Regarding RS-232 for controlling a Yamaha receiver:
Per the Yamaha 232 specs, you will need a null modem.
A few examples of command strings (enter the string EXCLUDING the quotations):

serialstring "Master Volume +" "\x0207A1A\x03"
serialstring "Master Volume -" "\x0207A1B\x03"
serialstring "Mute ON" "\x0207EA2\x03"
serialstring "Mute OFF" "\x0207EA3\x03"
serialstring "Input: Phono" "\x0207A14\x03"
serialstring "Input: CD" "\x0207A15\x03"
serialstring "Input: Tuner" "\x0207A16\x03"
serialstring "Input: CD-R" "\x0207A19\x03"
serialstring "Input: MD/TAPE" "\x0207AC9\x03"
serialstring "Input: DVD" "\x0207AC1\x03"
serialstring "Input: D-TV/LD" "\x0207A54\x03"
serialstring "Input: CBL/SAT" "\x0207AC0\x03"
serialstring "Input: VCR1" "\x0207A0F\x03"
serialstring "Input: VCR2/DVR" "\x0207A13\x03"
serialstring "Input: V-Aux" "\x0207A55\x03"
serialstring "Input: 6 Chan ON" "\x0207EA4\x03"
serialstring "Input: 6 Chan OFF" "\x0207EA5\x03"
serialstring "Input Mode: Auto" "\x0207EA6\x03"
serialstring "Input Mode: DD" "\x0207EA7\x03"
serialstring "Input Mode: DTS" "\x0207EA8\x03"
serialstring "Input Mode: Digital" "\x0207EA9\x03"
serialstring "Input Mode: Analog" "\x0207EAA\x03"
serialstring "Input Mode: AAC" "\x0207E3B\x03"
serialstring "Z2 Volume +" "\x0207ADA\x03"
serialstring "Z2 Volume -" "\x0207ADB\x03"
serialstring "Z2 Mute ON" "\x0207EA0\x03"
serialstring "Z2 Mute OFF" "\x0207EA1\x03"
serialstring "Z2 Input: Phono" "\x0207AD0\x03"
serialstring "Z2 Input: CD" "\x0207AD1\x03"
serialstring "Z2 Input: Tuner" "\x0207AD2\x03"
serialstring "Z2 Input: CD-R" "\x0207AD3\x03"
serialstring "Z2 Input: MD/TAPE" "\x0207ACF\x03"
serialstring "Z2 Input: DVD" "\x0207ACD\x03"
serialstring "Z2 Input: D-TV/LD" "\x0207AD9\x03"
serialstring "Z2 Input: CBL/SAT" "\x0207ACC\x03"
serialstring "Z2 Input: VCR1" "\x0207AD6\x03"
serialstring "Z2 Input: VCR2/DVR" "\x0207AD7\x03"
serialstring "Z2 Input: V-Aux" "\x0207AD8\x03"
serialstring "Power ON" "\x0207A1D\x03"
serialstring "Power OFF" "\x0207A1E\x03"
serialstring "Main Power ON" "\x0207E7E\x03"
serialstring "Main Power OFF" "\x0207E7F\x03"
serialstring "Z2 Power ON" "\x0207EBA\x03"
serialstring "Z2 Power OFF" "\x0207EBB\x03"
serialstring "Z3 Power ON" "\x0207AED\x03"
serialstring "Z3 Power OFF" "\x0207AEE\x03"
serialstring "Z3 Mute ON" "\x0207E26\x03"
serialstring "Z3 Mute OFF" "\x0207E66\x03"
serialstring "Z3 Volume +" "\x0207AFD\x03"
serialstring "Z3 Volume -" "\x0207AFE\x03"
serialstring "Z3 Input: Phono" "\x0207AF1\x03"
serialstring "Z3 Input: CD" "\x0207AF2\x03"
serialstring "Z3 Input: Tuner" "\x0207AF3\x03"
serialstring "Z3 Input: CD-R" "\x0207AF5\x03"
serialstring "Z3 Input: MD/TAPE" "\x0207AF4\x03"
Serialstring "Z3 Input: DVD" "\x0207AFC\x03"
serialstring "Z3 Input: D-TV/LD" "\x0207AF6\x03"
serialstring "Z3 Input: CBL/SAT" "\x0207AF7\x03"
serialstring "Z3 Input: VCR1" "\x0207AF9\x03"
serialstring "Z3 Input: VCR2/DVR" "\x0207AFA\x03"
Serialstring "Z3 Input: V-Aux" "\x0207AF0\x03"
serialstring "OSD: Off" "\x0207EB0\x03"
serialstring "OSD: Short" "\x0207EB1\x03"
serialstring "OSD: Full" "\x0207EB2\x03"
serialstring "Sleep: OFF" "\x0207EB3\x03"
serialstring "Sleep: 120 min" "\x0207EB4\x03"
serialstring "Sleep: 90 min" "\x0207EB5\x03"
