https://en.wikipedia.org/wiki/RS-232#Timing_signals

Timing signals

Some synchronous devices provide a clock signal to synchronize data
transmission, especially at higher data rates. Two timing signals are
provided by the DCE on pins 15 and 17.

Except the Model 90 has NO pins from 14 - 19. Not "N/C", the pins are
NOT populated...

Alternatively, the DTE can provide a clock signal, called transmitter
timing (TT), on pin 24 for transmitted data. Data is changed when the
clock transitions from OFF to ON, and read during the ON to OFF
transition. TT can be used to overcome the issue where ST must traverse
a cable of unknown length and delay, clock a bit out of the DTE after
another unknown delay, and return it to the DCE over the same unknown
cable delay. Since the relation between the transmitted bit and TT can
be fixed in the DTE design, and since both signals traverse the same
cable length, using TT eliminates the issue. TT may be generated by
looping ST back with an appropriate phase change to align it with the
transmitted data. ST loop back to TT lets the DTE use the DCE as the
frequency reference, and correct the clock to data timing.

Synchronous clocking is required for such protocols as SDLC, HDLC, and
X.25.

https://en.wikipedia.org/wiki/RS-232#Secondary_channel

A secondary data channel does not seem to have enough pins...

On Friday, 2 July 2021 at 17:57:07 UTC+1, Louis Ohland wrote:
> https://en.wikipedia.org/wiki/RS-232#Timing_signals
>
> Timing signals
>
> Some synchronous devices provide a clock signal to synchronize data
> transmission, especially at higher data rates. Two timing signals are
> provided by the DCE on pins 15 and 17.
>
> Except the Model 90 has NO pins from 14 - 19. Not "N/C", the pins are
> NOT populated...
>
> Alternatively, the DTE can provide a clock signal, called transmitter
> timing (TT), on pin 24 for transmitted data. Data is changed when the
> clock transitions from OFF to ON, and read during the ON to OFF
> transition. TT can be used to overcome the issue where ST must traverse
> a cable of unknown length and delay, clock a bit out of the DTE after
> another unknown delay, and return it to the DCE over the same unknown
> cable delay. Since the relation between the transmitted bit and TT can
> be fixed in the DTE design, and since both signals traverse the same
> cable length, using TT eliminates the issue. TT may be generated by
> looping ST back with an appropriate phase change to align it with the
> transmitted data. ST loop back to TT lets the DTE use the DCE as the
> frequency reference, and correct the clock to data timing.
>
> Synchronous clocking is required for such protocols as SDLC, HDLC, and
> X.25.
>
> https://en.wikipedia.org/wiki/RS-232#Secondary_channel
>
> A secondary data channel does not seem to have enough pins...

The "secondary channel" is most likely for a "control channel" using Async protocol.

https://avitech.com.au/?page_id=766

IBM PC DB25 Pinout (including current loop)

This section is largely adapted from a posting on Vintage Computer
Forums by “vwestlife”.

The original IBM 8-bit ISA serial port card (and exact clones) supported
current loop, using some of the pins not normally used by RS-232C. As
Wikipedia says, “The original IBM PC serial port card had provisions for
a 20 mA current loop”, and “on the original IBM PC, a male D-sub was an
RS-232-C DTE port (with a non-standard current loop interface on
reserved pins)”.

18 Receive + (Current Loop) (RD+)
20 Data Terminal Ready (DTR)
25 Receive – (Current Loop) (RD-)

The Model 90 DB25M lacks Pin 18 and 25. So, I'd agree with the 8590
SSI's statement that the PS/2 does NOT support current loop.

>> https://en.wikipedia.org/wiki/RS-232#Secondary_channel
>> A secondary data channel does not seem to have enough pins...
> The "secondary channel" is most likely for a "control channel" using
Async protocol.

Secondary Request To Send (SRTS) 19
Secondary Clear To Send (SCTS) 13
Secondary Carrier Detect (SDCD) 12

Tim, the DB25M lacks Pin 19.

Searching for 15 pin configurations, nothing yet [or at all?]

