Two special configuration options are available for serial connections between a virtual machine and the host or between two virtual machines. These options are of interest primarily to developers who are using debugging tools that communicate over a serial connection.

The first option must be set in the virtual machine settings editor (VM > Settings > Serial Port) or the VMware Management Interface (Configure Hardware > Edit next to the serial port). This option is useful when the serial port is being used by the guest operating system in polled mode as opposed to interrupt mode. Polled mode causes the virtual machine to consume a disproportionate share of processor time. This makes the host and other guests run sluggishly.

To restore performance for applications on the host, check the Yield CPU on poll check box. This configuration option forces the affected virtual machine to yield processor time if the only task it is trying to do is poll the virtual serial port.

To use the second option, power off the virtual machine and close the console window, then use a text editor to add the following line to your virtual machine's configuration file (.vmx):

serial<n>.pipe.charTimePercent = <x>

This option is useful if you want to squeeze every possible bit of speed from your serial connection over a pipe to the virtual machine. In principle, there is no limit on the output speed — the speed at which the virtual machine sends data through the virtual serial port. In practice, the output speed depends on how fast the application at the other end of the pipe reads data inbound to it.

<n> is the number of the serial port, starting from 0. So the first serial port is serial0.

<x> is any positive integer. It specifies the time taken to transmit a character, expressed as a percentage of the default speed set for the serial port in the guest operating system. For example, a setting of 200 forces the port to take twice as long per character, or send data at half the default speed. A setting of 50 forces the port to take only half as long per character, or send data at twice the default speed.

You should first use the guest operating system to configure the serial port for the highest setting supported by the application you are running in the virtual machine.

Once the serial port speed is set appropriately in the guest operating system, experiment with this setting. Start with a value of 100 and gradually decrease it until you find the highest speed at which your connection works reliably.