IMPEDANCE FOR SIGNALS IS COMPLICATED
IBM engineers the 1130 for 92 ohm impedance on cables, although in practice there are ribbon cables, traces on printed circuits, wire wrap on the backplane, and twisted pair wiring involved in the path between the 1130 and its internal disk drive. However, since the engineering goal was matching 92 ohm impedance, I will use that as a working start.
The 2310 Interface Board is where the twisted wire cable from the 1130 and a new twisted wire cable to the disk drive are connected, allowing the Virtual 2315 Cartridge Facility (V2315CF) to sit in between the signals of the 1130 disk controller and the internal disk drive (called 13SD or 2310). It has two IDC 40 pin ribbon cables that run from the 2310 Interface Board to the V2315CF main unit. That unit is a significantly modified version of the RK-05 Emulator from George Wiley, since the RK-05 is a derivative cousin of the 2310.
CURRENT TERMINATOR VALUES ARE SUBOPTIMUM
The RK-05 Emulator has a terminator board for the signals arriving there over the ribbon cables which uses 178 and 383 ohm resistors pulling up to 5V and down to ground respectively. That provides an impedance of about 122 ohms, which is a mismatch to both the IDC ribbon cable and to the 92 ohm IBM cabling on the 2310 Interface Board. We will use an impedance of 102 ohms for the IDC cable.
On the 2310 Interface Board, the mismatch between 92 ohms and 102 ohms is not too bad, particularly with the relatively slow rising edges from the 1130 and 2310 electronics over the size of the interface board. However, the mismatch between the 102 ohm ribbon cable and the 122 ohm terminator board is going to lead to some ringing, reflections and power loss.
CHANGING THE TERMINATOR BOARD TO IMPROVE SIGNAL INTEGRITY
If I switch the terminator resistors to 169 ohms and 249 ohms, still pulled up to 5 and down to ground, it will give me a 100 ohm impedance. I ordered the resistors and will remake the terminator board when the parts arrive.
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