To synchronize playback of your CS6X or CS6XR with an external MIDI device, you can use either the synthesizer’s internal clock (int) or MIDI clock signals from the external device (midi). At the menu “midi (other)” you can define this behaviour :

Select “int” if you are using the synthesizer as the master, or if you [...]

here’s a description how to setup external midiclock syncing for your Yamaha S90XS.
In the Quick Setup menu, you can choose setup 2.

This results in the following settings :
Local control: on
MIDI: auto
Clock Out: off
Arp MIDI Out Sw: all on
Perfect for synchronizing to midiclock .

The SU200 can be synced to midi clock by following the procedure described below :
1. Switch off all pads, and then press the [JOB] button to enter Job mode.
2. Press the [MIDI] button (pad 8 ) to enter MIDI Setup mode.
3. Set the MIDI clock (‘Sync’ on de the display) to ‘Ext’ :

4. Press [...]

The sophisticated sync features of the DX200 let you lock it in step with the rhythm of just about any other MIDI device, such as a computer, rhythm machine or external sequencer. No drift, no glitching, no slipping out of sync — the two machines stay perfectly in the same groove.
This is great for live [...]

On the Motif Rack XS you configure syncing to midi clock in the following menu :

Select midi (nr2).
Then Edit the MIDI Sync Parameter. Possible values are :
internal : Synchronization to internal clock. You can use this setting when this tone generator is to be used alone or as the master clock source for other equipment.
external [...]

The RS7000 Music Production Studio is quite advanced.

It can be synced to midiclock the following way :
In midi setup (page 1/2)

you can set the midi sync setting to “MIDI”
This way your RS7000 can be synced to an external
midi clock signal.

The Boss DR-3 can be synced to midiclock :
In the system menu, set the Sync parameter to auto :

The DR-3 will now sync to midiclock as soon as it has received the start command (which midiclock sends automatically when you press start).

Yamaha’s RY20 is a cool device.

Put the RY20 in MIDI Mode by pressing the [ MIDI ] button.
The “MIDI” indicator is now switched on.
The current mode is flashing below the “SYNC” prompt :
Switch from “INT” (internal clock, the default) to “MIDI” (external MIDI controller) by rotating the Jog Dial or by using the [ +1 [...]

TheRoland VG-99 and VB-99 can easily be synced to midi.
In the menu system/midi you can select the ’sync clock’ options :

The following settings are possible :
INTERNAL : the VG-99/VB-99 ignores any incoming midi clock commands : don’t use this setting if you want to synchronize with midiclock.
AUTO (USB) : The VG-99/VB-99 is automatically synchronized to [...]

Yamaha’s QY70 supports reception of midi clock signal.
You can edit the MIDI SYNC setting in the utility-midi menu :

When the MIDI SYNC parameter is set to “Internal” the QY70 is controlled by its own internal clock.
However, when set to “External” the QY70 is controlled by a MIDI clock signal received from
an external device such as [...]