Have a Dell R740 running esxi 6.7U3. 9-pin serial port is enabled.
cable is standard 9-pin from Dell.
trying to putty into Equallogic SAN from a VM on that host.
What is your problem? Have you already added the physical serial port to your VM as device and you cannot connect?
Besides of the obvious, there are several caveats with serial port virtualization. First of all - that's not a virtualization problem - the integrated serial ports on modern motherboards with Intel chipsets are not real serial ports, which you can use in the same way as a serial port of any older PC, they have only a subtype of features, you can use. I don't know, how that is working on an Intel C602 chipset, which the R740 seems to have. On Intel C620 chipsets e.g. it does definitely not work. That problem can be easily solved by some cheap PCI-serial card.
Next you face the problem, that you loose things between the physical port and the virtual port. Serial printing does work virtualized almost everytime, but I found serveral modules/devices with a serial connector, which you can only use, if the port is connected to a physical PC - as soon as you install any hypervisor or e.g. Vmware workstation on the same machine and try to work from a VM with the module/device attached to the same port over a virtual port you get problems. I have not tested, if it would work with PCI-passthhrough, but doubt, that the majority of PCI-serial cards will be able to work in PCI-passthrough mode.