Starting point for my substitute keyboard for the IBM 3179 terminal

KEYBOARDS USED WITH THE 3179 TERMINAL

There were three IBM part numbers for type M keyboards for use with this terminal - 1385151, 1389160, 1693640 - with the latter having a very similar Data Entry Keypunch key arrangement to my 3178 keyboards while the others are typewriter style arrangements.  

These use a 5 pin DIN connector but not the same as used on PC/AT keyboards, having a bidirectional serial protocol using a data and a clock line. The protocol is known to be the same as used with newer terminals such as PS/2. That also means the protocol is well described.

Serial protocol with the 3179 keyboard and terminal

Presumed commands from terminal to keyboard

A GOOD STARTING POINT FOR MY SUBSTITUTE

Whether I will attach one of my 3178 keyboards through a converter or just build a converter that talks to more modern systems, I can start with the open source code and documentation of the TMK keyboard project. It includes kicad PCB designs for converter boards as well as the firmware. 

This will not be a direct use since the purpose of the TMK code is to make use of physical keyboards such as what I am seeking, using them with modern systems via PS/2 or USB connections. In a sense, the reverse of what I want to do, which is use something modern to pretend to be the physical keyboard.

STILL SOME UNKNOWNS

There are still some unknowns even with everything I will know after reading over the TMK project information. It would be easiest to simply capture the dialog between a keyboard and a logic element, but I don't have the keyboard so that is moot. 

The Logic Element of my 3179 terminal will either interrogate the keyboard for its keyboard ID or will do a Reset which expects the keyboard ID to be returned. I don't know what to return that will satisfy the Logic Element, but I might be able to find out from others who have converted real keyboards to use with their PCs as they may have captured or be able to capture the returned ID. 

The keyboard is sent a request to adopt a scan code set, basically a mapping of physical keys to the returned code. Many modern type M keyboards will only speak one set, the modern one, but my keyboard may have worked with an earlier set. I don't know what it was or how to respond to the scan code set request message.

The 3179 Guide document mentions the need to set eight dip switches on the bottom of the keyboard to a particular configuration that matches what the Logic Element expects, or they will suffer a mis-configuration error. Perhaps this eight bit code is the terminal ID to be returned?

MISSING PROTOCOL SPECS FOR MY TYPE F KEYBOARD (IBM 3178 KEYBOARD)

I have a connector layout for the keyboard but it has much more than just the power, data and clock lines that are analogous to the 3179 keyboard. I see the additional signals

• Reset
• Data Available
• ACK
• CMD0
• CMD1
• PE
• PE

Unverified pin assignment for 3178 keyboard

Since the scan codes are purported to be unidirectional, keyboard to logic element, the additional lines may be to transfer requests to the keyboard. On the other hand, Data Available sure sounds like a signal going to the logic element. 

I may have to wire up a logic analyzer and capture some traffic between my 3178 Keyboard and its Logic Element in order to understand these pins more fully. The keyboard enclosure contains a clicker that can be switched on or off by the host, which may account for some of the commands that could come. Further, I would expect that there is a way to ask for the keyboard ID which is decimal 9 in this case.