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Terminal No. Abbrev. Signal name I/O Terminal No. Abbrev. Signal name I/O A1 SD Send data Output B1 RD Receive data Input A2 RS Request to send Output B2 CS Clear to send Input A3 ER Data terminal ready Input B3 DR Data set ready Output A4 SG Signal ground -- B4 SG Signal ground --- A5 NC Not used. -- B5 NC Not used. --- A6 SHLD Shield -- B6 SHLD Shield --- A7 NC Not used. -- B7 NC Not used. --- A8 FG Frame ground -- B8 FG Frame ground ---