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packet: add get value semantics
author Brett Gordon
date Tue, 13 Dec 2016 11:26:37 -0500
parents 3a4c972c92ec
children
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1 This document describes an extension to the LWWire protocol which provides
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2 virtual serial ports. The protocol comes in two parts. The first is the low
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3 level signalling protocol to transfer data on the ports and the second is
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4 the higher level "modem" protocol.
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5
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6 A low level port is simply an eight bit clean channel that allows traffic to
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7 pass over it bidirectionally. The low level signalling protocol does not
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8 care what that data is.
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9
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10 A port must be opened before it can be used. Once it is opened, it is
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11 connected with some sort of endpoint. That may be a modem simulation or some
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12 other endpoint specification. When communication is finished, the port is
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13 then closed. A port can be closed by either the client or the server.
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14
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15 LWWire Protocol Extension
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16 =========================
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17
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18 This extension is called VPORT and is assigned extension number 00. It
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19 defines the following operations in its extension space:
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20
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21 00 VPORT_OPEN
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22
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23 This request opens a virtual port. It takes the following parameters:
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24
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25 Octet Meaning
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26 ----- -------
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27 0-2 F3 00 00
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28 3 endpoint type
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29
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30 The endpoint type is 0 for a basic virtual modem as defined by this
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31 extension. Any other value is an extension number which may accept VPORT
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32 connections. In particular, the CCHAN extension is of interest here.
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33
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34 Octet Meaning
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35 ----- -------
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36 0 status
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37 1 port number assigned
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38 F0-FF reserved for private endpoint types
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39
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40 If the status is 0, it means the open was successful and a port was
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41 successfully allocated. Communication over the port can now begin.
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42
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43 Any other status is a failure and the port number value must be regarded as
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44 invalid. The server MUST send 0 for this port number in this instance. The
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45 status will be usually be one 1 if no ports are available to assign or 2 if
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46 ports are available but the requested endpoint type is not available.
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47
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48
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49 01 VPORT_CLOSE
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50
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51 This call informs the server that the client is finished with the specified
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52 port. The call looks as follows:
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53
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54 Octet Meaning
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55 ----- -------
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56 0-2 F3 00 01
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57 3 port number
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58
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59 The return from this call is exactly one octet, a status. If the status is
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60 zero, the port has been closed successfully. If it is 1, it means the port
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61 was not open in the first place.
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62
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63 When a port is closed, all data currently buffered on the server is cleared.
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64 It may or may not be delivered to its intended destination.
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65
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66
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67 02 VPORT_POLL
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68
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69 This call requests the server to provide information on the specified port.
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70 The call looks as follows:
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71
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72 Octet Meaning
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73 ----- -------
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74 0-2 F3 00 02
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75 03 the port to poll
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76
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77 The return looks as follows:
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78
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79 Octet Meaning
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80 ----- -------
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81 0 status
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82 1 write max
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83 2 read max
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84
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85 If the status is nonzero, then the port is not open and the write max and
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86 read max items should be ignored.
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87
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88 Otherwise, write max will be set to the maximum number of octets the server
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89 is willing to handle on a write operation and read max will be the number of
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90 bytes the server can provide in response to a read operation. If either
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91 number exceeds 255, then the server will set the respective value to 255. A
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92 value of 0 for the read max means the server has no data to send. A value of
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93 0 for the write max means the server's buffer is full and it is waiting for
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94 the endpoint to do something with it.
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95
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96
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97 03 VPORT_POLLMULTI
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98
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99 This operation is similar to VPORT_POLL except it allows polling multiple
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100 ports simultaneously. It will return 3 bytes for each port polled, organized
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101 as in the VPORT_POLL call. The ports are returned in the same order as
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102 specified in the request.
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103
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104 The request looks as follows:
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105
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106 Octet Meaning
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107 ----- -------
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108 0-2 F3 00 03
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109 3 number of ports to query
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110 4-n port nubmers to query
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111
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112 This operation can easily have quite a large request size and quite a large
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113 response size (maximum request size is 260 bytes and the maximum response is
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114 768 bytes). This call should only be used for a small number of active
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115 ports.
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116
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117 04 VPORT_READ
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118
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119 This request reads bytes from a virtual port. The request looks as follows:
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120
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121 Octet Meaning
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122 ----- -------
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123 0-2 F3 00 04
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124 3 port number
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125 4 maximum size to read in octets
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126
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127 The response is an LWWire variable length packet as follows:
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128
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129 Octet Meaning
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130 ----- -------
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131 0 status
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132 1 number of bytes returned (n)
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133 2-n returned data
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134
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135 If status is nonzero, then the read failed. This will usually be due to a
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136 lack of data (1) OR the port not being open (2). In this case, any
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137 subsequent octets in the packet can be ignored.
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138
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139 Otherwise, the packet will contain n bytes of data. The client must be
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140 certain not to request more bytes than it can handle.
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141
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142
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143 05 VPORT_WRITE
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144
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145 This request is used to send octets to a port. The request looks as follows:
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146
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147 Octet Meaning
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148 ----- -------
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149 0-2 F3 00 05
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150 3 port number
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151 4 number of bytes
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152 5-n the data to write
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153
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154 The response is as follows:
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155
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156 Octet Meaning
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157 ----- -------
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158 0 status
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159 1 bytes written
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160
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161 If status is zero, than the bytes written value will be valid. If all bytes
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162 were written, then it will match the number in the request. If this value is
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163 less than the requested number, it means some bytes were not written for
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164 whatever reason and the client must deal with queueing or buffering as
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165 appropriate.
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166
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167
6
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168 06 VPORT_READFIXED
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169
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170 This request reads bytes from a virtual port. The request looks as follows:
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171
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172 Octet Meaning
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173 ----- -------
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174 0-2 F3 00 04
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175 3 port number
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176 4 size to read in octets (N)
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177
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178 The response is looks as follows:
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179
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180 Octet Meaning
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181 ----- -------
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182 0 status
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183 1 number of bytes returned (n)
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184 2-... returned data
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185
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186 In all cases, there will be exactly N octets in the returned data. Any bytes
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187 not filled by actual data are undefined but should be zero.
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188
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189 If status is nonzero, then the read failed. This will usually be due to a
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190 lack of data (1) OR the port not being open (2). In this case, any
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191 subsequent octets in the packet can be ignored.
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192
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193 If (n) is less than (N), then there will be extra octets in the returned
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194 data section which must be ignored.
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195
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196 This option is provided as an alternative to the VPORT_READ call for systems
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197 which cannot easily handle a variable length packet safely. In particular, a
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198 115k "bitbanger" implementation on the Coco itself might choose to use this
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199 option to adding another 500+ bytes to its lwwire client driver. Ohter
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200 communication channels that do not have this limitation should prefer the
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201 variable length packet option with larger request sizes to minimize
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202 transaction count.
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203