CCS/TMS320F28027: Can XDS110 support three or four device programming?

Uchikoshi,

The XDS110 Debug Probe fully supports the recommended limit of 6 in between the probe and the device.
software-dl.ti.com/.../emu_xds_target_connection_guide.html

Since you are able to connect to a longer JTAG chain using other XDS technology variants, there is a chance the TMDSEMU110-U was designed with less strong buffers to drive the lines, thus giving the impression there is a hard limit. There isn't.

You can check this with an oscilloscope and compare the voltage levels and attenuation between the different environments to inspect possible weakness points. It may even be caused by ground spikes during switching.

In any case, check the reference I sent above and see if there is anything that may help you overcome this. One additional detail: some of the F28x designs used a strong pull down resistor on nTRST line (2.2kΩ IIRC) that used to affect only the XDS110 Debug Probe. This is mentioned in the reference below:
software-dl.ti.com/.../ccs_debugging_jtag_connectivity_issues.html

For some reference waveforms, check the following post:
e2e.ti.com/.../2442314

Hope this helps,
Rafael