to that 4*2 header. Then use
the jumper link as originally intended.
Tese avoid the use of a microcontroller (I feel it's inelegant to link a
PDP8/e clone to a mechanical TTY using a microcontroller iwth more
computing power than either device!), they also avoid any permanent mods
to the board. And you don't beve to worry about sorting out hardware flow
control.
-tony
