Measurement Computing Serial488/4 - Manual (Page 30)

Section 1 Introduction
2.18
4096 memory locations are available in its internal buffers. If
available memory drops below 4096 bytes, the Serial488/4 un-
asserts (low) the RTS+ signal. With RTS/CTS handshaking
selected, the RTS+ signal may also asserted and un-asserted under
program control by using the ( N2) and ( N1) handshake control
commands respectively.
If XON/XOFF handshaking ( G1) is selected, the RTS+ line
may either be programmed as either permanently asserted (high)
or to output sixteen times the baud rate clock. These options are
selected using the handshake control command (Nn).
If No handshaking (G2) is selected, the RTS+ line may either be
programmed as either permanently asserted (high), permanently
un-asserted (low), or to output a signal sixteen times the baud rate
clock. These options are selected using the handshake control
command (Nn).
RTS Request To Send (output) / 16 x Baud Clock (output)
The RTS- output is used as a hardw are handshake line w ith RS-
422 devices The pin functions identically to RTS+ except that its
polarity is inverted. This signal is low true.
GND Ground
This signal sets the ground reference point for the other RS-232
input and output signals.