On 7/2/2021 12:20, WBSTClarke wrote:
> On Friday, 2 July 2021 at 17:57:07 UTC+1, Louis Ohland wrote:
>> https://en.wikipedia.org/wiki/RS-232#Timing_signals
>>
>> Timing signals
>>
>> Some synchronous devices provide a clock signal to synchronize data
>> transmission, especially at higher data rates. Two timing signals are
>> provided by the DCE on pins 15 and 17.
>>
>> Except the Model 90 has NO pins from 14 - 19. Not "N/C", the pins are
>> NOT populated...
>>
>> Alternatively, the DTE can provide a clock signal, called transmitter
>> timing (TT), on pin 24 for transmitted data. Data is changed when the
>> clock transitions from OFF to ON, and read during the ON to OFF
>> transition. TT can be used to overcome the issue where ST must traverse
>> a cable of unknown length and delay, clock a bit out of the DTE after
>> another unknown delay, and return it to the DCE over the same unknown
>> cable delay. Since the relation between the transmitted bit and TT can
>> be fixed in the DTE design, and since both signals traverse the same
>> cable length, using TT eliminates the issue. TT may be generated by
>> looping ST back with an appropriate phase change to align it with the
>> transmitted data. ST loop back to TT lets the DTE use the DCE as the
>> frequency reference, and correct the clock to data timing.
>>
>> Synchronous clocking is required for such protocols as SDLC, HDLC, and
>> X.25.
>>
>> https://en.wikipedia.org/wiki/RS-232#Secondary_channel
>>
>> A secondary data channel does not seem to have enough pins...
>
> The "secondary channel" is most likely for a "control channel" using Async protocol.
>

On Friday, 2 July 2021 at 18:42:27 UTC+1, Louis Ohland wrote:
> >> https://en.wikipedia.org/wiki/RS-232#Secondary_channel
> >> A secondary data channel does not seem to have enough pins...
> > The "secondary channel" is most likely for a "control channel" using
> Async protocol.
> Secondary Request To Send (SRTS) 19
> Secondary Clear To Send (SCTS) 13
> Secondary Carrier Detect (SDCD) 12
>
> Tim, the DB25M lacks Pin 19.
>
> Searching for 15 pin configurations, nothing yet [or at all?]
> On 7/2/2021 12:20, WBSTClarke wrote:
> > On Friday, 2 July 2021 at 17:57:07 UTC+1, Louis Ohland wrote:
> >> https://en.wikipedia.org/wiki/RS-232#Timing_signals
> >>
> >> Timing signals
> >>
> >> Some synchronous devices provide a clock signal to synchronize data
> >> transmission, especially at higher data rates. Two timing signals are
> >> provided by the DCE on pins 15 and 17.
> >>
> >> Except the Model 90 has NO pins from 14 - 19. Not "N/C", the pins are
> >> NOT populated...
> >>
> >> Alternatively, the DTE can provide a clock signal, called transmitter
> >> timing (TT), on pin 24 for transmitted data. Data is changed when the
> >> clock transitions from OFF to ON, and read during the ON to OFF
> >> transition. TT can be used to overcome the issue where ST must traverse
> >> a cable of unknown length and delay, clock a bit out of the DTE after
> >> another unknown delay, and return it to the DCE over the same unknown
> >> cable delay. Since the relation between the transmitted bit and TT can
> >> be fixed in the DTE design, and since both signals traverse the same
> >> cable length, using TT eliminates the issue. TT may be generated by
> >> looping ST back with an appropriate phase change to align it with the
> >> transmitted data. ST loop back to TT lets the DTE use the DCE as the
> >> frequency reference, and correct the clock to data timing.
> >>
> >> Synchronous clocking is required for such protocols as SDLC, HDLC, and
> >> X.25.
> >>
> >> https://en.wikipedia.org/wiki/RS-232#Secondary_channel
> >>
> >> A secondary data channel does not seem to have enough pins...
> >
> > The "secondary channel" is most likely for a "control channel" using Async protocol.
> >

The DB25M supports Primary Channel Async/Sync and Handshake signals.