Model T Serial Cable

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Summary

The ideal cable to go from a PC to a Model 100, 102, 200, or 600, would be described as "9F/25M serial null-modem full-handshake".

This combination of connectors and wiring is sometimes called a "Serial Printer" cable, as opposed to a modem or null-modem cable.

That is an uncommon configuration to find in a single cable without needing adapters or needing to build it yourself custom.

The serial ports on Models 100, 102, 200, 600 (and NEC PC-8201 & PC-8300, Olivetti M10, and Kyotronic KC-85) have a DTE pinout, the same as a com port on a PC, but with a female connector, unlike a pc.

This is a backwards configuration from everything else today. This may have been the original standard, and IBM came along later and changed it by using male connectors for the COM ports on their PCs (probably to distinguish them from the printer port which used a female DB25), but the universal standard today is that DTE ports are male, and a DB25F (25-pin female) is either a parallel printer port or a DCE port on a peripheral (like a modem), never a DTE port on a host (a computer).

That means that usually you always need some kind of adapter between a PC and a M100/102/200/600, because none of the cables you are likely to have will have the right combination of wiring and connectors. If you have a null-modem cable, it will probably have the wrong connectors, in which case you will need either a gender-changer or a 9-to-25 adapter or both. If you have a cable with the right connectors, it is probably a straight-though, aka "modem" cable, in which case you will need at minimum a male-to-female null-modem adapter.

So a single cable with both the right wiring and the right connectors without needing a stack of adapters is a little bit special. They do make them, but you just have to search for them specially. Several ideal cables are linked below.

The ideal cable is this:

DE9F-DB25M Serial/RS-232 Null-Modem cable
looking at cable end RS-232-Null-Modem-Cable-DE9F-to-DB25M.jpg
female
Conn dsub9f.gif
male
Conn dsub25m.gif
DTE DTE
Signal DE9F DB25M Signal
RX 2 2 TX
TX 3 3 RX
DTR 4 6
8
DSR
DCD
SG 5 7 SG
DSR
DCD
6
1
20 DTR
RTS 7 5 CTS
CTS 8 4 RTS


Ideal Cables

These cables are wired null-modem, have all the connections for hardware flow-control, and the right physical connectors on both ends, all in one factory-molded piece. You don't need any null-modem adapters or gender-changer adapters.

DCD+DSR<-->DTR
No RI
DCD+DSR<-->DTR
S+PG
No RI
DCD+DSR<-->DTR
S+PG
No RI
Includes nuts (removable) on the 9-pin plug screws, so you can screw a usb-serial adapter together to the cable, no matter whether the usb-serial adapter has screws or nuts.
No DCD
No RI

Software Flow Control Only Cables

These cables actually work fine for everything(*). They are "less-ideal" only because the RTS/CTS hardware flow control lines are self-satisfied (aka shorted, faked, looped-back), or missing, or wired in some non-standard way like connecting DSR/DTR to CTS. But they have the correct physical plugs, and correct TX/RX wiring, and so they work fine for software flow-control.

And software flow-control is the only thing any of the old software ever used. The system rom doesn't even contain anything that accesses the RTS/CTS lines, not in BASIC nor TELCOM.

(*) The exception to "everything": The hardware in Models 100-600 can do RTS/CTS. The pins on the DB25 connector are wired up to the UART. But to use those signals, you have to manipulate the UART registers yourself from a machine language program. The only software I know of that does this to date is HTERM. So these cables are NOT suitable for HTERM. They ARE suitable for TELCOM and TPDD_Emulators.

DCD<-->RTS+CTS
RI<-->RI
DCD<-->RTS+CTS
RI<-->RI
DCD<-->RTS+CTS
No RI
DCD<-->RTS+CTS
No RI
DTR<-->DCD+RTS+CTS
No RI
DTR<-->DCD+RTS+CTS
No RI
DCD<-->RTS
DSR+CTS<-->DTR
No RI
This is a very unusual type of cable. If you don't have an HP or IBM plotter, I would not recommend getting this. But, strictly for the TX/RX and the physical connectors, as long as the software is ignoring RTS/CTS and DSR/DTR, it does work, so if you do have an HP or IBM plotter, then you could use this for both your plotter and your M100.

USB-Serial Adapters

Any one will work well enough. But some considerations are:

Models based on FTDI chips are generally better than the ones based on Prolific chips.

The serial cables all have screws in the plugs, so if you get a usb-serial adapter that has nuts (instead of screws) then the serial cable can be locked onto the usb-serial adapter with the screws.

Legend

For all the cable wiring notes above, the notes indicate how the cable differs from a canonical RS-232 DTE-to-DTE null-modem reference.
Any signal that is not mentioned, is wired according to the reference. IE, if TX or RX is not mentioned, then TX on one end of the cable is connected to RX on the other end of the cable.
Unless otherwise noted, the indicated wiring is symmetrical, the indicated connections are the same on both ends of the cable.

S+PG

The cable Shield (& connector shell) is connected to Protective Ground or "frame ground" (pin 1 on the DB25 connector, no pin on the DB9 just the connector shell)

RI<-->RI

Ring Iindicator is connected to Ring Indicator
DB25 pin 22 (RI) is connected to DE9 pin 9 (RI)

DCD<-->RTS+CTS

Request To Ssend is connected to Clear T Send on one end, and both are connected to Data Carrier Detect on the other end.
DB25 pins 4 (RTS) & 5 (CTS) are connected to each other, and to DE9 pin 1 (DCD).
DE9 pins 7 (RTS) & 8 (CTS) are connected to each other, and to DB25 pin 8 (DCD).

DCD+DSR<-->DTR

Data Carrier Detect is connected to Data Set Ready on one end, and both are connected to Data Terminal Rready on the other end.
DB25 pins 8 (DCD) & 6 (DSR) are connected to each other, and to DE9 pin 4 (DTR).
DE9 pins 1 (DCD) & 6 (DSR) are connected to each other, and to DB25 pin 20 (DTR).