seialstring "Sleep: 60 min" "\x0207EB6\x03"
serialstring "Sleep: 30 min" "\x0207EB7\x03"
serialstring "EX/ES: On (Matrix)" "\x0207EB8\x03"
serialstring "EX/ES: Off" "\x0207EB9\x03"
serialstring "EX/ES: Auto" "\x0207E7C\x03"
serialstring "EX/ES: Discrete" "\x0207E7D\x03"
serialstring "Night ON" "\x0207E9B\x03"
serialstring "Night OFF" "\x0207E9C\x03"
serialstring "Effect ON" "\x0207E27\x03"
serialstring "Stereo" "\x0207EE0\x03"
serialstring "DSP: Hall A" "\x0207EE1\x03"
serialstring "DSP: Hall B" "\x0207EE2\x03"
serialstring "DSP: Hall C" "\x0207EE3\x03"
serialstring "DSP: Live Concert" "\x0207EE6\x03"
serialstring "DSP: Freiburg" "\x0207EE8\x03"
serialstring "DSP: Royaumont" "\x0207EE9\x03"
serialstring "DSP: Village Gate" "\x0207EEA\x03"
serialstring "DSP: Bottom Line" "\x0207EEC\x03"
serialstring "DSP: Roxy Theater" "\x0207EED\x03"
serialstring "DSP: Arena" "\x0207EEF\x03"
serialstring "DSP: Disco" "\x0207EF0\x03"
serialstring "DSP: Game" "\x0207EF2\x03"
serialstring "DSP: 7ch Stereo" "\x0207EFF\x03"
serialstring "DSP: Pop/Rock" "\x0207EF3\x03"
serialstring "DSP: Classical" "\x0207EF5\x03"
serialstring "DSP: Mono Movie" "\x0207EF7\x03"
serialstring "DSP: Var Sports" "\x0207EF8\x03"
serialstring "DSP: Spectacle" "\x0207EF9\x03"
serialstring "DSP: Sci-Fi" "\x0207EFA\x03"
serialstring "DSP: Adventure" "\x0207EFB\x03"
serialstring "DSP: General" "\x0207EFC\x03"
serialstring "DSP: Normal" "\x0207EFD\x03"
serialstring "DSP: Enhanced" "\x0207EFE\x03"
serialstring "DSP: PLII Movie" "\x0207E67\x03"
serialstring "DSP: PLII Music" "\x0207E68\x03"
serialstring "DSP: NEO:6 Cinema" "\x0207E69\x03"
serialstring "DSP: NEO:6 Music" "\x0207E6A\x03"
serialstring "DSP: 2ch Dir Stereo" "\x0207EC1\x03"
serialstring "DSP: 2ch Stereo" "\x0207EC0\x03"
serialstring "DSP: THX Music" "\x0207EC3\x03"
serialstring "Tuner Preset Pg A" "\x0207AE0\x03"
serialstring "Tuner Preset Pg B" "\x0207AE1\x03"
serialstring "Tuner Preset Pg C" "\x0207AE2\x03"
serialstring "Tuner Preset Pg D" "\x0207AE3\x03"
serialstring "Tuner Preset Pg E" "\x0207AE4\x03"
serialstring "Tuner Preset 1" "\x0207AE5\x03"
serialstring "Tuner Preset 2" "\x0207AE6\x03"
serialstring "Tuner Preset 3" "\x0207AE7\x03"
serialstring "Tuner Preset 4" "\x0207AE8\x03"
serialstring "Tuner Preset 5" "\x0207AE9\x03"
serialstring "Tuner Preset 6" "\x0207AEA\x03"
serialstring "Tuner Preset 7" "\x0207AEB\x03"
serialstring "Tuner Preset 8" "\x0207AEC\x03"
serialstring "Tuner Band FM" "\x0207EBC\x03"
serialstring "Tuner Band AM" "\x0207EBD\x03"
serialstring "Auto Tuning Up" "\x0207EBE\x03"
serialstring "Auto Tuning Down" "\x0207EBF\x03"
serialstring "Speaker Relay A ON" "\x0207EAB\x03"
serialstring "Speaker Relay A OFF" "\x0207EAC\x03"
serialstring "Speaker Relay B ON" "\x0207EAD\x03"
serialstring "Speaker Relay B OFF" "\x0207EAE\x03"
serialstring "Home Preset Mem A" "\x0207E2B\x03"
serialstring "Home Preset Mem B" "\x0207E2C\x03"
serialstring "Home Preset Mem C" "\x0207E2D\x03"
serialstring "Home Preset Mem D" "\x0207E2E\x03"
serialstring "Home Preset Mem E" "\x0207E2F\x03"
serialstring "Home Preset Mem F" "\x0207E20\x03"
serialstring "Home Preset Rcl A" "\x0207E35\x03"
serialstring "Home Preset Rcl B" "\x0207E36\x03"
