The Speedtronic Mark IV Maintenance Manual has the details of making changes.
You will need a RS-232 serial cable, and a computer with a serial port and basic communication program (MS-Windows HyperTerminal works just fine, or any basic terminal emulator program that can connect to a serial port).
There is a serial port adapter (25-pin to 25-pin) that was provided with the Mark IV Service Kit. There are instructions on how to use the adapter in the manual. (I believe the adapter is a null modem adapter, so if you have a serial cable that is a null modem cable it should work--except that sometimes there is a jumper between a couple of pins on the Mark IV end of the cable that has to be installed. Sorry, it's been a long time and I'm away from home where I better instructions written down somewhere.)
You will need to set the communications parameters per the instructions in the manual. The editor program is completely text-based. There are instructions in the manual for checking rungs and changing rungs and making simple analog rungs (compare; add; subtract; etc.). The thing to know about doing the latter is that the Mark IV does NOT use floating point math; there is some explanation of that in the manual if I recall correctly (again, it's been a long, long time).
I believe this topic has been covered a couple of times on control.com. So, you can use the Search feature of control.com to look for previous threads about editing rungs in Mark IV. It's not difficult; really the hardest part is getting your computer and terminal emulator program to communicate with the Mark IV. Read the manual about the editor program and how to do whatever it is you need to do, and write down the steps you need to make in a procedure for once you get communications established--which I can pretty much guarantee if you've never communicated with a Mark IV before is going to take many hours if you don't have the right cable and the adapter and probable a couple of serial cable adapters (like a gender change, for example. I know LOTS of people (myself included) that bought a 25-pin connector and made a cable that has all the right connections. (I used an ohm meter to "ring out" the pin-pin connections on the provided Mark IV serial port adapter and then wired the connector appropriately. I know people who have cut the appropriate end off a 25-pin serial cable and connected it to the appropriate end of a 9-pin serial cable (to be connected to the computer), baring and twisting wires together and putting tape on them (or using butt-splices or heat-shrink tubing to insulate the connections). Whatever you choose, it's the hardest part--getting your terminal emulator to communicate with the Mark IV using a RS-232 serial cable.
Again, RTFM (Read The Fine Manual). Most everything you need to know is in the Mark IV Maintenance Manual. (No; I don't have an electronic copy of the manual to send anyone.)
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by Jeff Kerns