Timeout due to dependency when starting getty service

When trying to start a getty service for a realport created tty, the system fails to start because of a timeout due to a missing dependency. Using putty, I can open the serial device directly and see the data being sent to the port and being echoed back when using the loopback plug. But I cannot start a getty service for the port so that I can connect some terminals and get the normal login prompt.

The journal entry is:
[root@localhost system]# systemctl start serial-getty@tty_dgrp_a_20.service
A dependency job for serial-getty@tty_dgrp_a_20.service failed. See ‘journalctl -xe’ for details.

The journal entry is:
Jan 25 12:15:20 localhost.localdomain systemd[1]: dev-tty_dgrp_a_20.device: Job dev-tty_dgrp_a_20.device/start timed out.
Jan 25 12:15:20 localhost.localdomain systemd[1]: Timed out waiting for device /dev/tty_dgrp_a_20.
Subject: A start job for unit dev-tty_dgrp_a_20.device has failed
Defined-By: systemd
Support: Red Hat Customer Experience & Engagement - Red Hat Customer Portal

A start job for unit dev-tty_dgrp_a_20.device has finished with a failure.

The job identifier is 698883 and the job result is timeout.

Jan 25 12:15:20 localhost.localdomain systemd[1]: Dependency failed for Serial Getty on tty_dgrp_a_20.
Subject: A start job for unit serial-getty@tty_dgrp_a_20.service has failed
Defined-By: systemd
Support: Red Hat Customer Experience & Engagement - Red Hat Customer Portal

A start job for unit serial-getty@tty_dgrp_a_20.service has finished with a failure.

The job identifier is 698882 and the job result is dependency.

Looking in /var/log/messages doesn’t shed any more light on the problem either.
Jan 25 12:15:20 localhost systemd[1]: dev-tty_dgrp_a_20.device: Job dev-tty_dgrp_a_20.device/start timed out.
Jan 25 12:15:20 localhost systemd[1]: Timed out waiting for device /dev/tty_dgrp_a_20.
Jan 25 12:15:20 localhost systemd[1]: Dependency failed for Serial Getty on tty_dgrp_a_20.
Jan 25 12:15:20 localhost systemd[1]: serial-getty@tty_dgrp_a_20.service: Job serial-getty@tty_dgrp_a_20.service/start failed with result ‘dependency’.
Jan 25 12:15:20 localhost systemd[1]: dev-tty_dgrp_a_20.device: Job dev-tty_dgrp_a_20.device/start failed with result ‘timeout’.

The systemd file is just a copy of the default serial-getty@.service file with the execute line changed to:
ExecStart=-/sbin/agetty -o ‘-p – \u’ --keep-baud 9600 - vt100

If anyone can help me track down what dependency the system is complaining about it would be greatly appreciated.

It seems that including the “Legacy UNIX Compatibility” when installing Red Hat Enterprise Linux causes an unknown dependency. Leaving that out of the OS installation allows things to work normally.