serialstring "Home Preset Rcl C" "\x0207E37\x03"
serialstring "Home Preset Rcl D" "\x0207E38\x03"
Serialstring "Home Preset Rcl E" "\x0207E39\x03"
serialstring "Home Preset Rcl F" "\x0207E3A\x03"
serialstring "Vol Preset Mem A" "\x0207E6B\x03"
serialstring "Vol Preset Mem B" "\x0207E6C\x03"
serialstring "Vol Preset Mem C" "\x0207E6D\x03"
serialstring "Vol Preset Mem D" "\x0207E6E\x03"
serialstring "Vol Preset Mem E" "\x0207E6F\x03"
serialstring "Vol Preset Mem F" "\x0207E60\x03"
serialstring "Vol Preset Rcl A" "\x0207E75\x03"
serialstring "Vol Preset Rcl B" "\x0207E76\x03"
serialstring "Vol Preset Rcl C" "\x0207E77\x03"
serialstring "Vol Preset Rcl D" "\x0207E78\x03"
serialstring "Vol Preset Rcl E" "\x0207E79\x03"
serialstring "Vol Preset Rcl F" "\x0207E7A\x03"
serialstring "Z2 Vol Memory A" "\x0207E87\x03"
serialstring "Z2 Vol Memory B" "\x0207E88\x03"
serialstring "Z2 Vol Memory C" "\x0207E89\x03"
serialstring "Z2 Vol Memory D" "\x0207E8A\x03"
serialstring "Z2 Vol Memory E" "\x0207E8B\x03"
serialstring "Z2 Vol Memory F" "\x0207E8C\x03"
serialstring "Z2 Vol Recall A" "\x0207E8D\x03"
serialstring "Z2 Vol Recall B" "\x0207E8E\x03"
serialstring "Z2 Vol Recall C" "\x0207E8F\x03"
serialstring "Z2 Vol Recall D" "\x0207E90\x03"
serialstring "Z2 Vol Recall E" "\x0207E91\x03"
serialstring "Z2 Vol Recall F" "\x0207E92\x03"
serialstring "Z3 Vol Memory A" "\x0207E20\x03"
serialstring "Z3 Vol Memory B" "\x0207E21\x03"
serialstring "Z3 Vol Memory C" "\x0207E22\x03"
serialstring "Z3 Vol Memory D" "\x0207E23\x03"
serialstring "Z3 Vol Memory E" "\x0207E24\x03"
serialstring "Z3 Vol Memory F" "\x0207E25\x03"
serialstring "Z3 Vol Recall A" "\x0207E60\x03"
serialstring "Z3 Vol Recall B" "\x0207E61\x03"
serialstring "Z3 Vol Recall C" "\x0207E62\x03"
serialstring "Z3 Vol Recall D" "\x0207E63\x03"
serialstring "Z3 Vol Recall E" "\x0207E64\x03"
serialstring "Z3 Vol Recall F" "\x0207E65\x03"
serialstring "DC1 Trig Ctrl: Z1" "\x0207E32\x03"
serialstring "DC1 Trig Ctrl: Z2" "\x0207E33\x03"
Serialstring "DC1 Trig Ctrl: Z3" "\x0207E31\x03"
serialstring "Zone 1 DC1 Trig ON" "\x0207E71\x03"
serialstring "Zone 1 DC1 Trig OFF" "\x0207E72\x03"
serialstring "Zone 2 DC1 Trig ON" "\x0207E73\x03"
serialstring "Zone 2 DC1 Trig OFF" "\x0207E74\x03"
serialstring "Zone 3 DC1 Trig ON" "\x0207E83\x03"
serialstring "Zone 3 DC1 Trig OFF" "\x0207E84\x03"
serialstring "Dual Mono: Main" "\x0207E93\x03"
serialstring "Dual Mono: Sub" "\x0207E94\x03"
serialstring "Dual Mono: All" "\x0207E95\x03"
serialstring "DC2 Trig Ctrl: Z1" "\x0207E96\x03"
serialstring "DC2 Trig Ctrl: Z2" "\x0207E97\x03"
serialstring "DC2 Trig Ctrl: Z3" "\x0207E9F\x03"
serialstring "DC2 Trig Ctrl: Z OR" "\x0207E98\x03"
serialstring "Zone 1 DC2 Trig ON" "\x0207E3C\x03"
serialstring "Zone 1 DC2 Trig OFF" "\x0207E3D\x03"
serialstring "Zone 2 DC2 Trig ON" "\x0207E3E\x03"
serialstring "Zone 2 DC2 Trig OFF" "\x0207E3F\x03"
serialstring "Zone 3 DC2 Trig ON" "\x0207E85\x03"
serialstring "Zone 3 DC2 Trig OFF" "\x0207E86\x03"
serialstring "Speaker B: Zone 1" "\x0207E28\x03"
serialstring "Speaker B: Zone 2" "\x0207E29\x03"
serialstring "Z2 Speaker Out ON" "\x0207E99\x03"
serialstring "Z2 Speaker Out OFF" "\x0207E9A\x03"