NEW REFERENCE DOCUMENT OBTAINED
I found a new manual that defines the 370 channel specifications. It is a replacement for the earlier document but has a similar name. It is GA22-6974-x IBM System/360 and System/370 I/O Interface Channel to Control Unit Original Equipment Manufacturers' Information.
CHANGES FROM 360
One of the enhancements was to provide a two byte wide data path through the addition of a third cable which hosts a second set of Bus In and Bus Out signals. To differentiate use of this from the usual two cable single byte interface, IBM added some control tags in the Bus cable. These are Mark 0 Out, Mark 0 In, Mark 1 Out, Mark 1 In, Mark Out Parity and Mark In Parity. They are ignored by most control units but Mark 0 In is used for command retry.
IBM added Data Out and Data In tags to support a high speed feature. These are ignored by control units that don't use the facility. I will probably need to implement these just in case future uses of this channel will attach to very high speed devices.
IBM also added a Disconnect In tag that signals some I/O errors. It was a previously reserved signal on the cable thus it will only be generated by control units designed to work with 370 channels. Only some control units have this feature.
Additional Protocol Sequences
Disconnect In is used to assert that a control unit has an internal malfunction such as a processor error that won't allow it to use normal channel protocols to respond, thus the channel should abandon efforts to communicate with this control unit.
Mark 0 In is used to request Command Retry, where the control unit asks the channel to send a command another time because something stopped it from working properly the first time. The logic behind this is that the control unit expects that it may be able to successfully execute the command at a later time. This is either immediately or when the control unit signals it is no longer busy.
High Speed Transfer alternates the use of Service Out and Data Out tags (and correspondingly Service In and Data In) which allows operation at higher rates than the interlock specs dictated using only Service Out/In.