On Fri, Jan 31, 2025 at 9:36 PM Fred Cisin via cctalk <cctalk@classiccmp.org> wrote: > > On Fri, 31 Jan 2025, Chuck Guzis via cctalk wrote: > > As far as RS232 and history, I submit that current-loop has more of a > > place in telegraphy, as it's far more suited to long distances and is > > less susceptible to signal noise. Many early 1970s serial interfaces > > offered the choice between 20ma or 60ma current loop or RS232C. > > up to the 5150 "Asynchronous Communication Adapter" > > The "free" serial adapter used by IBM to discourage use of the eighth slot > was probably the same one.
It was. The techref shows the jumper to fit so it can assert the data buffer enable signal if it's placed in slot 8 of a 5160. Annoyingly the 5155 cabinet is about 1/4" too small, the async adapter won't fit in slot 8, it bangs into the disk drives. To select between RS232 and current loop you have to remove the card from the machine and turn a jumper block round. A couple of months ago I worked out how to modify the card so that you didn't have to do this Specifically : You don't have to add any components or cut any traces, just add (quite a few) jumper wires. There are no changes to the wiring to the DB25 You can then fit the original IBM jumper block and select RS232 or current loop, the board behaves just like an unmodifed one But you can also fit a specially-wired jumper block, whereupon The board will work in RS232 mode with standard RS232 cables. Or with a specially wired cable (and RTS asserted from the board) it works in current loop mode. I've modified both the boards in my 5155. -tony
