Stanford Research Systems SIM918 Operating And Service Manual page 24

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1.6.2.2 Serial settings
ground carries the return currents of digital control signals, power-
intensive analog amplifiers, and the power supplies, whereas the
output voltage references to a stable signal ground (Section 2.2.1).
When operating in the SIM900, the three grounds are tied together
in the SIM900 Mainframe. Signal Ground and Power Ground are
connected through back-to-back Schottky diodes, so they cannot be
more than 0 35 V apart. The three ground lines should be sepa-
rately wired to a single, low-impedance ground source at the power
supply.
The initial serial port settings at power-on are: baud rate 9600, 8 bits,
no parity, 1 stop bit, and no flow control. The baud rate of the SIM918
cannot be changed. Flow control is not implemented in the SIM918.
The parity may be changed with the PARI command.
SIM918 Precision Current Preamplifier
Getting Started